Audio Glossary

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  1. acid A chemical compound that can give up a hydrogen ion. Compare base and buffer.
  2. acid rain Rainfall with a low pH. Acid rain is a consequence of the release of sulfur dioxide and other pollutants into the atmosphere, where they are converted to acids that then fall back to Earth in rain or snow.
  3. acrosome The front tip of a sperm, which contains enzymes that digest the outer covering of the unfertilized ovum.
  4. actin A protein found in the cytoskeleton, in muscle tissue, and in bacterial flagella.
  5. actin filament One of the two types of filaments that make up muscle, consisting of two molecules of the protein actin. Compare myosin filament.
  6. action potential An electrical signal generated by the flow of ions across the plasma membrane of a neuron. Action potentials are self-sustaining and can travel down a neuron in only one direction.
  7. activation energy The small input of energy required for a chemical reaction to proceed.
  8. active carrier protein A protein in the plasma membrane of a cell that, using energy from an energy storage molecule such as ATP, changes its shape to transfer a molecule across the plasma membrane. Compare passive carrier protein.
  9. active immunity The immunity of an organism to a pathogen that depends on the production of antibodies specific to that pathogen by the organism's own body. Compare passive immunity.
  10. active site The specific region on the surface of an enzyme where substrate molecules bind.
  11. active transport Movement of molecules that requires an input of energy. Compare passive transport.
  12. adaptation A characteristic of an organism that improves that organism's performance in its environment.
  13. adaptive evolution The process by which natural selection improves the match between organisms and their environment over time.
  14. adaptive radiation An evolutionary expansion in which a group of organisms takes on new ecological roles and forms new species and higher taxonomic groups.
  15. adenosine triphosphate See ATP.
  16. adrenal gland One of a pair of endocrine glands that sit atop the mammalian kidney and which release the hormones epinephrine and norepinephrine.
  17. adrenaline See epinephrine.
  18. aerobic Of or referring to a metabolic process or organism that requires oxygen gas. Compare anaerobic.
  19. aerobic respiration A general term used to describe a series of oxidation reactions that use oxygen to produce ATP.
  20. allele One of several alternative versions of a gene. Each allele has a DNA sequence different from that of all other alleles of the same gene.
  21. allele frequency The proportion (percentage) of a particular allele in a population.
  22. allopatric speciation The formation of new species from populations that are geographically isolated from one another. Compare sympatric speciation.
  23. altruism A behavior that benefits another individual but has a cost to the individual performing the behavior.
  24. alveolus (pl. alveoli) A small sac in the mammalian lung where gas exchange takes place.
  25. amino acid An organic compound that has an amino group, a carboxyl group, and a variable R group attached to a single carbon atom. Proteins are polymers of amino acids.
  26. amniocentesis A procedure in which a needle is inserted through the abdomen into the uterus to extract a small amount of amniotic fluid from the pregnancy sac that surrounds the fetus; this fluid contains fetal cells that can be used to test for genetic disorders.
  27. amnion A sac filled with a watery fluid that surrounds and protects the developing vertebrate embryo.
  28. amplification The chain of events set in motion when the binding of a single hormone molecule to a receptor activates thousands of proteins in the target cell.
  29. anabolic See biosynthetic.
  30. anaerobic Of or referring to a metabolic process or organism that does not require oxygen gas. Compare aerobic.
  31. analogous Of or referring to a characteristic shared by two groups of organisms because of convergent evolution, not common descent. Compare homologous.
  32. anaphase The stage of mitosis during which sister chromatids separate and move to opposite poles of the cell.
  33. anchoring junction A protein structure that acts as a “hook” between two animal cells or between a cell and the extracellular matrix.
  34. androgen One of a class of steroid hormones that stimulate cells to develop the characteristics of maleness. The primary androgen is testosterone. Compare estrogen.
  35. angiosperms The flowering plants, a group that includes most plants on Earth today; named for the protective tissues covering the plant's embryo in the seed. Compare gymnosperms.
  36. Animalia The kingdom made up of animals, multicellular eukaryotes that have evolved specialized tissues, organs and organ systems, body plans, and behaviors.
  37. annual A plant that completes its entire life cycle in one year.
  38. antagonistic Of or referring to the opposing effects of two hormones that together serve to regulate a process.
  39. antenna complex An arrangement of chlorophyll molecules in the thylakoid membrane of a chloroplast that harvests energy from sunlight.
  40. antibody A protein that is produced by a B cell and binds specifically to a particular antigen.
  41. anticodon A sequence of three nitrogen bases on a transfer RNA molecule that can bind to a particular codon on an mRNA molecule. Compare codon.
  42. antigen A characteristic membrane protein or molecule produced by an invading pathogen that is recognized by particular lymphocytes and the antibodies produced by those lymphocytes.
  43. anus The opening of the digestive system through which undigested food and other solid wastes leave the body.
  44. apical dominance Inhibition of the growth of lateral buds by the apical meristem.
  45. apical meristem A region of rapidly dividing cells in the tips of plant branches and roots that gives rise to new stem and root tissues.
  46. apoptosis Programmed or intentional cell death.
  47. appendicular skeleton A collective term for the bones of the arms, legs, and pelvis. Compare axial skeleton.
  48. aquaculture Managed agricultural systems in which fish and shellfish are cultivated for human consumption.
  49. aquifer An underground body of water that is sometimes bounded by impermeable layers of rock.
  50. Archaea A domain of microscopic, single-celled prokaryotes that arose after the Bacteria.
  51. artery A blood vessel that carries blood from the heart. Compare vein.
  52. arthropods A group of animals characterized by a hard exoskeleton; includes millipedes, crustaceans, insects, and spiders.
  53. artificial selection A process in which only individuals that possess certain characteristics are allowed to breed; used to guide the evolution of crop plants and domestic animals in ways that are advantageous for people.
  54. asexual reproduction The production of genetically identical offspring without the exchange of genetic material with another individual. Compare sexual reproduction.
  55. atmospheric cycle A type of nutrient cycle in which the nutrient enters the atmosphere easily. Compare sedimentary cycle.
  56. atom The smallest unit of a chemical element that still has the properties of that element.
  57. atomic mass number The sum of the number of protons and neutrons found in the nucleus of an atom of a particular chemical element.
  58. atomic number The number of protons found in the nucleus of an atom of a particular chemical element.
  59. ATP Adenosine triphosphate, a molecule that is commonly used by cells to store energy and to transfer energy from one chemical reaction to another.
  60. atrioventricular (AV) node A signaling center in the heart that relays the signal for cardiac muscle to contract from the sinoatrial node to the ventricles.
  61. atrium (pl. atria) A chamber in a heart that receives blood from the body and pumps it into the ventricle. Compare ventricle.
  62. auditory canal The tubular opening leading from the ear pinna to the eardrum.
  63. autoimmune disease A disease caused when the body destroys some of its own tissues that the immune system misidentifies as foreign tissues.
  64. autosome Any chromosome that is not a sex chromosome. Compare sex chromosome.
  65. auxin A plant hormone that causes apical dominance and has other important effects, such as controlling the way plants bend toward sunlight and stimulating the differentiation of vascular tissue.
  66. axial skeleton A collective term for the skull, the spinal column, and the ribs. Compare appendicular skeleton.
  67. axon An extension of a neuron that carries action potentials through an animal's body.