Audio Glossary

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  1. radiation: A means of exchanging heat in the form of light or infrared (heat) waves.
  2. radioisotope: An unstable, radioactive form of an element that decays to more stable forms at a constant rate over time.
  3. rain shadow: An area on the side of a mountain facing away from moist prevailing winds where little rain or snow falls.
  4. rainforest: A forest that receives high rainfall.
  5. rangeland: An open area where domesticated animals such as cattle or sheep can roam.
  6. reabsorption: Removal of valuable solutes and water from the filtrate in the tubules of the kidney before they can leave the kidney in the urine.
  7. reaction center: A formation within an antenna complex where electrons become excited and are passed to an electron transport chain.
  8. receptor: A protein that facilitates the transmission of a signal after binding to a specific signaling molecule. Receptors may be found inside the cell or embedded in the plasma membrane.
  9. receptor-mediated endocytosis: A form of endocytosis in which receptor proteins embedded in the plasma membrane of a cell recognize certain surface characteristics of materials to be brought into the cell by endocytosis.
  10. recessive: Of or referring to an allele that does not have a phenotypic effect when paired with a dominant allele. Compare dominant.
  11. recombination: A collective term for the processes of fertilization, crossing-over, and independent assortment of chromosomes, all of which result in new combinations of alleles.
  12. red blood cell: A type of cell that circulates freely with the blood and that contains an oxygen-binding pigment, hemoglobin, that greatly increases the amount of oxygen that the blood can carry.
  13. redox reaction: A chemical reaction in which electrons are transferred from one molecule or atom to another.
  14. reduction: The gain of electrons by one atom or molecule from another. Compare oxidation.
  15. reflex arc: A neuronal connection in the spinal cord between a sensory cell and a muscle cell that allows a simple stimulus to be translated rapidly into a movement.
  16. regulatory DNA: A DNA sequence that can turn the expression of a particular gene or group of genes on or off. Regulatory DNA sequences interact with regulatory proteins to control gene expression.
  17. regulatory protein: A protein that signals whether or not a particular gene or group of genes should be expressed. Regulatory proteins interact with regulatory DNA to control gene expression.
  18. releaser: A simple stimulus that triggers a fixed behavior.
  19. replicate: An independent run or performance of an experiment.
  20. repressor protein: A protein that prevents the expression of a particular gene or group of genes.
  21. reproductive cloning: A technology used to produce an offspring that is an exact genetic copy (a "clone") of another individual. The first two steps in reproductive cloning are the same as those in therapeutic cloning, but stem cells are not removed from the embryo. Instead, the embryo is transferred to the uterus of a surrogate mother, where, if all goes well, the birth of a healthy offspring ultimately results; this offspring is genetically identical to the individual who provided the donor nucleus. Compare therapeutic cloning.
  22. reproductive isolation: A condition in which barriers to reproduction prevent or strongly limit two or more populations from reproducing with one another. Many different kinds of reproductive barriers can result in reproductive isolation, but it always has the same effect: no or few genes are exchanged between the reproductively isolated populations.
  23. respiration: See aerobic respiration.
  24. resting potential: The electrical state of an axon when it is not transmitting an action potential.
  25. restriction enzyme: Any of a number of enzymes that cut DNA molecules at a specific target sequence; a key tool of DNA technology.
  26. retina: A field of photoreceptors in the eye that allows organisms to form an image.
  27. reversible binding: The capacity of a molecule to bind and to release another molecule under different conditions.
  28. RFLP analysis: A method of DNA technology in which restriction enzymes are used to cut an organism's genome into small pieces, which are sorted by size using gel electrophoresis. Next, a DNA probe is used to form a profile, whose pattern depends on the number and size of the fragments that can bind to the probe.
  29. ribonucleic acid: See RNA.
  30. ribosomal RNA (rRNA): A type of RNA that is an important component of ribosomes.
  31. ribosome: A particle composed of proteins and RNA at which new proteins are synthesized. Ribosomes can be either attached to the endoplasmic reticulum or free in the cytosol.
  32. ring species: A species whose populations loop around a geographic barrier (such as a mountain chain) and in which the populations at the two ends of the loop are in contact with one another, yet cannot interbreed.
  33. river: An aquatic biome, the communities of which live in relatively narrow bodies of fresh water that move continuously in a single direction.
  34. RNA: Ribonucleic acid; a polymer of nucleotides that is necessary for the synthesis of proteins in living organisms.
  35. RNA polymerase: The key enzyme in DNA transcription, which links together the nucleotides of the RNA molecule specified by a gene
  36. rod: A type of photoreceptor cell in the vertebrate eye that detects light and dark and functions well in dim light. Compare cone.
  37. root: A plant organ that serves to anchor the plant, absorb water and nutrients from the soil, transport food and water, and (often) store food.
  38. root cap: A layer of cells at the base of the root that protects the root apical meristem.
  39. root hair: An absorptive structure formed by root dermal cells.
  40. root system: A collection of fingerlike growths into the soil that absorb water and nutrients; one of the two basic systems of the plant body.
  41. rooting reflex: The reflexive search for a nipple by a human infant when its cheek or lips are touched lightly.
  42. rough ER: A region of the endoplasmic reticulum that has attached ribosomes. Compare smooth ER.
  43. rRNA: See ribosomal RNA.
  44. rubisco: The enzyme that catalyzes the first reaction of carbon fixation in photosynthesis.