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    Example and Directions
    Words (or words that have the same definition) The definition is case sensitive (Optional) Image to display with the definition [Not displayed in Glossary, only in pop-up on pages] (Optional) Caption for Image (Optional) External or Internal Link (Optional) Source for Definition
    (Eg. "Genetic, Hereditary, DNA ...") (Eg. "Relating to genes or heredity") The infamous double helix CC-BY-SA; Delmar Larsen
    Glossary Entries
    Word(s) Definition Image Caption Link Source
    abdominopelvic cavity body cavity that fills the lower half of the trunk and holds the kidneys and the digestive and reproductive organs        
    abiotic factor nonliving aspect of the environment such as sunlight, air, or water        
    ABO blood group system system of red blood cell antigens controlled by a single gene with three common alleles on chromosome 9 that produces types A, B, AB, and O blood        
    absolute dating carbon-14 or other method of dating fossils that gives an approximate age in years        
    absorption process in which substances such as nutrients pass into the blood or lymph        
    accessory organ of digestion digestive organ that releases substances into the duodenum for chemical digestion but through which food does not pass; liver, gallbladder, or pancreas        
    acclimatization development of reversible changes to environmental stress over a relatively short period of time        
    acid solution with a pH lower than 7        
    acid rain low-pH precipitation that forms when certain primary pollutants in the atmosphere combine with water        
    acne common skin disorder in which pimples, blackheads, nodules, or other skin lesions occur when bacteria infect sebum-clogged pores        
    acquired immunodeficiency syndrome (AIDS) disorder characterized by frequent opportunistic infections that may eventually develop in people who are infected with human immunodeficiency virus (HIV)        
    actin thin protein filament in muscle cells that slides over a thick myosin filament to produce a muscle contraction        
    action potential reversal of electrical charge across the membrane of a resting neuron that travels down the axon of the neuron as a nerve impulse        
    activation energy energy needed to start a chemical reaction        
    active immunity ability to resist a specific pathogen that results when an adaptive immune response to the pathogen produces memory lymphocytes for that pathogen        
    active transport movement of substances across a plasma membrane that requires energy        
    acute disease short-term illness        
    adaptation genetically based trait that has evolved because it helps living things survive and reproduce in a given environment        
    adaptive immune system subset of the immune system that makes tailored attacks against specific pathogens or tumor cells such as the production of antibodies that match specific antigens        
    adaptive radiation process by which a single species evolves into many new species to fill available niches        
    Addison’s disease disorder characterized by hyposecretion of the adrenal cortex hormone cortisol, generally because the immune system attacks and destroys the adrenal gland        
    adhesion in the context of human reproduction, process in which a sperm "sticks" to an egg due to sperm-receptor proteins on the egg        
    adipose tissue type of connective tissue that is composed mainly of fat-storage cells called adipocytes; also called body fat        
    admixture situation in which new DNA sequences enter a population’s gene pool through interbreeding with individuals in another population        
    adolescence period of transition between childhood and adulthood during which significant physical, cognitive, and psychosocial changes occur        
    adolescent growth spurt (AGS) period of rapid physical growth that occurs during puberty        
    adrenal cortex outer layer of the adrenal gland that produces steroid hormones such as cortisol and aldosterone        
    adrenal gland one of a pair of glands located on top of the kidneys that secretes hormones such as cortisol and adrenaline        
    adrenaline non-steroid catecholamine hormone produced by the medulla of the adrenal glands that stimulates the fight-or-flight response        
    adrenal medulla central part of an adrenal gland that is surrounded by the adrenal cortex and that produces catecholamine hormones including adrenaline        
    adulthood life stage of the human organism between adolescence and death        
    aerobic referring to anything, such as a process or an organism, that requires oxygen        
    aerobic exercise any physical activity in which muscles are used well below their maximum contraction strength but for a relatively long period of time, consuming a large amount of oxygen        
    age-related macular degeneration (AMD) common cause of blindness that occurs because cells of the macula die        
    age-sex structure number of individuals of each sex and age or age group in a population        
    agonist any psychoactive drug that increases the activity of particular neurotransmitters        
    Air Quality Index (AQI) assessment of the levels of pollutants in the outdoor air that is based on their human health effects        
    alcoholic fermentation type of anaerobic respiration that includes glycolysis followed by the conversion of pyruvic acid to ethanol and carbon dioxide and the formation of NAD+        
    algal bloom excessive growth of algae in bodies of water because of high levels of nutrients, usually from fertilizer in runoff        
    allele one of two or more different versions of the same gene        
    allele frequency how often an allele occurs in a gene pool relative to the other alleles for that gene        
    Allen’s rule ecogeographic rule stating that animals living in hotter climates generally have longer extremities than closely related animals living in colder climates        
    allergen any substance, typically an antigen, that causes an allergy        
    allergy disorder in which the immune system makes an inflammatory response to a harmless antigen        
    allopatric speciation evolution of a new species that occurs when some individuals become geographically separated from the rest of their species        
    alveolus (plural, alveoli) one of a cluster of tiny sacs at the ends of bronchioles in the lungs where pulmonary gas exchange takes place        
    amino acid small molecule that is a building block of proteins        
    amnion membrane forming a fluid-filled sac that surrounds and protects an embryo and fetus        
    anabolic reaction endothermic reaction in organisms        
    anaerobic referring to anything, such as a process or an organism, that does not require oxygen        
    anaerobic exercise any physical activity in which muscles are used at close to their maximum contraction strength but for a relatively short period to time, consuming a small amount of oxygen        
    anaerobic respiration type of cellular respiration, such as fermentation, that does not require oxygen        
    analogous structure structure that is similar in unrelated organisms because it evolved to perform the same function and not because it was inherited from a common ancestor        
    anaphase third phase of mitosis during which sister chromatids separate and move to opposite poles of the cell        
    anaphylaxis severe, systemic allergic reaction that is considered a medical emergency        
    anatomically modern human any early member of the species Homo sapiens who was virtually indistinguishable in physical traits from humans today but was not necessarily behaviorally modern        
    anatomy study of the structure of the body        
    androgen general term for a male sex hormone such as testosterone        
    anemia disorder in which the blood has an inadequate volume of red blood cells, which are needed to carry oxygen to cells        
    angina chest pain or pressure that occurs when heart muscle cells do not receive adequate blood flow and become starved of oxygen        
    animal heterotrophic, multicellular eukaryote with cells that lack cell walls; member of the Animal Kingdom        
    anorexia nervosa eating disorder in which people consistently eat very little, resulting in extremely low body weight and often other signs and symptoms of starvation        
    antagonist any psychoactive drug that decreases the activity of particular neurotransmitters        
    anterior pituitary front lobe of the pituitary gland that synthesizes and secretes pituitary hormones        
    anthropogenic biome human-made terrestrial biome such as cropland, rangeland, or a dense settlement        
    anthropoid any non-prosimian primate; monkey, ape, or human        
    antibiotic drug drug that kills bacteria and usually cures bacterial infections and diseases        
    antibiotic resistance ability to withstand antibiotic drugs that has evolved in some bacteria        
    antibody large, Y-shaped protein produced by B cells that recognizes and binds to a nonself antigen in a humoral immune response        
    anticodon string of three nucleotide bases that are complementary to the three nucleotide bases of a codon for an amino acid        
    antigen molecule that the immune system identifies as either self or nonself and, if the latter, may respond to by forming antibodies        
    anus final part of the large intestine with an opening to the outside for feces to pass through        
    anxiolytic type of psychoactive drug that has a tranquilizing effect and inhibits anxiety        
    Apgar test simple assessment given to an infant immediately after birth to evaluate its transition from the uterus to the outside world        
    aphotic zone water in an aquatic biome that is deeper than 200 meters and where too little sunlight penetrates for photosynthesis to take place        
    appendicular skeleton bones of the upper and lower limbs, shoulder girdle, and pelvic girdle        
    appendix vestigial organ that projects from the colon of the large intestine and has no apparent function in adults        
    aquatic biome any water-based biome; may be a freshwater or saltwater biome        
    aquifer underground layer of rock that stores water        
    arboreal referring to trees, as in arboreal, or tree-living, animal        
    Archaea one of two prokaryotic domains; includes organisms that live in extreme environments        
    archaic human member of any now-extinct species of Homo that existed between about 500,000 years ago and the evolution of Homo sapiens, including the species Homo neanderthalensis, Homo floresiensis, and Homo denisova        
    areolar connective tissue common form of loose connective tissue found in the skin and mucous membranes and around blood vessels and internal organs        
    artery type of blood vessel that carries blood away from the heart and toward the lungs or body        
    artificial selection process in which organisms evolve traits useful to humans because people select which individuals are allowed to reproduce and pass on their genes to successive generations        
    assisted reproductive technology (ART) collection of medical procedures in which eggs and sperm are removed from an infertile couple and manipulated in ways that increase the chances of fertilization occurring, such as in vitro fertilization        
    asthma chronic inflammatory disease of the respiratory system in which airways periodically become inflamed, causing swelling and narrowing of the airways, which makes breathing difficult        
    atherosclerosis condition in which plaque builds up inside arteries, eventually causing the lumen inside to narrow and the arterial walls to stiffen        
    athlete’s foot infection of the skin between the toes by the fungus Trichophyton        
    atom smallest particle of an element that still has the properties of that element        
    ATP (adenosine triphosphate) energy-carrying molecule that cells use to power their metabolic processes        
    atrium (plural, atria) one of the two upper chambers of the heart that pumps blood to the ventricle below it        
    atrophy decrease in the size of a structure, such as a decrease in the size of a muscle through nonuse        
    Australopithecus genus of early hominins that is thought to be ancestral to the human genus Homo        
    autoimmune disease type of disease, such as type 1 diabetes, in which the immune system attacks the body’s own cells as though they were pathogens        
    autonomic nervous system division of the peripheral nervous system that controls involuntary activities        
    autosomal trait any genetic trait controlled by a gene on an autosome        
    autosome any of the chromosomes numbered 1 to 22 in humans that contains genes for characteristics unrelated to sex determination        
    autotroph organism that uses energy directly from the sun or from chemical bonds to produce organic compounds from simple inorganic molecules        
    axial skeleton division of the skeleton that includes the skull, rib cage, and vertebral column        
    axon long extension of the cell body of a neuron that transmits nerve impulses to other cells        
    baby boom period marked by a temporary but dramatic increase in the birth rate        
    Bacteria one of two domains of prokaryotes, some of which cause human diseases        
    balance ability to sense and maintain an appropriate body position        
    bariatric surgery weight-loss surgery for obesity that modifies the digestive system to limit the intake of food and/or absorption of nutrients from food        
    barrier method type of contraception in which a device such as a condom or diaphragm is used to physically block sperm from entering the uterus        
    basal cell carcinoma most common type of skin cancer that occurs in basal cells of the epidermis and rarely metastasizes        
    basal metabolic rate (BMR) amount of energy needed to keep the body functioning at rest        
    base solution with a pH higher than 7        
    B cell type of lymphocyte that fights infections by forming antibodies        
    Bergmann’s rule ecogeographic rule stating that, within a broadly distributed taxonomic group, populations or species of larger size are found in colder environments, whereas populations or species of smaller size are found in warmer environments        
    bile fluid produced by the liver and stored in the gall bladder that is secreted into the small intestine to help digest lipids and neutralize acid from the stomach        
    bilirubin brown pigment secreted into bile by the liver that is a byproduct of catabolism of dead red blood cells and is excreted in feces by the large intestine        
    binding site region of a protein responsible for binding with another molecule        
    binge eating disorder eating disorder in which people repeatedly binge on a large amount of food, followed by feelings of guilt but not purging and usually leading to excessive weight gain        
    binomial nomenclature method of naming species with two names, consisting of the genus name and species name        
    bioaccumulation increasing concentration of a nonbiodegradable substance as it passes from lower to higher trophic levels in an ecosystem        
    biochemical compound any carbon-based chemical compound that helps make up the cells of living things or carry out life processes        
    biochemical reaction chemical reaction that occurs inside living things        
    biodegradable referring to a substance such as a pollutant that breaks down in the environment due to natural processes        
    biodiversity all of the variety of life that exists on Earth, including the diversity of genes, species, and ecosystems        
    biodiversity hotspot area with a high level of biodiversity that is under serious threat from human actions        
    biogeochemical cycle interconnected pathways through which water or a chemical element such as carbon or nitrogen is continuously recycled through the biotic and abiotic components of ecosystems        
    biogeography study of how and why organisms live where they do        
    biomass amount of organic matter present in an individual organism or in all the organisms at a given trophic level        
    biomass energy energy obtained from living or recently living things, such as wood or biogas made from crops such as sugarcane        
    biome group of similar ecosystems with the same general type of physical environment anywhere in the world        
    biopsy surgical removal of a tissue specimen for analysis in a medical laboratory, usually to diagnose cancer        
    biosphere every part of Earth where life exists, including land, water, and air        
    biotechnology use of technology to change the genetic makeup of living things in order to produce useful products        
    biotic factor living aspect of the environment, including any organism of the same or different species        
    bipedalism walking on two legs        
    birth rate number of births in one year per 1,000 people in a population        
    birthweight weight of a baby at birth        
    Black Death mid-14th century Eurasian plague pandemic that was one of the deadliest pandemics in human history        
    bladder infection common type of urinary tract infection in which the bladder becomes infected, usually by bacteria but occasionally by fungi        
    blastocoele fluid-filled cavity within a blastocyst        
    blastocyst fluid-filled ball of cells that develops a few days after fertilization in the process of blastulation        
    blastulation process during the germinal stage in which the solid ball of cells called the morula changes into a fluid-filled ball of cells called the blastocyst        
    blind experiment scientific experiment in which subjects do not know in which experimental group they have been placed to help avoid bias in the outcome        
    blood fluid connective tissue that circulates throughout the body through blood vessels        
    blood-brain barrier highly selective membrane formed of epithelial cells that separates circulating blood from extracellular fluid in the brain and spinal cord        
    blood group system all of the genes, alleles, and possible genotypes and phenotypes that exist for a particular set of blood type antigens        
    blood pressure measure of the force exerted by circulating blood on the walls of arteries        
    blood type genetic characteristic associated with the presence or absence of antigens on the surface of red blood cells        
    blood vessel hollow, tube-like structure through which blood flows in the cardiovascular system; vein, artery, or capillary        
    body cavity fluid-filled space inside the body that holds and protects internal organs        
    body mass index (BMI) estimate of the fat content of the body that is calculated by dividing a person’s weight (in kilograms) by the square of the person’s height (in meters)        
    bolus lump of swallowed food        
    bone marrow soft connective tissue in spongy bone that produces blood cells        
    bone remodeling continuous, lifelong process in which existing bone is resorbed by osteoclasts and new bone is made by osteoblasts        
    bone repair process in which bone heals itself following a bone fracture        
    bone tissue hard connective tissue in bones that consists mainly of mineralized collagen matrix; also called osseous tissue        
    Bowman’s capsule structure surrounding the glomerulus of a nephron in a kidney into which substances that are filtered out of blood are passed to the renal tubule        
    brain central nervous system organ inside the skull that is the control center of the nervous system        
    brain stem lowest part of the brain that connects the brain with the spinal cord and controls unconscious functions such as heart rate and breathing        
    Broca’s area speech center in the brain that controls speech production        
    bronchus (plural, bronchi) one of many tubes of various sizes that carry air between the trachea and the alveoli in the lungs        
    bulbourethral gland one of a pair of glands in the male reproductive system that secretes a fluid to help lubricate the urethra and neutralize any urine it may contain before ejaculation occurs; also called Cowper’s gland        
    bulimia nervosa eating disorder in which people recurrently binge on large amounts of food, following by purging the food from the body, for example, by vomiting        
    calcitonin endocrine hormone secreted by the thyroid gland that helps regulate calcium homeostasis by stimulating bones to absorb calcium from the blood        
    Calorie unit used to measure the energy in food, where 1 Calorie is the amount of energy needed to raise 1 kilogram of water by 1 degree Celsius; also called kilocalorie        
    Cambrian explosion spectacular burst of new life that occurred at the start of the Paleozoic Era        
    cancer disease that occurs when the cell cycle is no longer regulated and cells divide out of control        
    cancer staging classifying cancer based on such factors as how large a tumor is and the extent to which the cancer has spread        
    candidiasis infection of the mouth or vagina that is caused by the yeast Candida        
    canine tooth one of four pointed teeth on either side of the front teeth that are used for tearing foods        
    capillary smallest type of blood vessel that connects arterioles and venules and that transfers substances between blood and tissues        
    carbohydrate major class of biochemical compounds that includes sugars, starches, glycogen, cellulose, and chitin        
    carbon cycle biogeochemical cycle through which carbon is recycled through the biotic and abiotic components of ecosystems        
    carcinogen anything that can cause cancer        
    cardiac cycle contraction of the heart’s atria and ventricles to produce a single heartbeat, consisting of diastole and systole        
    cardiac muscle involuntary, striated muscle found only in the walls of the heart; also called myocardium        
    cardiovascular disease (CVD) class of diseases that involve the heart or blood vessels        
    cardiovascular system organ system consisting of the heart, blood vessels, and blood that transports materials to and from cells of the body        
    carnivore consumer that eats animals        
    carotene pigment in the epidermis that gives skin a yellowish tint, especially in skin with low levels of melanin        
    carpal tunnel syndrome musculoskeletal disorder that occurs when a nerve becomes compressed between carpal bones in the wrist, leading to reduced innervation of the thumb and first two fingers        
    carrying capacity (K) largest population size that can be supported in an area without harming the environment        
    cartilage dense connective tissue that provides a smooth surface for the movement of bones at joints        
    cartilaginous joint partly movable joint in which bones are joined by cartilage        
    case-control study observational study in which a group of subjects with a trait (cases) is compared retrospectively with a group of subjects without the trait (controls)        
    catabolic reaction exothermic reaction in organisms        
    catecholamine any non-steroid hormone, such as adrenaline, that is produced by the medulla of the adrenal gland and that stimulates the fight-or-flight response        
    cell basic unit of structure and function of living things        
    cell body central part of a neuron that contains the nucleus and other cell organelles        
    cell cycle repeating series of events that a cell goes through during its life including growth,        
    cell division process in which a parent cell divides to form two daughter cells        
    cell theory theory that all living things are made of cells, all life functions occur within cells, and all cells come from pre-existing cells        
    cellular respiration process in which cells break down glucose and make ATP for energy        
    cellulose complex carbohydrate (polysaccharide) that is an important structural component of the cell walls of plants and many algae        
    Cenozoic Era era of "modern life," also called the age of mammals, which began about 65 million years ago and still continues in the present        
    central nervous system (CNS) one of two main divisions of the nervous system that includes the brain and spinal cord        
    central obesity type of obesity in which most of the excess fat is stored in the abdomen, placing the obese person at greater risk of adverse health consequences of obesity, such as type 2 diabetes and cardiovascular disease        
    centriole organelle in eukaryotic cells that helps organize the chromosomes before cell division occurs, so that each daughter cell has the correct number of chromosomes after the cell divides        
    centromere region of sister chromatids where they are joined together        
    cerebellum part of the brain below the cerebrum and behind the brain stem that coordinates body movements        
    cerebral cortex highly folded, thin outer layer of the cerebrum where most information processing in the brain takes place        
    cerebrospinal fluid clear fluid produced by the brain that forms a thin layer within the meninges and provides protection and cushioning for the brain and spinal cord        
    cerebrum largest part of the brain that controls conscious functions such as reasoning and sight        
    cervical cancer cancer of the cervix of the uterus, usually caused by infection with human papillomavirus (HPV)        
    cervix neck of the uterus that protrudes down into the vagina and through which a canal connects the vagina and uterus        
    Chargaff’s rules observations by Erwin Chargaff that concentrations of the four nucleotide bases in nucleic acids differ among species and that, within a species, the concentrations of adenine and thymine are always about the same, as are the concentrations of cytosine and guanine        
    chemical bond force holding together atoms in a molecule that occurs when atoms share electrons        
    chemical digestion chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by blood or lymph        
    chemical equation symbolic way of showing what happens in a chemical reaction        
    chemical reaction molecular process that changes some chemical substances into others        
    chemoautotroph producer that uses energy in chemical bonds to produce organic molecules by chemosynthesis        
    chemoreceptor type of sensory receptor that responds to chemicals        
    chemosynthesis process of using the energy in chemical bonds to make organic molecules from inorganic substances        
    chemotaxis movement of a living structure in response to a chemical signal, as when chemical signals from an egg direct the movement of sperm toward the egg        
    chemotherapy treatment of cancer using one or more drugs that kill cancer cells        
    childhood stage of the human organism between birth and adolescence        
    chitin tough, complex carbohydrate (polysaccharide) that makes up the cell walls of fungi and the exoskeletons of insects and other arthropods        
    chlamydia sexually transmitted infection by the bacterium Chlamydia trachomatis that is the most common bacterial STI in the United States        
    chorion membrane formed from extraembryonic cells that helps form the fetal portion of the placenta        
    chromatid one of two identical copies of a chromosome that are joined together at a centromere before a cell divides        
    chromatin grainy material that DNA forms when it is not coiled into chromosomes        
    chromosomal alteration mutation that changes chromosome structure        
    chromosome coiled structure made of DNA and proteins that is the form in which the genetic material of a cell goes through cell division        
    chronic disease long-term or lifelong disease        
    chronic obstructive pulmonary disease (COPD) lung disease characterized by chronic poor airflow, most often following years of tobacco smoking        
    chyme thick, semi-liquid mixture that food in the gastrointestinal tract becomes by the time it leaves the stomach        
    cilia (singular, cilium) short, hairlike projections that extend from some cells, such as cells lining the airways        
    circadian rhythm regular change in biology or behavior that occurs in a 24-hour cycle        
    clade group of related organisms that includes an ancestor and all of its descendants        
    cladogram branching diagram representing related organisms in which each branch represents the emergence of new traits that separate one group of organisms from the rest        
    cleavage first several mitotic divisions of a fertilized egg that lead to the formation of a solid ball of cells called a morula        
    climate average weather in an area over a long period of time        
    climate change any change in average weather condition on Earth that lasts for at least several decades; often used to refer to ongoing global warming        
    cline line on a map connecting populations that have the same frequency of an allele or of a genetic trait, showing the geographic distribution of the allele or trait        
    clitoris small, sensitive external female organ that is part of the vulva and may lead to sexual arousal when stimulated        
    coagulation process by which blood changes from a liquid to a gel to form a blood clot        
    coal naturally occurring renewable solid fossil fuel that produces the most pollution and greenhouse gases when it is burned        
    cochlea coiled, fluid-filled tube in the inner ear that changes mechanical sound vibrations and positional information to nerve impulses that travel to the brain        
    codominance relationship between two alleles for the same gene in which both alleles are expressed equally in the phenotype of the heterozygote        
    codon group of three nitrogen bases in nucleic acids that makes up a code "word" of the genetic code        
    coevolution process in which two interacting species evolve together, with each species influencing the other’s evolution        
    coextinction phenomenon in which extinction of one species causes a second species to go extinct because the first species is a necessary resource for the second species        
    cohort study observational study in which a group of subjects selected at the start of the study (the cohort) is followed prospectively into the future        
    collecting duct one of a network of ducts in a kidney where additional water may be reabsorbed from urine        
    colon main part of the large intestine between the small intestine and rectum where water and salts are removed from liquid food wastes to form feces        
    commensalism symbiotic relationship between two species in which one species benefits while the other species is unaffected        
    community all the populations of all the species that live and interact in an ecosystem        
    compact bone dense outer layer of bone that is very hard and strong        
    comparative anatomy study of the similarities and differences in the structures of different species        
    comparative embryology study of the similarities and differences in the embryos of different species        
    competitive exclusion principle principle of ecology stating that two different species cannot occupy the same niche in the same place for very long        
    complementary base pair pair of nucleotide bases that bond together; either adenine and thymine (or uracil) or cytosine and guanine        
    complement system innate immune response that consists of a cascade of proteins that complement the killing of pathogens by antibodies        
    complex carbohydrate polysaccharide such as starch, glycogen, cellulose, or chitin        
    compound substance with a unique, fixed composition that consists of two or more elements        
    connective tissue tissue made up of cells that form the body’s structure, such as bone and cartilage, and in which cells are separated by non-living material called extracellular matrix        
    consensus science science undertaken in areas in which a great deal of evidence has already accumulated        
    consumer organism that consumes other organisms to obtain organic compounds        
    contraception any method or device used to prevent pregnancy; also called birth control        
    control center component of a homeostatic control mechanism that monitors a variable and sends signals to the effector as needed to keep the variable in homeostasis        
    coronary artery one of two arteries that supply the cells of the heart with oxygen and nutrients        
    coronary artery disease class of diseases that result from atherosclerosis of coronary arteries; includes angina and myocardial infarction        
    corpus luteum ovarian structure that forms from a follicle after it matures and ovulates an egg        
    correlation association between variables in which a change in one variable is associated with a change in the other variable        
    cortical bone tissue type of bone tissue that is smooth and dense and makes up the outer layer of bones        
    corticosteroid any steroid hormone produced by the cortex of the adrenal gland; includes mineralocorticoids, glucocorticoids, and androgens        
    cortisol glucocorticoid hormone produced by the cortex of the adrenal gland that helps control metabolic rate, suppression of the immune system, and other functions        
    cranial cavity cavity that fills most of the upper part of the skull and contains the brain        
    cranium upper part of the skull that encloses and protects the brain; also called braincase        
    Crohn’s disease inflammatory bowel disease that may affect any part of the gastrointestinal tract from the mouth to the anus        
    crossbridge cycling sequence of molecular events that forms crossbridges between myosin and actin filaments in muscle fibers, allowing for muscle contraction        
    cross-contamination contamination of a food by microbes in another food with which it comes into contact either directly or indirectly        
    crossing-over exchange of genetic material between homologous chromosomes when they are closely paired during meiosis I        
    cross-sectional study observational study in which data are collected from a sample of subjects just once at a certain point in time        
    culture entire way of life of a group of people        
    Cushing’s disease disorder in which there is hypersecretion of the adrenal cortex hormone cortisol, most commonly due to a tumor of the pituitary gland        
    cystic fibrosis autosomal recessive genetic disease that causes the production of sticky mucus that clogs respiratory and digestive organs, causing frequent lung infections and malabsorption of nutrients        
    cytokine chemical released by injured, infected, or immune cells that triggers inflammation or other immune responses        
    cytokinesis splitting of the cytoplasm to form daughter cells when a cell divides        
    cytoplasm all of the material inside the plasma membrane of a cell (excluding organelles), consisting largely of cytosol        
    cytoskeleton structure of filaments and tubules in the cytoplasm that provides a cell with an internal framework        
    cytosol watery substance that makes up cytoplasm inside cells        
    data (singular, datum) observations collected in a scientific investigation in order to test a hypothesis        
    dead zone area in the ocean or other body of water where low oxygen levels following excessive growth of algae have killed virtually all aquatic organisms        
    death rate number of deaths per year per 1,000 people in a population        
    deciduous teeth first set of 20 teeth that emerge in infancy and early childhood and are lost and replaced by permanent teeth during middle childhood; also called baby teeth        
    decomposer organism that breaks down the remains of dead organisms or other organic wastes        
    demographic transition major population shift in which the death rate falls and the population growth rate increases, followed later by the birth rate falling and the population growth rate decreasing        
    demography scientific study of human populations        
    dendrite extension of the cell body of a neuron that receives nerve impulses from other neurons        
    dependent variable variable in a scientific experiment that is affected by another variable, known as the independent variable; also called outcome variable        
    depressant type of psychoactive drug that calms the brain, reduces anxious feelings, and induces sleepiness        
    dermis inner layer of skin that is made of tough connective tissue and contains blood vessels, nerve endings, hair follicles, and glands        
    desalination process in which salt is removed from ocean water to produce fresh water for human use        
    descriptive statistics statistics such as the mean and standard deviation that describe and summarize data        
    developmental adjustment nongenetic phenotypic change that occurs during development in infancy or childhood and that may persist into adulthood and become irreversible        
    diabetes mellitus disease caused by problems with the pancreatic hormone insulin, which lead to high blood glucose levels and symptoms such as excessive thirst and urination; includes type 1 and type 2 diabetes        
    diabetic nephropathy progressive kidney disease caused by damage to capillaries in the glomeruli of the kidneys due to poor blood sugar control in people with diabetes        
    diaphragm large, dome-shaped muscle below the lungs that allows breathing to occur when it alternately contracts and relaxes        
    diastole part of a heartbeat in which the atria contract and pump blood into the ventricles, while the ventricles relax and fill with blood from the atria        
    differentiation process by which unspecialized cells become specialized into one of many different types of cells, such as neurons or epithelial cells        
    diffusion type of passive transport that does not require the help of transport proteins        
    digestion process of breaking down food into nutrients that can be absorbed by blood or lymph        
    digestive system organ system that breaks down food, absorbs nutrients, and eliminates any remaining wastes        
    diploid having two of each type of chromosome, or twice the number of chromosomes as a haploid gamete        
    directional selection type of natural selection for a polygenic trait in which one of two extreme phenotypes is selected for, resulting in a shift in the phenotypic distribution toward that extreme        
    disaccharide sugar consisting of two monosaccharides        
    disease condition associated with impairment of normal body functioning, generally causing signs or symptoms of illness        
    disease burden impact of a disease on a population as measured by financial cost, mortality, morbidity, or other indicators        
    disease outbreak event in which two or more people contract an infectious disease from a common source of infection        
    disruptive selection type of natural selection for a polygenic trait in which phenotypes in the middle of the phenotypic distribution are selected against, resulting in two overlapping phenotypes, one at each end of the distribution        
    diverticulitis disease in which one or more pouches (diverticula) in the large intestine become infected and inflamed        
    diverticulosis condition in which pouches called diverticula form in the wall of the large intestine        
    DNA (deoxyribonucleic acid) double-stranded DNA that makes up genes and chromosomes        
    DNA replication natural process of copying DNA prior to cell division        
    domain taxon in the revised Linnaean system that is larger and more inclusive than the kingdom        
    dominant referring to an allele that masks the presence of another allele for the same gene when the two alleles occur together in a heterozygote, or referring to a trait that is controlled by such an allele        
    dormancy state in which a plant slows down cellular activity and may shed its leaves        
    dorsal cavity major human body cavity that includes the head and the posterior (back) of the trunk and holds the brain and spinal cord        
    double-blind experiment scientific experiment in which neither the subjects nor the researchers know in which group subjects have been placed to help avoid bias in the outcome        
    double helix double spiral shape of the DNA molecule        
    doubling time time it would take a population to double in size at its current rate of growth        
    drowning respiratory impairment from being in or under a liquid, whether or not death results        
    drug abuse use of a drug without the advice of a medical professional and for reasons not originally intended        
    drug addiction compulsive use of a substance such as a psychoactive drug despite negative consequences that such use may entail        
    drug dependence condition in which stopping use of a substance such as a psychoactive drug produces physical and/or psychological symptoms        
    duodenum first and shortest of three parts of the small intestine where most chemical digestion occurs        
    ear special sensory organ that collects and amplifies sound waves and information on body position and transforms them into nerve impulses that travel to the brain        
    early adulthood first stage of human adulthood that coincides more or less with the 20s and early 30s        
    early childhood stage of a human organism between infancy and middle childhood that covers the ages 1-5 years        
    early modern human any member of the species Homo sapiens who lived prior to about 10,000 years ago        
    eating disorder mental illness in which people feel compelled to eat in a way that causes physical, mental, and/or emotional health problems        
    ecological succession changes through time in the numbers and types of species in an ecosystem        
    ecology study of how living things interact with each other and their environment        
    ecosystem all the biotic and abiotic factors in an area and their interactions        
    ecosystem service any of the goods or services that healthy ecosystems provide to ensure the survival of living things on Earth        
    ectoderm outer germ layer of an embryo that will eventually give rise to skin and nerve cells        
    effector component of a homeostatic control mechanism, such as a gland or an organ, that acts on a signal from the control center to move the variable back toward the set point        
    egg gamete produced by a female        
    ejaculation process in males in which muscle contractions propel sperm from the epididymes and out through the urethra in semen        
    ejaculatory duct one of two tubes in the male reproductive system that joins the vas deferens with the urethra and carries semen during ejaculation        
    electromagnetic force atomic force holding together oppositely charged particles (electrons and protons) and repelling particles of the same charge (two electrons or two protons)        
    electron transport third and final stage of cellular respiration in which electrons from NADH and FADH2 move along an electron-transport chain and protons (H+) are transferred across the membrane of a mitochondrion, thereby driving the synthesis of ATP        
    element pure substance that cannot be broken down into other types of substances        
    elimination process in which wastes pass out of the body        
    embryo stage of growth and development that occurs from implantation in the uterus through the eighth week after fertilization        
    embryoblast mass of cells inside a blastocyst that will eventually develop into an embryo        
    embryonic disc two-layered disc of cells that forms by about 12 days after fertilization and is an early stage in embryo formation        
    emergency contraception any form of birth control that is used after unprotected vaginal intercourse        
    emerging infectious disease new infectious disease that has no prior history in a human population        
    emigration movement of individuals out of a population        
    empathogen type of psychoactive drug that produces feelings of empathy with other people        
    emphysema lung disease, usually caused by smoking, in which walls of alveoli break down, so less gas can be exchanged in the lungs        
    encephalization evolutionary enlargement of the brain relative to body size        
    encephalization quotient measure for comparing brain sizes that corrects for differences in body size        
    endemic disease disease that is present year-round in a population        
    endemic species species native to a given region and not found naturally anywhere else on Earth        
    endocrine gland any gland of the endocrine system, which is the system of glands that release their hormones into the blood        
    endocrine hormone chemical messenger molecule secreted by a gland of the endocrine system into the blood        
    endocrine system human body system of glands that release hormones into the blood        
    endoderm inner germ layer of the embryo that eventually gives rise to cells of organs and glands such as the lungs and thyroid        
    endometriosis disease in which endometrial tissue grows outside the uterus, typically causing pain and bleeding        
    endometrium innermost layer of the uterus that builds up during each menstrual cycle and helps nourish the embryo if fertilization occurs or is shed from the uterus as menstrual flow if fertilization does not occur        
    endoplasmic reticulum (ER) organelle in eukaryotic cells that helps make and transport proteins and lipids        
    endosymbiotic theory theory that eukaryotic organelles such as mitochondria evolved from ancient, free-living prokaryotes that invaded primitive eukaryotic cells        
    endothermic reaction chemical reaction that absorbs energy        
    energy ability to do work        
    energy density amount of Calories a food provides per unit of mass or volume        
    energy homeostasis balance between the energy consumed in food and the energy expended in metabolism and physical activity        
    enteric division division of the autonomic nervous system that controls digestive functions        
    enzyme protein that speeds up biochemical reactions; biological catalyst        
    epidemic large-scale disease outbreak in a population        
    epidemiology study of the patterns, causes, and effects of diseases in human populations        
    epidermis outer layer of skin that consists mainly of epithelial cells and lacks nerve endings, blood vessels, and other structures        
    epididymis (plural, epididymes) one of two male reproductive organs where sperm mature and are stored until they leave the body during ejaculation        
    epididymitis inflammation of the epididymis, which may be acute or chronic        
    epistasis situation in which a gene affects the expression of another gene or genes        
    epithelial tissue tissue made up of cells that line inner and outer body surfaces, such as skin and mucous membranes        
    erectile dysfunction (ED) disorder characterized by the regular and repeated inability of a sexually mature male to obtain or maintain an erection of the penis        
    erection state in which the penis becomes stiff and erect, usually during sexual arousal, as its columns of spongy tissue become engorged with blood        
    esophagus long, narrow, tube-like digestive organ through which food passes from the pharynx to the stomach        
    essential nutrient any nutrient that cannot be synthesized in the human body in adequate amounts for normal functioning so it must be obtained from food        
    estrogen female sex hormone secreted mainly by the ovaries        
    eukaryote organism that has cells containing a nucleus and other organelles        
    eukaryotic cell cell that contains a nucleus and other organelles        
    euphoria intense feeling of well-being and happiness        
    euphoriant type of psychoactive drug that brings about a state of euphoria        
    evidence any type of data that may be used to test a hypothesis        
    evolution change in the genetic characteristics of living things over time        
    exchange pool part of a biogeochemical cycle that holds an element or water for a short period of time        
    excretion process of removing wastes and excess water from the body        
    excretory system organ system that removes wastes and excess water from the body and includes the kidneys, large intestine, liver, skin, and lungs        
    exocrine gland gland such as a sweat gland, salivary gland, or mammary gland that secretes a substance into a duct that carries the secretion to the outside of the body        
    exon region of DNA or RNA that codes for a protein        
    exothermic reaction chemical reaction that releases energy        
    experiment type of scientific investigation that is performed under controlled conditions and in which the researcher typically manipulates a variable (independent variable) to see how it affects another variable (dependent variable)        
    exponential growth pattern of population growth in which a population starts out growing slowly but grows at an increasing rate as population size increases        
    extinction event in which a species completely dies out and no members of the species remain anywhere on Earth        
    eye special sensory organ that collects and focuses light to form images and transforms the images into nerve impulses that travel to the brain        
    facilitated diffusion diffusion with the help of transport proteins        
    Fallopian tube one of two female reproductive organs that carry eggs from an ovary to the uterus and are the site where fertilization usually takes place        
    fast-twitch muscle fiber type of skeletal muscle cell that is mainly responsible for anaerobic activities such as weight lifting        
    fatty acid simple lipid molecule that consists mainly of chains of carbon and hydrogen atoms and that may exist alone or as a monomer of larger lipid molecules        
    feces solid waste that remains after food is digested and that is eliminated from the body through the anus        
    fermentation type of anaerobic respiration that includes glycolysis followed by the conversion of pyruvic acid to one or more other compounds and the formation of NAD+        
    fertility awareness method contraceptive method in which a woman monitors signs of ovulation and avoids unprotected vaginal intercourse around the time of ovulation when fertilization is most likely to occur        
    fertility rate average number of livebirths per woman by the end of the childbearing years        
    fertilization union of two haploid gametes (egg and sperm), resulting in a diploid zygote        
    fetal viability point in fetal development at which a fetus is likely to be able to survive outside the uterus        
    fetus prenatal human organism between the embryonic stage and birth (weeks 8 through 38 after fertilization)        
    fiber indigestible carbohydrates such as cellulose that is needed in the human diet for normal bowel movements, among other functions        
    fibrous connective tissue type of connective tissue that is found in skin, tendons, and ligaments and that is composed of parallel bundles of collagen fibers, making it tough and elastic        
    fibrous joint immovable joint in which bones are connected by collagen fibers; also called a suture        
    fight-or-flight response involuntary human body response mediated by the nervous and endocrine systems that prepares the body to fight or flee from perceived danger        
    fitness in the context of evolution by natural selection, the relative ability of an organism to survive and produce fertile offspring        
    flexibility exercise any physical activity that stretches and lengthens muscles        
    folic acid micronutrient needed for good health and reproduction; also called vitamin B9        
    follicle anatomical structure that consists of a small cluster of cells, surrounding a central cavity        
    follicle-stimulating hormone (FSH) pituitary gland hormone that stimulates the ovaries to secrete estrogen and mature follicles        
    fontanel "soft spot" on an infant’s skull that gradually ossifies after birth        
    food organic molecules that organisms use for energy, building materials, and other purposes        
    foodborne disease any disease that is transmitted via food; also called food poisoning        
    food chain diagram that represents a single pathway by which energy flows through organisms an ecosystem        
    food web diagram that represents multiple interacting pathways by which energy flows through organisms in an ecosystem        
    foramen ovale temporary opening between the right and left atria of the fetal heart that allows blood to bypass the pulmonary (lung) circulation until the fetus starts breathing air after birth        
    force of evolution any one of four factors (mutation, gene flow, genetic drift, and natural selection) that changes allele frequencies in a population’s gene pool        
    fossil preserved remains or traces of an organism that lived in the past        
    fossil fuel nonrenewable energy resource that forms over millions of years from dead plants and produces pollutants and greenhouse gases when it burns; coal, petroleum, or natural gas        
    fossil record record of life on Earth as told by the discovery and analysis of fossils        
    frameshift mutation deletion or insertion of one or more nucleotides that changes the reading frame of the genetic code        
    free radical highly reactive chemical such as OH- that can damage DNA and contribute to diseases such as cancer        
    freshwater biome aquatic biome such as a pond, lake, stream, or river in which the water contains little or no salt        
    frontal lobe part of each hemisphere of the cerebrum that controls executive functions such as reasoning and language        
    frontier science science undertaken in new areas at the frontier of knowledge in a particular field        
    frostbite tissue destruction, usually in the extremities, that occurs when tissue freezes        
    fungi (singular, fungus) organisms in the Fungus Kingdom that includes molds, mushrooms, and yeasts, some of which cause human disease        
    Galápagos Islands group of 16 small volcanic islands in the Pacific Ocean 966 kilometers (600 miles) off the west coast of South America, where Charles Darwin made some of his most important observations during his voyage on the HMS Beagle        
    gallbladder sac-like organ that stores bile from the liver and secretes it into the duodenum of the small intestine as needed for digestion        
    gamete reproductive cell produced during meiosis that has the haploid number of chromosomes; sperm or egg        
    gametogenesis development of a haploid cell into a gamete (either sperm or egg)        
    ganglion (plural, ganglia) type of nervous tissue that acts as a relay point for messages transmitted through nerves of the peripheral nervous system        
    gas exchange biological process through which gases are transferred across cell membranes to either enter or leave the blood        
    gastroenteritis acute and usually self-limiting infection of the gastrointestinal tract by pathogens; also known as infectious diarrhea        
    gastrointestinal (GI) tract organs of the digestive system through which food passes during digestion, including the mouth, pharynx, esophagus, stomach, and small and large intestines        
    gastrulation process in which the two-layer embryonic disc develops a third cell layer (mesoderm) and a primitive gut        
    gene unit of DNA on a chromosome that may be encoded with the instructions for a protein        
    gene cloning process of isolating and making copies of a gene        
    gene expression use of a gene to make a protein        
    gene flow change in allele frequencies that occurs when individuals move into or out of a population        
    gene pool all the genes in all of the members of a population        
    generalist organism that can thrive in a wide variety of environments and make use of a variety of different resources, such as consuming many different types of food        
    general sense sense of touch, which lacks specialized sensory organs and is monitored instead by sensory receptors all over the body        
    gene theory theory that many of the characteristics of living things are controlled by genes that are passed from parents to offspring        
    gene therapy experimental method of curing genetic disorders by inserting normal genes into cells with mutant genes        
    genetic code universal code of three-base codons that encodes genetic instructions for the amino acid sequences of proteins        
    genetic disorder disease, syndrome, or other abnormal condition caused by a mutation in one or more genes or by a chromosomal alteration        
    genetic drift random change in allele frequencies that occurs in a small population        
    genetic engineering using biotechnology to change the genetic makeup of an organism        
    genetics science of heredity        
    genetic trait characteristic that is encoded in DNA        
    genital herpes sexually transmitted infection caused by herpes simplex virus (usually HSV-2) that is characterized by periodic outbreaks of blisters on the genitals        
    genital warts small, rough growths on the genitals caused by a sexually transmitted infection with human papillomavirus (HPV)        
    genotype alleles an individual inherits at a particular genetic locus        
    genus taxon above the species in the Linnaean classification system; group of closely related species        
    geologic time scale timeline of Earth based on major events in geology, climate, and the evolution of life        
    geothermal energy renewable, nonpolluting source of energy that can be obtained from Earth’s naturally occurring internal heat        
    germinal stage earliest and shortest stage of the human lifespan, lasting about a week from fertilization until the new organism implants in the endometrium of the uterus        
    germline mutation mutation that occurs in a gamete        
    germ theory of disease theory that contagious diseases are caused by the transmission of microorganisms that grow and reproduce in their hosts and make them sick        
    gestational carrier woman who agrees to receive a transplanted embryo from a couple, carry it to term, and then give the infant to the couple after birth        
    ghrelin hormone produced in the stomach and small intestine that normally stimulates appetite        
    giardiasis type of gastroenteritis caused by a single-celled protozoan parasite named Giardia lambliathat typically spreads through contaminated food or water via a fecal-oral route        
    glial cell nervous tissue cell that provides support for neurons and helps them transmit nerve impulses        
    global warming recent rise in Earth’s average surface temperature generally attributed to an enhanced greenhouse effect caused mainly by human actions        
    glomerulus (plural, glomeruli) network of capillaries in the nephron of a kidney where substances are filtered out of the blood        
    glucagon endocrine hormone secreted by alpha cells of the pancreas that signals cells to break down stored glycogen to glucose, thereby increasing the level of blood glucose        
    glucose simple six-carbon sugar, or monosaccharide, that is the nearly universal food for life        
    glucose-6-phosphate dehydrogenase (G6PD) red blood cell enzyme involved in metabolizing glucose, deficiency in which is a genetic adaptation to malaria        
    glycogen complex carbohydrate (polysaccharide) that animals use to store energy        
    glycolysis first stage of cellular respiration in which glucose is split, in the absence of oxygen, to form two molecules of pyruvate (pyruvic acid) and two (net) molecules of ATP        
    goiter abnormal enlargement of the thyroid gland        
    Golgi apparatus organelle in eukaryotic cells that processes proteins and prepares them for use both inside and outside the cell        
    gonad one of a pair of organs that secrete sex hormones and produce gametes; testis in males and ovary in females        
    gonorrhea common sexually transmitted infection that is caused by the bacterium Neisseria gonorrhoeae        
    gradualism model of the timing of evolution in which evolutionary change occurs at a slow and steady pace        
    Grave’s disease autoimmune disorder in which abnormal antibodies produced by the immune system stimulate the thyroid gland to secrete excessive quantities of its hormones        
    gray matter type of nervous tissue that is found only in the brain and spinal cord and that is important for information processing        
    greenhouse effect natural feature of Earth’s atmosphere that occurs when greenhouse gases in the atmosphere absorb and re-radiate heat back down to Earth’s surface, making the surface temperature warmer than it otherwise would be        
    greenhouse gas naturally occurring or anthropogenic gas such as carbon dioxide that allows the greenhouse effect to warm Earth’s surface        
    green revolution worldwide advances in agriculture, including increased use of machinery and chemicals, that occurred between 1950 and 1984 and greatly increased global food production        
    groundwater fresh water that exists in the ground, either in the soil or in rock layers below the surface        
    growing season period of time each year when it is warm enough and wet enough for plants to grow        
    growth hormone hormone secreted by the anterior pituitary gland that stimulates growth in cells all over the body        
    habitat natural environment in which a species lives and to which it is adapted        
    habitat loss destruction or disruption of a natural habitat, most often due to human actions such agriculture, forestry, mining, or urbanization        
    hair filament made of tightly packed, keratin-filled keratinocytes that grows out a hair follicle in the dermis of the skin        
    hair follicle structure in the dermis of skin where a hair originates        
    hair root part of a hair that is located within the hair follicle and consists of living keratinocytes        
    hair shaft part of a hair that is visible above the surface of the skin and consists of dead keratinocytes        
    hallucinogen type of psychoactive drug that causes hallucinations and other perceptual anomalies, as well as subjective changes in thoughts, emotions, and consciousness        
    haploid having only one chromosome of each type, or half the number of chromosomes of a diploid cell        
    Hardy-Weinberg theorem founding principle of population genetics that proves allele and genotype frequencies do not change in a population in which the forces of evolution (mutation, natural selection, genetic drift, and migration) are not operating        
    Hashimoto’s thyroiditis autoimmune disorder in which the immune system destroys the thyroid gland, causing insufficient secretion of thyroid hormones        
    hearing ability to sense sound waves        
    heart muscular organ in the chest that pumps blood through blood vessels when it contracts        
    heart attack blockage of blood flow to heart muscle tissues that may result in the death of cardiac muscle fibers        
    heat index (HI) number that combines air temperature and relative humidity to indicate how hot the air feels due to the humidity        
    helminth parasitic worm such as tapeworm, pinworm, or hookworm        
    hematopoiesis process in which red blood cells, white blood cells, and platelets are produced by red bone marrow        
    hemisphere one of two halves (left and right) of the cerebrum of the human brain        
    hemodialysis medical procedure for patients with kidney failure in which wastes and excess water are artificially filtered out of blood by passing it through a machine        
    hemoglobin oxygen-binding protein containing iron that is the principal component of red blood cells        
    hemoglobin C abnormal hemoglobin variant found mainly in West and North Africa that is a genetic adaptation to malaria        
    hemoglobin E abnormal hemoglobin variant found mainly in South and Southeast Asia that is a genetic adaptation to malaria        
    hemoglobin S abnormal hemoglobin variant found in sub-Saharan Africa, the Middle East, and India that is the single most effective genetic adaptation to malaria        
    hemolytic disease of the newborn (HDN) type of anemia in which red blood cells in a Rhesus positive fetus are destroyed by antibodies to the Rhesus antigen produced by a Rhesus negative mother        
    hemophilia any of several genetic disorders that cause dysfunction in the blood-clotting process, leading to uncontrolled bleeding from even minor injuries        
    herbivore consumer that eats producers such as plants or algae        
    herd immunity protection from infection of nonvaccinated people in a population because most of the people around them are vaccinated and immune to the infection        
    heterotroph organism that obtains energy by consuming other organisms        
    heterozygote organism that inherits two different alleles for a given gene        
    high-altitude sickness acute illness that results from hypoxia at high altitude and includes symptoms such as fatigue, shortness of breath, headache, and dizziness        
    histamine chemical released by injured or infected cells or by certain immune system cells that triggers inflammation or an allergic reaction        
    homeobox gene one of many genes that regulate early development of the organism        
    homeostasis condition in which a system such as the human body is maintained in a more-or-less steady state        
    homeostat physiological mechanism that helps maintain homeostasis in an organism; also called homeostatic mechanism        
    homeostatic imbalance condition in which cells may not get everything they need or toxic wastes may accumulate because of the failure of a homeostatic mechanism        
    hominid any member of the family of the primate order that includes humans and great apes        
    hominin any human or human ancestor that evolved after the human lineage diverged from the chimpanzee lineage about 6 million years ago        
    Homo genus in which the only living species is Homo sapiens        
    Homo denisova extinct species of archaic humans that lived in Asia beginning about 400,000 years ago and interbred with Neanderthals and early modern Homo sapiens; also called Denisovans        
    Homo erectus early species in the genus Homo that first evolved about 1.9 million years ago in East Africa, migrated to Eurasia before going extinct about 800,000 years ago, and was probably ancestral to Homo heidelbergensis        
    Homo floresiensis extinct species of archaic humans that lived on Flores Island in Indonesia from about 100,000 to 60,000 years ago and were very small in size        
    Homo habilis first known species in the genus Homo that lived from about 2.5 to 1.4 million years ago in East Africa and probably was ancestral to Homo erectus        
    Homo heidelbergensis transitional species in the genus Homo that probably evolved from Homo erectus about 800,000 years ago and evolved into Homo neanderthalensis and early Homo sapiensaround 200,000 years ago        
    homologous chromosomes pair of chromosomes that have the same size and shape and contain the same genes        
    homologous structure structure that is similar in related species because it was inherited from a common ancestor; or structure that develops from the same undifferentiated embryonic tissue in males and females of the same species, such as the testis and ovary in humans        
    Homo neanderthalensis extinct species of archaic humans that lived in Europe and Asia between about 400,000 and 30,000 years ago; also called Neanderthals        
    homozygote organism that inherits two alleles of the same type for a given gene        
    hormonal contraception use of hormones such as estrogen and/or progesterone to prevent pregnancy by interfering with ovulation        
    host species that is harmed in a parasitic relationship with another species        
    human biology scientific study of the anatomy, physiology, genetics, evolution, and ecology of the human species        
    human genome all of the DNA of the human species        
    Human Genome Project international science project that sequenced all 3.3 billion base pairs of the human genome        
    human immunodeficiency virus (HIV) virus transmitted through body fluids or mucous membranes that infects and destroys helper T cells and may eventually cause acquired immunodeficiency syndrome (AIDS)        
    human papillomavirus (HPV) sexually transmitted virus that may cause genital warts and cervical cancer        
    hunting and gathering type of subsistence strategy that includes hunting wild animals and gathering wild plants and that was utilized by humans from Homo erectus until at least 10,000 years ago        
    hunting response process of alternating vasoconstriction and vasodilation in extremities that are exposed to cold        
    hydrogen bond relative weak chemical bond that forms between molecules of the same substance, such as between molecules of water        
    hydropower renewable, nonpolluting energy that can be obtained from moving water and used to generate electricity        
    hyperopia vision problem in which close objects are out of focus but distant vision is unaffected; also called farsightedness        
    hypersecretion secretion of more than the normal amount of a substance, such as secretion of too much hormone by an endocrine gland        
    hypertension persistently high blood pressure, generally defined as 140/90 mm Hg or higher        
    hyperthermia dangerous increase in core body temperature above 40.6 degrees C (105 degrees F)        
    hyperthyroidism disorder in which the thyroid gland produces excessive amounts of hormones        
    hypertrophy increase in the size of a structure, such as an increase in the size of a muscle through exercise        
    hyperventilation breathing more quickly and shallowly than normal        
    hyposecretion secretion of less than the normal amount of a substance, such as an endocrine hormone        
    hypothalamus part of the brain that secretes hormones and connects the brain with the endocrine system        
    hypothermia dangerous decrease in core body temperature below 34.4 degrees C (94 degrees F)        
    hypothesis logical answer to a scientific question that may be tested in a scientific investigation        
    hypothyroidism disorder in which the thyroid gland produces inadequate amounts of hormones        
    hypoxia relative lack of oxygen, as, for example, at high altitude        
    ileum final of three parts of the small intestine where vitamin B12 and bile salts are absorbed        
    immigration movement of individuals into a population        
    immune surveillance function of the immune system in which it identifies and eliminates tumor cells        
    immune system organ system that defends the body form pathogens and cancer        
    immunity ability to resist a pathogen due to the formation of memory lymphocytes (B or T cells) to that specific pathogen        
    immunization deliberate exposure of a person to a pathogen in order to provoke an immune response and the formation of memory cells specific to that pathogen        
    immunodeficiency inability of the immune system to fight off pathogens that a normal, healthy immune system would be able to resist because the immune system is damaged        
    immunotherapy treatment for an allergy in which a patient is gradually desensitized to an allergen through periodic injections with increasing amounts of the allergen; or treatment for cancer that attempts to stimulate the immune system to destroy cancer cells        
    implantation process in which a blastocyst embeds in the endometrium lining the uterus        
    incisor one of eight (four upper and four lower) blade-like teeth at the front of the mouth that are used to slice off pieces of food        
    incomplete dominance relationship between the alleles for a gene in which one allele is only partly dominant to the other allele, producing an intermediate phenotype        
    incubation period length of time between infection with a pathogen or other agent of disease and the first appearance of symptoms        
    independent variable variable in a scientific experiment that is manipulated by the researcher to investigate its effect on another variable, known as the dependent variable; also called the manipulated variable        
    infancy first year of life after birth in humans        
    infant human being between birth and the first birthday        
    infant mortality rate annual number of infant deaths per 1,000 live births in a population, often used as a reliable indicator of the overall level of health of the population        
    infection invasion of an organism’s tissues by pathogens        
    infectious disease any disease caused by pathogens; also called communicable or contagious disease        
    infectious dose amount of a pathogen that must infect an organism to cause disease        
    inferential statistics statistics such as the t-test that help researchers interpret data to test hypotheses        
    infertility failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse        
    inflammation response of the innate immune system that establishes a physical barrier against the spread of infection and repairs tissue damage while causing redness, swelling, and warmth        
    inflammatory bowel disease type of disease in which the immune system attacks the intestines, causing diarrhea and abdominal pain; for example, Crohn’s disease or ulcerative colitis        
    inheritance of acquired characteristics mistaken idea of Jean Baptiste Lamarck that evolution occurs through the inheritance of traits that an organism develops in its own life time        
    innate immune system subset of the immune system that makes generic attacks such as inflammation against invading pathogens        
    insoluble fiber nondigestible carbohydrates in food that do not dissolve in water but provide bulk and stimulate peristalsis in the digestive tract        
    insulin endocrine hormone secreted by beta cells of the pancreas that normally helps cells take up glucose from the blood, thereby decreasing the blood glucose level        
    integumentary system human body system that includes the skin, nails, and hair        
    interneuron type of neuron that carries nerve impulses back and forth between sensory and motor neurons        
    interphase stage of the eukaryotic cell cycle when the cell grows, synthesizes DNA, and prepares to divide        
    interspecific competition relationship between two species that depend on the same limiting resources in their ecosystem        
    intrauterine device (IUD) T-shaped contraceptive structure containing copper or a hormone that is inserted into the uterus by a physician and may be left in place for months or years        
    intrauterine growth restriction (IUGR) abnormally slow growth of a fetus, which may be due to maternal, fetal, or placental factors        
    intromission process in which a male’s penis deposits sperm in a female’s vagina        
    intron region of DNA or RNA that does not code for a protein        
    invasive species species that is introduced (usually by human actions) into a new habitat where it may out-compete native species        
    ion atom that has gained or lost electrons so it has more or fewer electrons than protons and a negative or positive charge        
    isometric referring to a muscle contraction in which muscle tension increases but muscle length remains the same        
    isotonic referring to a muscle contraction in which muscle length decreases but muscle tension remains the same        
    isotope alternate form of an atom, with the same number of protons but a different number of neutrons        
    jejunum middle of three parts of the large intestine where most absorption of nutrients occurs        
    joint structure where two or more bones of the skeleton come together        
    keratin tough, fibrous protein in skin, hair, and nails        
    keratinocyte type of epithelial cell found in the skin, hair, and nails that produces keratin        
    keystone species species that plays an especially important role in its community so that major changes in its numbers affect the populations of many other species        
    kidney one of a pair of organs of the excretory and urinary systems that filters wastes and excess water out of blood and forms urine        
    kidney failure loss of the ability of nephrons in the kidney to function fully due to a progressive kidney disease such as diabetic nephropathy or polycystic kidney disease        
    kidney stone solid crystal that forms in a kidney from minerals such as calcium in urine        
    kingdom largest and most inclusive taxon in the original Linnaean classification system        
    Koch’s postulates set of four criteria for identifying the microorganism that causes a particular infectious disease, first proposed by Robert Koch in the 19th century        
    Krebs cycle second stage of aerobic respiration in which two pyruvate (pyruvic acid) molecules from the first stage react to form ATP, NADH, and FADH2        
    kwashiorkor severe undernutrition syndrome caused by lack of protein and Calories and characterized by edema of the ankles, feet, and abdomen        
    labia (singular, labium) "lips" of the vulva, consisting of folds of tissue that protect the urethral and vaginal openings        
    labor general term for the process of childbirth, which includes three stages: dilation of the cervical canal, birth of the child, and delivery of the placenta (afterbirth)        
    lactase enzyme needed to digest the milk sugar lactose        
    lactase persistence ability to produce lactase and digest milk after early childhood        
    lactation production of breastmilk to feed an infant        
    lactic acid fermentation type of anaerobic respiration that includes glycolysis followed by the conversion of pyruvic acid to lactic acid and the formation of NAD+        
    lactose disaccharide sugar that makes up 2-8 percent of milk by weight        
    lactose intolerance inability to digest the lactose in milk, generally because of lack of the enzyme lactase        
    language communication of thoughts and feelings through a system of arbitrary symbols called words that are represented by vocal sounds and in many cases also by written marks        
    lanugo fine, colorless hair that covers a fetus’s face and body until it is shed close to the time of birth        
    large intestine organ of the digestive system that removes water and salts from food waste and forms solid feces for elimination        
    larynx organ of the respiratory system between the pharynx and trachea that is also called the voice box because it contains the vocal cords that allow the production of vocal sounds        
    last universal common ancestor (LUCA) hypothetical early cell (or group of cells) that gave rise to all subsequent life on Earth        
    lateralization concentration of particular functions in one hemisphere of the cerebrum of the brain        
    law of conservation of mass law stating that mass is neither created nor destroyed in chemical reactions        
    law of independent assortment Mendel’s second law stating that factors controlling different characteristics are inherited independently of each other        
    law of segregation Mendel’s first law stating that the two factors controlling a characteristic separate and go to different gametes        
    leptin hormone produced mainly by fat cells that normally inhibits appetite by inducing a feeling of satiety        
    leukemia group of cancers of the blood-forming tissues in bone marrow        
    leukocyte white blood cell produced by bone marrow to fight infections        
    Leydig cell type of cell found between seminiferous tubules in the testes that produces and secretes testosterone        
    life cycle series of stages a sexually reproducing organism goes through from one generation to the next        
    life expectancy average time an individual is expected to live, or the average age at death        
    ligament band of fibrous connective tissue that holds bones together        
    limiting factor any factor that constrains the population size of a species in an ecosystem        
    linked genes genes that are located on the same chromosome        
    Linnaean classification system system of classifying organisms based on observable physical traits; consists of a hierarchy of taxa, from the kingdom to the species        
    lipid class of biochemical compounds that includes fats and oils        
    liver organ of digestion and excretion that secretes bile for lipid digestion and breaks down excess amino acids and toxins in the blood        
    locus position of a gene on a chromosome        
    logistic growth pattern of population growth in which growth slows and population size levels off as the population approaches the carrying capacity        
    lower gastrointestinal (GI) tract part of the GI tract that includes the small and large intestines        
    lung one of two paired organs of the respiratory system in which gas exchange takes place between the blood and the atmosphere        
    lung cancer malignant tumor characterized by uncontrolled cell growth in tissues of the lung        
    luteinizing hormone (LH) pituitary gland hormone that stimulates the testes to secrete testosterone and the ovaries to secrete estrogen        
    lymph fluid that leaks out of capillaries into spaces between cells and circulates in the vessels of the lymphatic system        
    lymphatic system system of the body that produces lymphocytes and filters pathogens from lymph and blood        
    lymph node one of many small structures located along lymphatic vessels where pathogens are filtered from lymph and destroyed by lymphocytes        
    lymphocyte type of leukocyte produced by the lymphatic system that is a key cell in the adaptive immune response to a specific pathogen or tumor cell        
    macroevolution evolutionary change that occurs at or above the level of the species, generally over many generations        
    macronutrient nutrient such as carbohydrates, proteins, lipids, or water that is needed by the body in relatively large amounts        
    major histocompatibility complex (MHC) set of molecules normally found on virtually all nucleated human cells that provide a way for the immune system to recognize body cells as self        
    malaria common parasitic disease caused by Plasmodium protozoa and transmitted by Anophelesmosquitoes in tropical and subtropical regions of the world        
    malnutrition poor nutritional status due to inadequate or imbalanced nutrient intake; includes unbalanced nutrition, overnutrition, and undernutrition        
    mammary gland gland in the female breast that produces milk for offspring        
    mandible lower jaw bone        
    marasmus severe form of undernutrition characterized by very low weight and caused by extremely low intakes of macronutrients and energy        
    marine biome aquatic biome in the salt water of the ocean        
    mass extinction extinction event in which many if not most species disappear from Earth over a relatively short period of time        
    maxilla one of two fixed bones making up the upper jaw        
    mechanical digestion physical breakdown of chunks of food into smaller pieces by organs of the digestive system        
    mechanoreceptor type of sensory receptor that responds to mechanical forces        
    meiosis type of cell division in which the number of chromosomes is reduced by half and four haploid cells result        
    melanin brown pigment produced by melanocytes in the skin that gives skin most of its color and prevents UV light from penetrating the skin        
    melanocyte special skin cell that is responsible for producing melanin        
    melanoma rare but most serious type of skin cancer that affects melanocytes and usually metastasizes if not treated        
    melanosome small organelle in a melanocyte that synthesizes, stores, and transports melanin        
    memory cell lymphocyte (B or T cell) that retains a "memory" of a specific pathogen after an infection is over and thus provides immunity to the pathogen        
    menarche beginning of menstruation; first monthly period in a female        
    Mendelian inheritance inheritance of a trait controlled by a single gene with two alleles, one of which may be dominant to the other        
    meninges three-layered membrane that encloses and protects the brain and spinal cord and contains cerebrospinal fluid        
    menopause cessation of a woman’s menstrual cycles, usually by age 52        
    menstrual cycle monthly cycle of processes and events in the ovaries and uterus of a sexually mature human female until menopause        
    menstruation process in which the endometrium of the uterus is shed from the body during the first several days of the menstrual cycle; also called monthly period or menses        
    mesoderm middle germ layer of an embryo that will eventually give rise to bone and muscle cells        
    Mesozoic Era age of dinosaurs that lasted from about 245 to 65 million years ago        
    messenger RNA (mRNA) type of RNA that copies genetic instructions from DNA in the nucleus and carries them to the cytoplasm        
    metabolic syndrome condition that includes abdominal obesity, high blood pressure, high blood glucose and triglyceride levels, and low blood HDL levels; major risk factor for cardiovascular diseases and type 2 diabetes        
    metabolism sum of all the biochemical reactions in an organism        
    metaphase second phase of mitosis during which chromosomes line up at the equator of the cell        
    metastasis (plural, metastases) new cancer that forms at a distant site when cancer cells from a primary tumor travel through the bloodstream        
    metastasize for cancer cells to travel from a primary tumor through the bloodstream and form a new cancer at a distant site        
    microevolution evolutionary change that occurs over a relatively short period of time within a population        
    micronutrient nutrient such as a vitamin or mineral that is needed by the body in relatively small amounts        
    microvillus (plural, microvilli) one of many tiny projections covering each villus in the mucosa lining the small intestine that increases its absorptive surface        
    middle adulthood second stage of human adulthood that lasts from the mid-30s to the mid-60s        
    middle childhood stage of a human organism between early childhood and pre-adolescence that covers the ages 6 to 10 years        
    mineral chemical element such as calcium or potassium that is needed in relatively small amounts for proper body functioning        
    mitochondrion (plural, mitochondria) organelle in eukaryotic cells that makes energy available to the cell in the form of ATP molecules        
    mitosis process in which the nucleus of a eukaryotic cell divides        
    mixed nerve nerve of the peripheral nervous system that contains both sensory and motor neurons so it can transmit signals to and from the central nervous system        
    model representation of part of the real world        
    model organism nonhuman species that is extensively studied to understand particular biological phenomena        
    molar one of twelve teeth with cusps in the back of the mouth behind the premolars that are used for crushing and grinding food        
    molecular clock use of DNA (or protein) differences to measure how long it has been since related species diverged from a common ancestor        
    molecule smallest part of a compound that has the properties of that compound        
    monoculture growing the same crop repeatedly, which depletes soil nutrients and may attract plant pests        
    monomer small molecule that joins together with many others like it to form a larger molecule called a polymer        
    monosaccharide simple six-carbon sugar, such as glucose, that exists alone or as a monomer of a complex carbohydrate        
    morphology form and structure of an organism        
    morula solid ball of cells that forms during cleavage, when a fertilized egg undergoes its first several mitotic divisions        
    motor nerve nerve of the peripheral nervous system that transmits information from the central nervous system to muscles, organs, and glands        
    motor neuron type of neuron that carries nerve impulses from the central nervous system to muscles and glands; also called efferent neuron        
    mucous membrane epithelial tissue that lines inner body surfaces and body openings and produces mucus        
    mucus slimy substance produced by mucous membranes that traps pathogens, particles, and debris        
    multiple allele trait trait controlled by one gene with more than two alleles        
    muscle contraction increase in the tension or decrease in the length of a muscle that occurs when muscle fibers receive a nervous stimulus        
    muscle fiber long-thin muscle cell that has the ability to contract, or shorten        
    muscle strain injury in which muscle fibers tear due to overstretching of a muscle        
    muscle tissue tissue made up of cells that can contract; smooth, skeletal, or cardiac muscle tissue        
    muscular dystrophy genetic neuromuscular disorder caused by defective proteins in muscle cells and characterized by death of skeletal muscles and progressive weakness        
    muscular system human body system that includes all the muscles of the body        
    musculoskeletal disorder injury to muscles or tendons caused by biomechanical stresses        
    mutagen environmental factor that causes mutations        
    mutation change in the sequence of bases in DNA or RNA        
    mutualism type of symbiotic relationship between two species in which both species benefit        
    myasthenia gravis genetic neuromuscular disorder caused by the immune system blocking acetylcholine receptors on muscle cells and characterized by progressive muscle weakness and fatigue        
    myelin sheath lipid layer around the axon of a neuron that allows nerve impulses to travel more rapidly down the axon        
    myocardial infarction (MI) damage to heart muscle from death of myocardial cells that occurs when blood flow is blocked to part of the heart; also called heart attack        
    myocyte type of muscle cell that makes up smooth muscle tissue        
    myopia vision problem in which distant objects are out of focus but close vision is unaffected; also called nearsightedness        
    myosin thick protein filament in a muscle cell that pulls on a thin actin filament to produce a muscle contraction        
    MyPlate visual guide for balanced eating that was created in 2011 by the US Department of Agriculture to replace MyPyramid        
    nail accessory organ of the skin made of sheets of dead keratinocytes at the distal ends of the fingers and toes        
    nail bed pink skin under the nail plate that is visible through the nail        
    nail matrix deep layer of epidermal tissue at the proximal end of a nail where nail growth occurs        
    nail plate visible part of a nail that is external to the skin        
    nail root portion of a nail that is under the surface of the skin at the proximal end of the nail        
    nasal cavity large, air-filled space in the skull above and behind the nose that helps conduct air in and out of the body as part of the upper respiratory tract        
    natural gas naturally occurring nonrenewable gaseous fossil fuel consisting primarily of methane that is less polluting to burn than other fossil fuels        
    natural resource something supplied by nature that helps support life        
    natural selection force of evolution in which some living things produce more offspring than others so the characteristics of organisms change over time        
    negative feedback loop control mechanism that serves to reduce an excessive response and keep a variable within its normal range        
    neonate newborn infant from birth to the age of four weeks        
    nephron one of the million tiny structural and functional units of the kidney that filters blood and forms urine        
    nerve type of nervous tissue that consists of many cable-like bundles of axons and makes up the majority of the peripheral nervous system        
    nerve impulse electrical signal transmitted by the nervous system        
    nervous system human organ system that transmits electrical signals throughout the body to coordinate all of the body’s voluntary and involuntary activities        
    nervous tissue tissue made up of neurons that carry electrical messages and glial cells that support neurons        
    neural tube structure that forms in an embryo and eventually develops into the brain and spinal cord        
    neurogenesis formation of new neurons by cell division        
    neuroimmune system part of the immune system that protects the central nervous system        
    neuromuscular disorder muscle disorder that occurs due to problems with the nervous control of muscle contractions or with muscle cells themselves        
    neuromuscular junction chemical synapse where a motor neuron transmits a signal to a muscle fiber to initiate a muscle contraction        
    neuron functional unit of the nervous system that transmits nerve impulses; also called nerve cell        
    neurotransmitter type of chemical that carries nerve impulses from the axon of a neuron to another cell across the synapse        
    neurulation process in which an embryo develops structures that will eventually become the nervous system        
    niche role of a species in its ecosystem that includes all the ways the species interacts with the biotic and abiotic factors of the ecosystem        
    nicotine highly addictive psychoactive stimulant drug that is found in tobacco and tobacco smoke        
    nitrogen cycle biogeochemical cycle through which nitrogen is recycled through the biotic and abiotic components of ecosystems        
    nitrogen fixation process of changing nitrogen gas to nitrates that is carried out by nitrogen-fixing bacteria in the soil or in the roots of legumes        
    nociceptor type of sensory receptor that responds to pain        
    node of Ranvier one of the regularly spaced gaps between myelin sheaths along an axon that allow nerve impulses to travel very rapidly        
    nonbiodegradable unable to be broken down in the environment by natural processes        
    nondisjunction failure of replicated chromosomes to separate during meiosis II, resulting in some gametes with a missing chromosome (or part of a chromosome) and some with an extra chromosome (or part of a chromosome)        
    non-essential nutrient nutrient that can be synthesized by the body in sufficient quantities for normal functioning so it does not need to be obtained from food        
    noninfectious disease any disease caused by genetic and/or environmental factors other than pathogens; also called noncommunicable disease        
    non-Mendelian inheritance inheritance of traits that have a more complex genetic basis than one gene with two alleles and complete dominance        
    nonpoint-source pollution contamination of the environment by a pollutant that enters the environment from multiple sources        
    nonrenewable resource natural resource that exists in a fixed amount and cannot be replenished at all or cannot be replenished quickly enough to keep pace with human use of the resource        
    non-steroid hormone type of endocrine hormone that is made of amino acids and binds with a receptor on the plasma membrane of a target cell        
    normal range spread of values around the set point of a biological variable such as body temperature that is considered insignificant in terms of health        
    nuclear energy nonrenewable energy resource produced in a nuclear power plant by fission reactions in a radioactive fuel, usually uranium        
    nuclear force atomic force holding together protons and neutrons in the nucleus of an atom that is stronger than the electromagnetic force repelling positively charged protons from each other        
    nucleic acid class of biochemical compounds that includes DNA and RNA        
    nucleotide small molecule containing a sugar, phosphate group, and nitrogen base that is a building block of nucleic acids        
    nucleus (plural, nuclei) organelle inside eukaryotic cells that contains most of the cell’s DNA and acts as the control center of the cell        
    nutrient substance the body needs for energy, building materials, or control of body processes        
    nutrient density how much of a given nutrient is provided by a particular food, relative to the mass of the food or the amount of Calories it provides        
    nutrition process of taking in nutrients in food and using them for growth, metabolism, and repair        
    nutrition facts label label on packaged food that lists the nutrient content per serving of the food and also its ingredients        
    obesity disease in which the body mass index (BMI) is greater than 30.0 kg/m2        
    observation anything that is detected through human senses or with instruments and measuring devices that enhance human senses        
    observational study type of nonexperimental scientific investigation in which the researcher measures characteristics in a sample but does not attempt to manipulate or control variables of interest        
    occipital lobe part of each hemisphere of the cerebrum that is dedicated almost solely to vision        
    old age last stage of human adulthood that lasts from the mid-60s until death        
    olfactory receptor chemoreceptor in the nasal passages that detects chemicals in the air        
    omnivore consumer that eats both plants and animals        
    oogenesis process of producing eggs in the ovaries of a female fetus        
    oogonium (plural, oogonia) diploid stem cell in an ovary that undergoes mitosis to begin the process of oogenesis        
    opioid drug derived from the opium poppy or a synthetic version of such a drug, including heroin and painkillers such as codeine, morphine, or OxyContin        
    opportunistic disease disease that rarely occurs except in people with a compromised immune system, such as people infected with HIV        
    organ structure composed of more than one type of tissue that performs a particular function, such as the brain, kidney, or heart        
    organelle structure within the cytoplasm of a eukaryotic cell that is enclosed within a membrane and performs a specific job        
    organic compound compound found in or derived from living things that contains mainly carbon        
    organism individual living thing        
    organogenesis process in which organs develop in an embryo        
    organ system group of organs that work together to do a certain job        
    osmosis diffusion of water molecules across a membrane        
    ossification process in which cartilage is changed into bone        
    osteoarthritis joint disorder that results from the breakdown of joint cartilage and bone, most often due to wear and tear on joints        
    osteoblast type of bone cell that makes and mineralizes bone matrix        
    osteocalcin endocrine hormone secreted by bone cells that helps to regulate blood glucose and fat deposition        
    osteoclast type of bone cell that breaks down bone, dissolves its minerals, and releases them into the blood        
    osteocyte type of bone cell that helps regulate the formation and breakdown of bone tissue        
    osteogenic cell type of stem cell that can divide and differentiate to form new bone cells        
    osteoporosis disorder that generally occurs in older adults in which bones lose minerals, become brittle, and break easily        
    ovarian cycle events of the menstrual cycle that occur in the ovaries, including maturation of a follicle, ovulation, and development of the corpus luteum        
    ovarian follicle functional unit of an ovary that consists of a nest of epithelial cells surrounding an egg        
    ovary one of a pair of female reproductive organs that produces eggs and secretes estrogen        
    over-harvesting use of a natural resource at an unsustainable rate, such as over-fishing or overly intensive use of soils for farming        
    overpopulation condition in which population size is too large to be supported by available resources without damaging the environment        
    ovulation release of a secondary oocyte from an ovary about half way through the menstrual cycle        
    oxytocin endocrine hormone secreted by the pituitary gland that functions during childbirth to stimulate uterine contractions        
    pacemaker type of cells in the heart that create electrical signals to stimulate heart muscles to contract        
    Paleolithic "old stone" stage of human technological development that characterized humans starting about 2-5 million years ago with Homo habilis and continued through early modern Homo sapiens        
    paleontologist scientist who finds and studies fossils to learn about evolution and understand past life        
    Paleozoic Era age of "old life" from 544-245 million years ago that began with the Cambrian explosion and ended with the Permian extinction        
    pancreas dual endocrine and exocrine gland near the stomach that secretes insulin and glucagon, which regulate blood glucose, as well as enzymes that aid in digestion        
    pancreatic islet one of millions of clusters of cells in the pancreas that secrete endocrine hormones such as insulin and glucagon; also called islet of Langerhans        
    pancreatitis painful inflammation of the pancreas due to gallstones, chronic alcohol use, or other cause        
    pandemic infectious disease epidemic that spreads across multiple populations or continents or even worldwide        
    Pangaea supercontinent that formed during the Permian Period and included all of Earth’s major land masses        
    papillary layer upper layer of the dermis with papillae extending upward into the epidermis        
    Pap smear medical test in which cells are scraped from the cervix and examined under a microscope in order to detect cancer cells if they are present        
    paradigm shift radical change in science in which current theories are abandoned and new ideas take their place        
    paralysis loss of sensation and movement in part of the body, such as may occur with a stroke or spinal cord injury        
    parasite species that lives in or on another species, called the host, and causes harm to the host while benefitting from the relationship        
    parasitism symbiotic relationship between two species in which one species (the parasite) benefits while the other species (the host) is harmed        
    parasympathetic division division of the autonomic nervous system that returns the body to normal after the fight-or-flight response and maintains homeostasis at other times        
    parathyroid gland one of a pair of small endocrine glands in the neck that secretes hormones that regulate blood calcium        
    parietal lobe part of each hemisphere of the cerebrum that controls touch, reading, and arithmetic        
    Parkinson’s disease degenerative brain disorder caused by progressive death of neurons in the midbrain, resulting in muscular symptoms of tremor, rigidity, slowness of movement, and postural instability        
    passive immunity short-term immunity to a particular pathogen that results when antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen        
    passive transport movement of substances across a plasma membrane that does not require energy        
    pathogen disease-causing agent such as a bacterium, virus, fungus, or protozoan        
    pectoral girdle paired clavicles (collar bones) and scapulas (shoulder blades) that together form the shoulders and attach the arms to the trunk; also called shoulder girdle        
    pedigree chart showing how a trait is passed from generation to generation within a family        
    pelvic girdle paired, fused bones (ilium, pubis, and ischium) that form the hips and attach the legs to the trunk        
    pelvic inflammatory disease (PID) infection that spreads from the vagina to the upper reproductive organs and may lead to ectopic pregnancies or infertility        
    penis male reproductive organ containing the urethra, through which semen and urine pass out of the body        
    peptic ulcer sore that develops in the lining of the stomach or duodenum most often caused by infection with the bacterium Helicobacter pylori        
    periosteum tough, fibrous membrane that covers the outer surface of bones        
    peripheral artery disease (PAD) narrowing of peripheral arteries, usually in the legs, due to atherosclerosis and generally causing intermittent pain in the legs when walking        
    peripheral immune system part of the immune system that protects all of the body except for the central nervous system (which is protected by the neuroimmune system)        
    peripheral nervous system (PNS) one of two major divisions of the nervous system that consists of all the nervous tissue that lies outside the central nervous system        
    peristalsis rapid, involuntary, wave-like contractions of smooth muscles that push food through the gastrointestinal tract and urine through the urinary tract        
    permanent teeth second set of 32 teeth that emerge mainly during middle childhood and most of which replace deciduous (baby) teeth after they are lost; consist of 8 incisors, 4 canines, 8 premolars, and 12 molars        
    Permian extinction extinction event at the end of the Paleozoic Period that was the biggest mass extinction that had occurred on Earth until then        
    petroleum naturally occurring nonrenewable liquid fossil fuel found in reservoirs and rocks below Earth’s surface that is the world’s primary fuel source for transportation; also called crude oil        
    pH scale that is used to measure acidity, on which 7 is neutral, less than 7 is acidic, and greater than 7 is basic        
    phagocytosis process in which certain leukocytes engulf and break down solid particles such as pathogens or debris        
    pharmacogenomics study of how genetic variation affects individual responses to therapeutic drugs        
    pharynx tubular organ that connects the mouth and nasal cavity with the larynx and through which air and food pass        
    phenotype characteristics of an organism that depend on how the organism’s genotype is expressed        
    phenotypic plasticity ability to change the phenotype in response to changes in the environment        
    phenylthiocarbamide (PTC) harmless compound that is used to test people for taster status because it tastes bitter to tasters and has no taste to nontasters        
    phospholipid type of lipid that contains a phosphate group and is the main component of the cell membranes of all living things        
    phospholipid bilayer double layer of phospholipid molecules that makes up a plasma membrane        
    phosphorus cycle biogeochemical cycle in which phosphorus is continuously recycled through biotic and abiotic components of ecosystems        
    photic zone area in an aquatic biome extending to a maximum depth of 200 meters that receives enough sunlight for photosynthesis        
    photoautotroph producer that uses energy from sunlight to produce organic molecules by photosynthesis        
    photoreceptor type of sensory receptor that responds to light        
    photosynthesis process of using light energy to make organic molecules from inorganic substances        
    phylogenetic tree diagram that shows how species are related to each other through common ancestors        
    phylogeny evolutionary history of a group of related organisms        
    physical exercise any bodily activity that enhances or maintains physical fitness and good health        
    physiology study of the functioning of the human organism        
    phytochemical any of a large group of plant chemicals that may be needed in small amounts for good health but are not (yet) classified as nutrients        
    pineal gland endocrine gland that secretes the hormone melanin, which regulates the sleep-wake cycle        
    pioneer species type of species that first colonizes a disturbed area        
    pituitary gland master gland of the endocrine system that secretes many hormones, the majority of which regulate other endocrine glands        
    placebo "fake" treatment that actually has no effect on health and is included in a study to control for the placebo effect        
    placebo effect psychologically based reaction to a treatment that occurs just because the subject is treated, even if the treatment is a placebo that has no physiological effect on the patient        
    placenta temporary organ that consists of a large mass of maternal and fetal blood vessels through which the mother’s and embryo’s or fetus’s blood exchange substances        
    placental insufficiency problem with the placenta that causes inadequate transfer of oxygen and nutrients to the fetus, causing intrauterine growth restriction        
    plasma straw-yellow fluid part of blood that contains many dissolved substances and blood cells        
    plasma membrane double layer of phospholipids that surrounds and encloses the contents of a cell; also called cell membrane        
    Plasmodium falciparum protozoan parasite that causes malaria when it infects a human host        
    platelet cell fragment in blood that helps blood clot        
    pleiotropy situation in which a single gene affects more than one trait        
    pneumonia disease in which the alveoli of the lungs become inflamed and filled with fluid, usually as a result of infection, causing symptoms such as shortness of breath, coughing, chest pain, and fever        
    point mutation change in a single nucleotide base in a gene        
    point-source pollution contamination of the environment that occurs when pollutants enter the environment from a single source        
    polarity difference in electrical charge between different parts of the same molecule        
    pollination fertilization in plants in which pollen is transferred from one flower to another, often with the help of a pollinator such as a bee        
    pollutant anything that causes pollution, either a substance or a form of energy        
    pollution introduction of contaminants into the natural environment, leading to adverse changes in ecosystems and potential damage to human health        
    polycystic kidney disease (PKD) genetic disorder in which multiple abnormal cysts develop and grow in the kidneys        
    polygenic trait trait controlled by more than one gene, each of which may have two or more alleles        
    polymer large molecule that consists of many small repeating units called monomers        
    polymerase chain reaction (PCR) biotechnology process that makes many copies of a gene or other DNA segment        
    polymorphism situation in which a gene has two or more alleles in a population at frequencies greater than 1 percent        
    polynucleotide chain of small molecules called nucleotides that alone or with a complementary chain makes up a nucleic acid (DNA or RNA)        
    polypeptide chain of amino acids that alone or with other such chains makes up a protein        
    polysaccharide chain of monosaccharides (simple sugars) that makes up a complex carbohydrate (such as starch or cellulose)        
    population all the organisms of the same species that live in the same area        
    population density average number of individuals in a population per unit of area        
    population distribution how the individuals in a population are arrayed over the area they inhabit        
    population genetics science focusing on the forces of evolution at the level of allele frequencies in populations        
    population growth rate (r) net change in population size per year per 100 people already present in the population that occurs because of births, deaths, and migrations        
    population projection prediction of future population growth or size based on assumptions about future birth and death rates        
    population pyramid bar graph that represents the age-sex structure of a population        
    positive feedback loop control mechanism that serves to intensify a response until an endpoint is reached        
    posterior pituitary back lobe of the pituitary gland that stores and secretes hypothalamic hormones        
    pre-adolescence stage of the human organism between middle childhood and adolescence that covers the ages 11 to 12 years, during which many children start going through puberty        
    precipitation water that falls from clouds in the atmosphere to Earth’s surface in the form of rain, snow, sleet, hail, or freezing rain        
    predation relationship between two species in which members of one species (the predator) consume members of the other species (the prey)        
    predator species that consumes the other species (the prey) in a predatory relationship        
    pregnancy carrying of one or more offspring from fertilization until birth        
    premolar one of eight cusped teeth in the sides of the jaws between the canine teeth and molars that are used for crushing food        
    presbyopia common vision problem associated with aging in which the eye gradually loses its ability to focus on close objects        
    preschool stage second part of early childhood that covers the ages 4 to 5 years        
    prey species that is consumed by the other species (the predator) in a predatory relationship        
    primary pollutant contaminant that is released directly into the environment from the source(s) of pollution        
    primary succession change over time in the numbers and types of species that live in an area that was never before colonized by living things        
    primate any member of the Primate Order of mammals, in which the human species is placed        
    prion infectious agent composed entirely of proteins that may cause a fatal neurodegenerative disease such as mad cow disease or kuru        
    producer organism that produces food for itself and other organisms        
    product substance that forms as the result of a chemical reaction        
    progesterone female sex hormone secreted mainly by the ovaries that helps maintain a successful pregnancy        
    prokaryote single-celled organism that lacks a nucleus        
    prokaryotic cell living cell without a nucleus        
    promoter region of a gene where RNA polymerase binds to initiate transcription of the gene        
    prophase first phase of mitosis during which chromatin condenses into chromosomes, the nuclear envelope breaks down, centrioles separate, and a spindle begins to form        
    prosimian any non-anthropoid primate; lemur, loris, or tarsier        
    prostate cancer tumor in the prostate gland of the male reproductive system that is the most common type of cancer in men        
    prostate gland gland in the male reproductive system that secretes fluid into semen and provides nourishing substances to sperm        
    protein class of biochemical compounds made of amino acids, with complex structures and diverse functions        
    protein synthesis process in which cells make proteins that includes transcription of DNA and translation of mRNA        
    protist any member of the Protist Kingdom, the kingdom in the Eukarya Domain that includes all eukaryotes except plants, animals, and fungi        
    proto-oncogene gene that normally promotes growth and reproduction of cells but may cause cancer if it mutates and becomes an oncogene        
    protozoan (plural, protozoa) animal-like, single-celled protist such as Giardia or Plasmodium species, both of which are common human parasites        
    pseudoscience claim, belief, or practice that is presented as scientific but does not adhere to the standards and methods of science        
    psychoactive drug drug that affects the central nervous system, generally by influencing neurotransmitters in the brain        
    puberty period during which humans become sexually mature        
    pulmonary circulation part of the cardiovascular system that carries blood between the heart and lungs        
    punctuated equilibrium model of the timing of evolution in which long periods of little evolutionary change are interrupted by bursts of rapid evolutionary change        
    Punnett square chart for determining the expected percentages of different genotypes in the offspring of two parents        
    pyruvate kinase (PK) enzyme needed for ATP production in red blood cells, deficiency in which is a genetic adaptation to malaria        
    qualitative data data that are expressed in words        
    quantitative data data that are expressed in numbers        
    race discrete category within the human species that groups together individuals based on a few readily observable traits such as skin color and hair texture        
    racism association of racial traits such as skin color with unrelated traits such as intelligence, often leading to prejudice and discrimination against people based only on how they look        
    radiation therapy treatment of cancer using ionizing radiation such as X rays to kill cancerous tissues        
    radon radioactive gas from underground rocks that may cause lung cancer        
    reactant starting substance in a chemical reaction        
    reading frame grouping of nitrogen bases in DNA into three-base codons        
    receptor protein on a cell membrane that binds with a hormone, neurotransmitter, or other chemical        
    recessive referring to an allele that is masked by the presence of another allele for the same gene when they occur together in a heterozygote; or referring to a trait controlled by such an allele        
    recombinant DNA DNA that results when DNA from two organisms is combined        
    rectum short part of the large intestine between the colon and anus where feces is stored until it is eliminated through the anus        
    red blood cell type of cell in blood that contains hemoglobin and carries oxygen        
    reflex rapid motor response to a sensory stimulus in which nerve impulses travel in an arc that includes the spinal cord but not the brain; in infants, may refer to one of several instinctive behaviors, such as crying and sucking, that help an infant survive        
    regulatory element region of DNA where a regulatory protein binds        
    regulatory protein protein that regulates gene expression        
    relative dating method of dating fossils by their location in rock layers that determines which fossils are older or younger but not their age in years        
    renal pelvis funnel-like end of a ureter where it enters the kidney and where urine collects before it is transported through the ureter        
    renal tubule tubular structure of a nephron in a kidney through which filtered substances pass and where some filtered substances are reabsorbed by the blood and additional substances are secreted from the blood        
    renewable resource natural resource that can be replenished by natural processes as quickly as humans use it        
    replacement fertility rate fertility rate at which women average only enough children by the end of their reproductive years to replace themselves and their partner in the population        
    replication reproducing a scientific investigation and getting the same result        
    reproduction process by which living things give rise to offspring        
    reproductive system organ system responsible for the production and fertilization of gametes and, in females, the carrying of a fetus        
    reservoir part of a biogeochemical cycle that holds an element or water for a long period of time        
    respiration exchange of gases between the body and the outside air        
    respiratory center one of several areas in the medulla and pons of the brain stem that help control unconscious breathing        
    respiratory system organ system that brings oxygen into the body and releases carbon dioxide and other waste gases into the atmosphere        
    respiratory tract continuous system of passages through which air flows into and out of the body        
    resting potential difference in electrical charge across the plasma membrane of a neuron that is not actively transmitting a nerve impulse        
    reticular layer lower layer of the dermis that gives the dermis strength and elasticity and contains many dermal structures such as glands and hair follicles        
    retina layer of photoreceptor cells at the back of the eye, including rods that are particularly sensitive to dim light and cones that are sensitive to light of different colors        
    Rhesus (Rh) blood group system system of blood cell antigens controlled by two genes with many alleles on chromosome 1, producing five common antigens of which the main antigen is the D (Rh+) antigen        
    ribcage bony "cage" enclosing the thoracic cavity and consisting of the ribs, thoracic vertebrae, and sternum        
    ribosomal RNA (rRNA) type of RNA that helps form ribosomes and assemble proteins        
    ribosome organelle inside all cells where proteins are made        
    rickets disorder in children caused by vitamin D deficiency and characterized by soft, weak bones and skeletal deformities        
    ringworm skin infection by the fungus Trichophyton that causes a characteristic ring-shaped rash        
    RNA (ribonucleic acid) single-stranded nucleic acid that helps make proteins        
    RNA world hypothesis hypothesis that RNA was the first biochemical molecule to evolve and that early life was based on RNA, rather than DNA or proteins        
    runoff precipitation that falls on land and flows over the surface of the ground        
    saliva fluid secreted by salivary glands that keeps the mouth moist and contains the digestive enzymes amylase and lipase        
    salivary gland one of many exocrine glands in the mouth that secrete saliva into the mouth through ducts        
    sample individuals or events that are actually selected for study in an experiment or other scientific investigation and generally are a subset of all possible cases that could be selected for study        
    sarcomere basic functional unit of skeletal and cardiac muscles, containing actin and myosin protein filaments that slide over one another to produce a shortening of the sarcomere and a muscle contraction        
    sarcopenia gradual decrease in the ability to maintain skeletal muscle mass that occurs in later adulthood        
    saturated fatty acid simple lipid molecule in which a chain of carbon atoms is bonded to as many hydrogen atoms as possible because adjacent carbon atoms in the chain share only single bonds        
    savanna hypothesis hypothesis that many human traits such as upright bipedalism evolved as adaptations to a savanna habitat        
    scavenger decomposer that consumes the soft tissues of dead animals such as the remains of a predator’s kill        
    science distinctive way of gaining knowledge about the natural world that tries to answer questions with evidence and logic        
    scientific investigation plan for asking questions and testing possible answers to them using evidence and logic        
    scientific law statement describing what always happens under certain conditions in nature        
    scientific method process typically followed in a scientific investigation that includes such steps as making observations, asking a question, forming a hypothesis, testing the hypothesis, drawing conclusions, and communicating results        
    scientific racism ideology popular until the early 20th century that race is a valid biological concept and that human behavior is partly determined by race        
    scientific theory broad explanation that is widely accepted because it is strongly supported by a great deal of evidence        
    scrotum pouch-like external structure of the male reproductive system, located behind the penis, that contains the testes, epididymes, and part of the vas deferens        
    sebaceous gland gland in the dermis of the skin that produces sebum, an oily substance that waterproofs the skin and hair        
    secondary pollutant contaminant in the environment that forms when primary pollutants react after being released into the environment        
    secondary sex characteristic trait that is different in males and females but is not directly involved in reproduction, such as male facial hair and female breasts        
    secondary succession change over time in the numbers and types of species that live in an area that was previously colonized by living things but has been disturbed        
    secondhand smoke smoke that enters the air from burning cigarettes or from the lungs of smokers        
    semen fluid containing sperm and glandular secretions, which nourishes sperm and carries them through the urethra and out of the body        
    seminal vesicle one of a pair of glands of the male reproductive system that secretes fluid into semen        
    seminiferous tubule one of the many tiny tubes contained within the testes where sperm are produced        
    sensor component of a homeostatic mechanism that senses the value of a variable and sends data on it to the control center        
    sensory nerve nerve of the peripheral nervous system that transmits information from sensory receptors in the body to the central nervous system        
    sensory neuron type of neuron that carries nerve impulses from sensory receptors in tissues and organs to the central nervous system; also called afferent neuron        
    sensory receptor specialized nerve cell that responds to a particular type of stimulus such as light or chemicals by generating a nerve impulse        
    Sertoli cell type of cell that lines the seminiferous tubules in the testes and plays several roles in sperm production        
    set point physiologically optimum value for a given biological variable such as body temperature        
    sex chromosome X or Y chromosome        
    sex hormone endocrine hormone secreted mainly by gonads that controls sexual development and reproduction        
    sex-linked gene gene located on a sex chromosome        
    sex-linked trait trait controlled by a gene located on a sex chromosome        
    sex ratio number of males per 100 females in a population        
    sexual dimorphism differences between the phenotypes of males and females of the same species        
    sexually transmitted infection (STI) infection caused by a pathogen that spreads mainly through sexual contact; also called sexually transmitted disease (STD)        
    sexual reproduction type of reproduction that involves the fertilization of gametes produced by two parents and produces genetically variable offspring        
    single nucleotide polymorphism (SNP) variation in just one nucleotide in alleles that are present in a population at frequencies greater than 1 percent        
    sinus rhythm normal, rhythmical beating of the heart        
    sixth mass extinction current mass extinction caused primarily by habitat loss due to human actions        
    skeletal muscle voluntary, striated muscle that is attached to bones of the skeleton and helps the body move        
    skeletal system human body system that consists of all the bones of the body as well as cartilage and ligaments and provides an internal framework for the body        
    skin major organ of the integumentary system that covers and protects the body and helps maintain homeostasis, for example, by regulating body temperature        
    skull part of the human skeleton that provides a bony framework for the head and includes bones of the cranium and face        
    sleep apnea disorder characterized by pauses in breathing during sleep, usually because of physical blockage of air flow        
    sliding filament theory theory that explains muscle contraction by the sliding of myosin filaments over actin filaments within muscle fibers        
    slow-twitch muscle fiber type of skeletal muscle cell that is mainly responsible for aerobic activities such as long-distance running        
    small intestine long, narrow, tube-like organ of the digestive system where most chemical digestion of food and virtually all absorption of nutrients take place        
    smooth muscle involuntary, nonstriated muscle that is found in the walls of internal organs such as the stomach        
    sodium-potassium pump active transport mechanism in which sodium ions are pumped out of a cell and potassium ions are pumped into the cell with the help of a carrier protein and energy from ATP        
    soil mixture of eroded rock, minerals, organic matter, and other materials that is essential for plant growth and forms the foundation of terrestrial ecosystems        
    soluble fiber nondigestible carbohydrates in food that dissolve in water and slow the absorption of nutrients from the GI tract, thereby reducing insulin spikes        
    somatic mutation mutation that occurs in a cell of the body other than a gamete        
    somatic nervous system division of the peripheral nervous system that controls voluntary activities        
    somatostatin endocrine hormone produced by the pancreas that inhibits the production of growth hormone by the pituitary and the secretion of insulin and glucagon by the pancreas        
    specialization evolution of different adaptations in competing species, which allows them to live in the same area without competing        
    special sense sense such as vision or hearing that has special sense organs that gather sensory information and change it into nerve impulses        
    speciation process by which a new species evolves        
    species group of organisms that are similar enough to mate and produce fertile offspring together        
    sperm gamete produced by a male organism        
    spermatogenesis process of producing sperm in the testes        
    spermatogonium (plural, spermatogonia) diploid stem cell in a testis that undergoes mitosis to begin the process of spermatogenesis        
    sphincter ring of muscles that can contract to close off an opening between structures, such as between the esophagus and stomach        
    spinal cavity long, narrow body cavity inside the vertebral column that runs the length of the trunk and contains the spinal cord        
    spinal cord thin, tubular bundle of nervous tissue that extends from the brainstem down the back to the pelvis and connects the brain with the peripheral nervous system        
    spleen secondary organ of the lymphatic system where blood and lymph are filtered        
    spongy bone light-weight, porous inner layer of bone that contains bone marrow        
    squamous cell carcinoma common type of skin cancer that affects squamous cells in the epidermis and rarely metastasizes        
    stabilizing selection type of natural selection for a polygenic trait in which phenotypes at both extremes of the phenotypic distribution are selected against, resulting in a narrowing of the range of phenotypic variation        
    starch type of complex carbohydrate (polysaccharide) that plants use to store energy        
    stem cell undifferentiated cell that can develop into specialized types of cells        
    sterilization surgical procedure that is generally irreversible and that makes it impossible for a woman to become pregnant or for a man to ejaculate viable, motile sperm        
    steroid type of lipid with a ring structure, such as cholesterol or a sex hormone        
    steroid hormone type of endocrine hormone that is made of lipids and crosses the plasma membrane to bind with a receptor inside a target cell        
    stimulant type of psychoactive drug that stimulates the brain and increases alertness and wakefulness        
    stimulus something that triggers a behavior or other response        
    stomach sac-like organ of the digestive system between the esophagus and small intestine in which both mechanical and chemical digestion take place        
    stroke cerebrovascular accident in which a broken artery or blood clot results in lack of blood flow to part of the brain, causing death of brain cells        
    stunting growth deficit diagnosed in children who are at least two standard deviations below the median height for their age in a reference population        
    subsistence strategy general way in which a group of people obtain food and the types of food they rely on most, such as hunting and gathering, farming, or fishing        
    substrate specific substance acted upon by a given enzyme        
    sugar short-chain soluble carbohydrate that is found in many foods, tastes sweet, and provides quick energy        
    sunburn reddening of the skin that occurs when the outer layer of the skin is damaged by UV light from the sun or tanning lamps        
    surrogate mother woman who agrees to become pregnant using a man’s sperm and her own egg, carries the fetus to term, and then gives up the baby at birth for adoption by the sperm donor and his partner        
    sustainable use use of a natural resource in a way that meets the needs of the present and also preserves the resource for use by future generations        
    sweat salty fluid secreted into ducts by sweat glands in the dermis that excretes wastes and helps cool the body; also called perspiration        
    sweat gland exocrine gland in the dermis of the skin that produces the salty fluid called sweat through a duct to the skin surface        
    symbiosis close relationship between organisms in two different species in which at least one of the organisms benefits from the relationship; mutualism, commensalism, or parasitism        
    sympathetic division division of the autonomic nervous system that controls the fight-or-flight response        
    sympatric speciation evolution of a new species that occurs without geographic separation first occurring between members of an original species        
    synapse space between the axon terminal of a neuron and an adjoining cell        
    synovial joint movable joint in which a fluid-filled synovial cavity separates bones at the joint        
    syphilis sexually transmitted infection caused by the bacterium Treponema pallidum that may eventually be fatal if untreated        
    systemic circulation part of the cardiovascular system that carries blood between the heart and body        
    systole part of a heartbeat in which the atria relax and fill with blood from the lungs and body, while the ventricles contract and pump blood out of the heart        
    T3 (triiodothyronine) endocrine hormone secreted by the thyroid gland that increases the rate of metabolism in cells throughout the body        
    T4 (thyroxine) endocrine hormone secreted by the thyroid gland that increases the rate of metabolism in cells throughout the body        
    target cell type of cell on which a particular hormone has an effect because it has receptor molecules for the hormone        
    taste bud small structure on the tongue containing chemoreceptor cells that sense chemicals in food        
    TATA box regulatory element that is part of the promotor of most eukaryotic genes        
    taxon (plural, taxa) level of organization in a biological classification system such as the Linnaean system; for example, species or genus        
    taxonomy science of classifying organisms        
    T cell type of lymphocyte that kills infected or cancerous cells (killer T cell) or helps regulate the immune response (helper T cell)        
    teething emergence of deciduous (baby) teeth during infancy and early childhood        
    telomere region of repetitive nucleotide sequences at the end of a chromosome that becomes shorter each time a cell divides        
    telophase last stage of mitosis during which chromosomes uncoil to form chromatin, the spindle breaks down, and new nuclear membranes form        
    temporal lobe part of each hemisphere of the cerebrum that controls hearing, memories, and sensations        
    tendinitis inflammation of a tendon when it is over-extended or worked too hard without rest        
    tendon tough connective tissue that attaches skeletal muscle to bones        
    terrestrial biome any land-based biome        
    testicular cancer cancer of the testes, which is relatively common in young men        
    testis (plural, testes) one of two male reproductive organs that produce sperm and secrete testosterone; male gonad        
    testosterone male sex hormone secreted mainly by the testes        
    thalamus part of the inner brain that is a major hub for nerve impulses traveling back and forth between the cerebrum and spinal cord        
    thalassemia type of inherited anemia caused by abnormal hemoglobin that is a genetic adaptation to malaria in the Mediterranean region, parts of Africa, and South and Southeast Asia        
    thermal pollution pollution by heat energy, typically occurring when heated water is discharged into a river or lake        
    thermoreceptor type of sensory receptor that senses temperature        
    thoracic cavity body cavity in the chest that holds the lungs and heart        
    thrifty gene hypothesis idea that "thrifty genes" evolved in some human populations because they allowed people to burn fewer Calories and store the rest as body fat, which would help them survive famines        
    thymus organ of the lymphatic system where lymphocytes called T cells mature        
    thyroid gland large endocrine gland in the neck whose hormones control the rate of cellular metabolism and help maintain calcium homeostasis        
    tissue group of cells, generally of the same kind, that perform a particular function in an organism        
    toddlerhood first part of early childhood that covers the ages 1 to 3 years        
    tonsil one of four paired secondary lymphatic organs that encircle the inner throat and filter pathogens out of lymph        
    touch ability to sense pressure, vibration, temperature, pain, and other tactile stimuli        
    toxoplasmosis common parasitic disease that may be a selective agent maintaining both Rhesus-positive (Rh+) and Rhesus-negative (Rh-) blood type alleles in human populations        
    trachea tubular organ of the respiratory system that carries air between the larynx and bronchi; also called windpipe        
    transcription process in which genetic instructions in DNA are copied to form a complementary strand of mRNA        
    trans fat type of harmful, artificial fat that is added to some foods to help preserve freshness and improve taste        
    transfer RNA (tRNA) type of RNA that brings amino acids to ribosomes where they are joined together to form proteins        
    transgenic crop crop that has been genetically modified with new genes that code for traits useful to humans        
    translation process in which genetic instructions in mRNA are "read" to synthesize a protein        
    transpiration process in which plants give off water vapor through tiny pores, called stomata, in their leaves        
    transport protein protein in a cell membrane that helps other substances cross the membrane to enter or leave the cell        
    tribe taxon between the subfamily and genus in the Linnaean system of classification        
    trichomoniasis common, sexually transmitted infection that is caused by the protozoan parasite Trichomonas vaginalis        
    triglyceride type of lipid that contains glycerol and makes up body fat in animals        
    trimester one of three, approximately three-month periods into which a pregnancy is divided        
    trophic level feeding position in a food chain or food web, such as producer, primary consumer, or secondary consumer        
    trophoblast outer cell layer of a blastocyst that implants in the uterus and eventually develops into the fetal portion of the placenta        
    tubal ligation surgical sterilization procedure in females in which the Fallopian tubes are blocked so sperm cannot reach and fertilize an egg        
    tumor abnormal mass of cells that may be cancerous        
    tumor-suppressor gene gene that normally inhibits division of abnormal cells and may cause cancer if it mutates        
    type 1 diabetes autoimmune disorder in which the immune system destroys insulin-secreting beta cells in the pancreas, leading to loss of glucose control and high levels of blood glucose        
    type 2 diabetes multifactorial disorder in which a combination of insulin resistance and impaired insulin production lead to loss of glucose control and high levels of blood glucose        
    typology system of discrete categories, such as races, that are sometimes used for classifying variation        
    ulcerative colitis inflammatory bowel disease that causes ulcers (sores) in the colon and rectum        
    umbilical cord long, narrow conduit containing two arteries and a vein that connects an embryo or fetus to the placenta        
    undernutrition insufficient intake of nutrients, generally leading to an individual being underweight for height (in adults) or underweight for age (in children)        
    universal donor individual with type O blood in the ABO blood group system who can donate blood to people of any ABO blood type        
    universal recipient individual with type AB blood in the ABO blood group system who can receive a blood donation from people of any ABO blood type        
    unsaturated fatty acid simple lipid molecule in which carbon atoms are not bonded to as many hydrogen atoms as possible because some adjacent carbon atoms in the chain share double or even triple bonds        
    upper gastrointestinal (GI) tract part of the gastrointestinal tract that includes the mouth, pharynx, esophagus, and stomach        
    urbanization shift in population from rural to urban areas, which is the process by which cities form and grow        
    urea waste product of protein catabolism that is mainly filtered from blood in the kidneys and excreted in urine        
    ureter muscular, tube-like organ of the urinary system that moves urine by peristalsis from a kidney to the bladder        
    urethra tube-like organ of the urinary system that carries urine out of the body from the bladder and, in males, also carries semen out of the body        
    uric acid waste product of nucleic acid catabolism that is mainly filtered from blood by the kidneys and excreted in urine        
    urinary bladder sac-like organ that stores urine until it is excreted from the body        
    urinary incontinence common chronic problem of uncontrolled leakage of urine        
    urinary system organ system that includes the kidneys, ureters, urinary bladder, and urethra and that is responsible for filtering waste products and excess water from the blood and excreting them from the body in urine        
    urination process in which urine leaves the body through the external urethral orifice        
    urine liquid waste product of the body that is formed by the kidneys and excreted by the other organs of the urinary system        
    uterine cycle events of the menstrual cycle that occur in the uterus, including menses and the buildup of the endometrium        
    uterus (plural, uteri) female reproductive organ in which first an embryo and then a fetus grows and develops until birth        
    vaccine substance containing a modified pathogen that does not cause disease but provokes an adaptive immune response, resulting in immunity to the pathogen        
    vacuole large sac-like organelle that stores and transports materials inside a cell        
    vagina female reproductive organ that receives sperm during sexual intercourse and provides a passageway for a baby to leave the mother’s body during birth        
    vaginitis inflammation of the vagina usually caused by an infection with microbes        
    variability-selection hypothesis idea that hominins evolved greater adaptability in response to climate change rather than evolving specializations for a particular climate        
    vas deferens one of a pair of thin tubes that transports sperm from an epididymis to an ejaculatory duct during ejaculation; also called sperm duct        
    vasectomy surgical sterilization procedure in males in which the vas deferens are blocked so sperm cannot be ejaculated        
    vasoconstriction narrowing of blood vessels so less blood can flow through them        
    vasodilation widening of blood vessels so more blood can flow through them        
    vector organisms such as an insect that spreads pathogens from host to host        
    vein type of blood vessel that carries blood toward the heart from the lungs or body        
    ventilation process of moving air into and out of the lungs; also called breathing        
    ventral cavity major human body cavity at the anterior (front) of the trunk that contains such organs as the lungs, heart, stomach, intestines, and internal reproductive organs        
    ventricle one of two lower chambers of the heart that pumps blood out of the heart        
    vernix thick waxy coating that forms on a fetus to protect the skin and that mostly disappears by birth        
    vertebra (plural, vertebrae) one of 33 small bones that make up the vertebral column        
    vertebral column flexible column of vertebrae that connects the trunk to the skull and encloses the spinal cord; also called spine or backbone        
    vesicle small sac-like organelle that stores and transports materials inside a cell        
    vesicle transport type of active transport in which substances are carried across a cell membrane by vesicles        
    vestigial structure structure such as the human tailbone or appendix that evolution has reduced in size because it is no longer needed        
    villi (singular, villus) microscopic, finger-like projections in a mucous membrane that form a large surface area for absorption        
    viral load amount of virus in a sample of an infected individual’s blood        
    virus tiny, nonliving particle that contains DNA but lacks other characteristics of living cells and may cause human disease        
    vision ability to sense light and see; also called sight        
    vitamin organic compound needed in small amounts for proper body functioning        
    vitamin D micronutrient synthesized in the skin and needed to absorb calcium for strong bones        
    vulva external female reproductive structures, including the clitoris, labia, and vaginal and urethral openings        
    wasting dangerously low levels of body fat and muscle mass, usually due to severe undernutrition        
    water cycle biogeochemical cycle through which water is recycled on, above, and below Earth’s surface; also called hydrological cycle        
    Wernicke’s area speech center in the brain that controls the ability to comprehend speech        
    wetland area that is saturated with water or covered by water for at least one season of the year and has plants that can grow in water-logged soil        
    white blood cell type of cell in blood that defends the body against invading microorganisms or other threats in blood or extracellular fluid        
    white matter type of nervous tissue that is found only in the brain and spinal cord and that connects and facilitates communication between gray matter areas        
    withdrawal contraceptive method with a high failure rate in which a man withdraws his penis from his partner’s vagina before ejaculation occurs; also called coitus interruptus        
    X-linked gene gene located on the X chromosome        
    X-linked trait trait controlled by a gene located on the X chromosome        
    yolk sac membranous sac attached to an embryo that provides a source of nutrients to the early embryo and becomes part of the primitive gut        
    zero population growth (ZPG) equilibrium condition in which the human population is neither increasing nor decreasing in size because the birth rate equals the death rate        
    zona pellucida protein layer surrounding the cell membrane of a human egg        
    zoonosis (plural, zoonoses) >infectious disease of nonhuman animals that can be transmitted to humans        
    zygote diploid cell that forms when two haploid gametes unite during fertilization        
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