Glossary

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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	https://bio.libretexts.org/	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-14^th 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 19^th 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 20^th 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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