Audio Glossary

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  1. calcitonin: A hormone produced by the mammalian parathyroid gland that promotes the storage of calcium in bones. Compare para thyroid hormone.
  2. Cambrian explosion: A major increase in the diversity of life on Earth that occurred about 530 million years ago, during the Cambrian period. The Cambrian explosion lasted 5 to 10 million years; during this time large and complex forms of most living animal phyla appeared suddenly in the fossil record.
  3. cancer: A group of diseases caused by rapid and inappropriate cell division.
  4. canopy: The habitat in the branches of forest trees.
  5. cap-and-trade system: An approach to pollution control in which a government sets a nationwide limit, or cap, on the amount of a pollutant that can be added to the environment each year. Each factory that emits the pollutant is given a certain number of "emission allowances" in each year. Unused allowances can be sold, traded, or saved (banked) for future use.
  6. capillary: A tiny blood vessel in a closed circulatory system, across the walls of which all exchange with the surrounding tissues takes place.
  7. carbohydrate: Any of a class of organic compounds that includes sugars and their polymers, in which each carbon atom is linked to two hydrogen atoms and an oxygen atom. See also sugar.
  8. carbon fixation: The process by which carbon atoms from carbon dioxide gas are incorporated into sugars; occurs in the chloroplasts of plants.
  9. carcinogen: A physical, chemical, or biological agent that causes cancer
  10. cardiac cycle: The series of events involved in the pumping of blood by the heart. The cardiac cycle consists of two phases: diastole and systole.
  11. cardiac muscle: The specialized muscle of the vertebrate heart. Cardiac muscle has bands like the muscles associated with the skeleton, and its muscle fibers are joined by interconnecting branches that help them to produce the coordinated contractions known as heartbeats.
  12. cardiovascular system: The closed circulatory system of vertebrates, which consists of a muscular, chambered heart and loops of blood vessels.
  13. carnivore: A consumer that relies on living animal tissues for nutrients. Compare herbivore.
  14. carpel: The female reproductive part of a flower, consisting of a stigma, style, and ovary. Carpels are the innermost of the four whorls of modified leaves that make up a flower. Compare sepal, petal, stamen.
  15. carrier: An individual that carries a disease-causing allele but does not get the disease.
  16. carrying capacity: The maximum population size that can be supported indefinitely by the environment in which a population is found.
  17. cartilage: A support structure found in animals made up of cells that secrete an extra cellular matrix rich in the protein collagen.
  18. catabolic: Of or referring to a chemical reaction that breaks down complex molecules to release energy for use by the cell. Compare biosynthetic.
  19. catalysis: A process in which a chemical substance (a catalyst) participates in a reaction in such a way as to lower the amount of activation energy required, greatly increasing the rate at which the reaction proceeds.
  20. catalyst: A molecule that speeds up a specific chemical reaction without being permanently altered in the process. Enzymes are protein catalysts.
  21. cell: The smallest self-contained unit of life, enclosed by a membrane.
  22. cell communication: The process by which one cell can affect the activities of another via signaling molecules.
  23. cell cycle: A series of distinct stages in the life cycle of a cell that culminate in cell division.
  24. cell junction: A structure connecting two cells that holds them together and allows them to communicate with one another.
  25. cell-mediated immunity: A form of immunity that involves the action of T cells. Compare humoral immunity.
  26. cell specialization: An organizational principle stating that different types of cells in a multicellular organism differ in their structure and function.
  27. cell wall: A support layer that lies outside the plasma membrane of the cells of many prokaryotes, fungi, and plants.
  28. cellulose: A carbohydrate produced by plants and some other organisms that makes up much of their cell walls.
  29. central nervous system (CNS): The portion of the nervous system devoted to the exchange and integration of information among neurons. The central nervous system consists of the brain and spinal cord. Compare peripheral nervous system.
  30. central vacuole: A large vacuole that usually occupies more than a third of a plant cell's total volume.
  31. centromere: physical constriction that holds sister chromatids together.
  32. centrosome: A protein structure in the cytosol that helps organize the mitotic spindle and defines the two poles of a dividing cell.
  33. cerebral cortex: The highly folded outer portion of the human brain that is specialized for coordinating information from sensory cells with the activity of muscles.
  34. cerebrum: The largest part of the human forebrain; includes the cerebral cortex.
  35. channel protein: A protein in the plasma membrane of a cell that forms an opening through which certain molecules can pass.
  36. chaparral: A terrestrial biome, the communities of which are characterized by shrubs and small nonwoody plants that grow in regions with mild summers and winters and low to moderate amounts of precipitation.
  37. character displacement: A process by which intense competition between species causes the forms of the competing species to evolve to become more different over time.
  38. chemical compound: An association of atoms of different chemical elements linked by covalent bonds.
  39. chemical reaction: A process that rearranges atoms in chemical compounds.
  40. chemoreceptor: A sensory cell that initiates an action potential in response to a chemical stimulus.
  41. chitin: A carbohydrate that serves as an important support material in the cell walls of fungi and in animal exoskeletons.
  42. chlorofluorocarbons (CFCs): Synthetic chemical compounds whose release into the atmosphere can damage the ozone layer.
  43. chlorophyll: A green pigment that is used to capture energy from light in photosynthesis.
  44. chloroplast: An organelle found in plants and algae that is the primary site of photosynthesis.
  45. cholesterol: A steroid constructed from saturated fats that is important in cell membranes and forms the molecular backbone of steroid hormones.
  46. chorionic villus sampling (CVS): A procedure in which a flexible tube is inserted through a woman's vagina and into her uterus. The tip of this tube is placed next to a cluster of cells that attaches the pregnancy sac to the wall of the uterus; cells are then removed from this cluster by gentle suction so they can be tested for genetic disorders.
  47. chromatid: Either of two identical, side-by-side copies of a chromosome that are linked at the centromere.
  48. chromatin: The combination of DNA and proteins that makes up chromosomes.
  49. chromosome: Any of several elongated structures found in the nucleus of a cell, each composed of DNA packaged with proteins. Chromosomes become visible under the microscope during mitosis and meiosis.
  50. chromosome theory of inheritance: A theory, supported by much experimental evidence, stating that genes are located on chromosomes.
  51. cilium (pl. cilia): A hairlike structure found in some eukaryotes that uses a rowing motion to propel the organism or to move fluid past the organism. Compare eukaryotic flagellum.
  52. circulatory system: A system of interconnected tubular vessels through which blood is pumped by the heart; the main internal transport system in animals.
  53. citric acid cycle: A series of oxidation reactions that produce high-energy electrons stored in NADH and release CO2 as a waste product. In eukaryotic cells, the citric acid cycle takes place in mitochondria.
  54. class: The level in the Linnaean hierarchy that is above orders and therefore comprises orders, but that is below phyla and so composes phyla.
  55. climate: The prevailing weather conditions experienced in an area over relatively long periods of time (30 years or more). Compare weather.
  56. climax community: A community, typical of a given climate and soil type, whose species are not replaced by other species. A climax community is the end point of succession for a particular location; in many cases, however, ongoing disturbances such as fire or windstorms prevent the formation of a stable climax community.
  57. clonal selection: The process by which a lymphocyte that has bound to its specific antigen reproduces rapidly, yielding large numbers of genetically identical lymphocytes with exactly the same antigen-recognizing proteins as the parent cell.
  58. clone (of a gene): A copy of a gene or other DNA sequence.
  59. clone (of an organism): A genetically identical copy of an individual organism, as produced by reproductive cloning.
  60. closed circulatory system: An internal transport system found in animals in which blood vessels carry blood throughout the body and back to the heart. Compare open circulatory system.
  61. cochlea: A coiled, fluid-filled tube in the inner ear.
  62. coding strand: Of the two strands in a DNA molecule, the strand whose DNA sequence is exactly duplicated by the sequence of bases in an mRNA molecule (except that U substitutes for T). Compare template strand.
  63. codominance: A situation in which the phenotype of a heterozygote is determined equally by each allele
  64. codon: A sequence of three nitrogen bases in an mRNA molecule. Each codon specifies either a particular amino acid or a signal to start or stop the translation of a protein. Compare anticodon.
  65. collagen: A protein produced by the connective tissues of animals.
  66. colon: The portion of the digestive system in which undigested material is prepared for release from the body. Also called large intestine.
  67. communication: A type of behavior that allows one individual to exchange information with another.
  68. community: An association of populations of different species that live in the same area.
  69. compact bone: The hard, white outer region of vertebrate bone. Compare spongy bone.
  70. comparative genomics: A field of scientific study that analyzes and compares the genomes of multiple species.
  71. competition: An interaction between two species in which each has a negative effect on the other.
  72. complement protein: A type of protein that circulates in the blood plasma and concentrates at sites of tissue damage, stimulating macrophage and neutrophil production and activity.
  73. complementary strand: A strand of DNA whose sequence of bases can pair (according to base-pairing rules) with the sequence of bases found in a focal DNA strand.
  74. concentration gradient: A change in the concentration of molecules from one location to another.
  75. conduction: The transfer of heat by direct contact between two materials.
  76. cone: A type of photoreceptor cell in the vertebrate eye that detects color and functions best in bright light. Compare rod.
  77. connective tissue: A type of animal tissue that connects and supports other cells in the body. Many connective tissues secrete an extra cellular matrix of nonliving material.
  78. consumer: An organism that obtains its energy by eating other organisms or their remains. Consumers include herbivores, carnivores, and decomposers. Compare producer.
  79. continental drift: The movement of Earth's continents over time.
  80. control: A treatment in an experiment in which the factor or factors being tested in the experiment are omitted, but other conditions are the same as in the experimental treatments.
  81. convection: The physical movement of heat in air or water.
  82. convection cell: A large and consistent atmo spheric circulation pattern in which warm, moist air rises and cool, dry air sinks. Earth has four stable giant convection cells (two in tropical regions and two in polar regions) and two less stable cells (located in temperate regions).
  83. convergent evolution: Evolutionary change that occurs when natural selection causes distantly related organisms to evolve similar structures in response to similar environmental challenges.
  84. convergent feature: A feature shared by two groups of organisms not because it was inherited from a common ancestor, but because it arose independently in the two groups.
  85. copulation: A behavior in which a male animal places his sperm directly into the body of a female; the typical mode of sperm transfer in land animals.
  86. coral reef: An aquatic biome, the communities of which form in warm, shallow waters located in the tropics. Corals are tiny animals that build up long-lasting structures, the reefs on which many of the other organisms in the community depend.
  87. cork cambium: A lateral meristem tissue that contributes to secondary growth by dividing to produce tough dermal tissues (including the outer surface of bark). Compare vascular cambium.
  88. corpus luteum (pl. corpora lutea): The cells of the ruptured follicle that remain behind in the ovary after ovulation to produce the hormone progesterone.
  89. covalent bond: A strong chemical linkage between two atoms based on the sharing of electrons. Compare hydrogen bond and ionic bond.
  90. Cretaceous extinction: A mass extinction that occurred 65 million years ago, wiping out many marine invertebrates and terrestrial plants and animals, including the last of the dinosaurs.
  91. crista (pl. cristae): A fold in the inner mitochon drial membrane.
  92. crossing-over: A physical exchange of genes between homologous chromosomes in which part of the genetic material inherited from one parent is replaced with the corresponding genetic material inherited from the other parent.
  93. cuticle: A waxy layer that covers aboveground plant parts, helping to prevent water loss and to keep enemies, such as fungi, from invading the plant.
  94. cynodont: A member of a group of mammal-like reptiles from which the earliest mammals arose, roughly 220 million years ago.
  95. cytokinesis: The stage following mitosis, during which the cell physically divides into two daughter cells.
  96. cytoplasm: The contents of a cell enclosed by the plasma membrane, but, in eukaryotes, excluding the nucleus. Compare cytosol.
  97. cytoskeleton: A complex network of protein filaments found in the cytosol of eukaryotic cells. The cytoskeleton maintains cell shape and is necessary for the physical processes of cell division and movement.
  98. cytosol: The contents of a cell enclosed by the plasma membrane, but, in eukaryotes, excluding all organelles. Compare cytoplasm.