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Biology LibreTexts

25.1: Glossary: A

  • Page ID
  • 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


    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


    development of reversible changes to environmental stress over a relatively short period of time


    solution with a pH lower than 7

    acid rain

    low-pH precipitation that forms when certain primary pollutants in the atmosphere combine with water


    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)


    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


    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


    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


    situation in which new DNA sequences enter a population’s gene pool through interbreeding with individuals in another population


    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


    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


    life stage of the human organism between adolescence and death


    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


    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


    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


    any substance, typically an antigen, that causes an 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


    membrane forming a fluid-filled sac that surrounds and protects an embryo and fetus

    anabolic reaction

    endothermic reaction in organisms


    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


    third phase of mitosis during which sister chromatids separate and move to opposite poles of the cell


    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


    study of the structure of the body


    general term for a male sex hormone such as testosterone


    disorder in which the blood has an inadequate volume of red blood cells, which are needed to carry oxygen to cells


    chest pain or pressure that occurs when heart muscle cells do not receive adequate blood flow and become starved of oxygen


    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


    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


    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


    large, Y-shaped protein produced by B cells that recognizes and binds to a nonself antigen in a humoral immune response


    string of three nucleotide bases that are complementary to the three nucleotide bases of a codon for an amino acid


    molecule that the immune system identifies as either self or nonself and, if the latter, may respond to by forming antibodies


    final part of the large intestine with an opening to the outside for feces to pass through


    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


    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


    underground layer of rock that stores water


    referring to trees, as in arboreal, or tree-living, animal


    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


    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


    chronic inflammatory disease of the respiratory system in which airways periodically become inflamed, causing swelling and narrowing of the airways, which makes breathing difficult


    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


    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


    decrease in the size of a structure, such as a decrease in the size of a muscle through nonuse


    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


    any of the chromosomes numbered 1 to 22 in humans that contains genes for characteristics unrelated to sex determination


    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


    long extension of the cell body of a neuron that transmits nerve impulses to other cells