h1>Audio Glossary

You need Adobe Flash Player to listen to the pronunciations.

  1. P generation: The parent generation of a genetic cross. Compare F1 generation and F2 generation.
  2. pacemaker: See sinoatrial node.
  3. pain receptor: A sensory cell that initiates an action potential in response to extreme heat or cold or to injury.
  4. pancreas: An organ associated with the digestive system that produces a number of important digestive enzymes as well as hormones.
  5. Pangaea: An ancient supercontinent that contained of all of the world's landmasses. Pangaea formed 250 million years ago and began to break apart 200 million years ago, ultimately yielding the continents we know today.
  6. parasite: An organism that lives in or on another organism (its host) and obtains nutrients from that organism. Parasites harm and may eventually kill their hosts, but do not kill them immediately.
  7. parathyroid gland: A gland lying next to the thyroid gland that produces parathyroid hormone.
  8. parathyroid hormone (PTH): A hormone that stimulates the release of calcium from bones and increases the reabsorption of calcium in the kidneys. Compare calcitonin.
  9. particulates: Microscopic particles in smoke that can penetrate the lung wall and enter the blood.
  10. passive carrier protein: The immunity of an organism to a pathogen that depends on antibodies specific to that pathogen that come from a source outside the organism's body. Compare active immunity.
  11. passive carrier protein: The immunity of an organism to a pathogen that depends on antibodies specific to that pathogen that come from a source outside the organism's body. Compare active immunity.
  12. passive transport: Movement of molecules from areas of lower concentration to areas of higher concentration without the expenditure of energy. Compare active transport.
  13. pathogen: An organism or virus that infects a host and causes disease, harming and in some cases killing the host.
  14. pcr: See polymerase chain reaction.
  15. pedigree: A chart that shows genetic relationships among family members over two or more generations of a family's history.
  16. penis: A reproductive structure used by male animals to introduce sperm directly into a female's reproductive tract.
  17. peptide bond: A covalent bond between the amino group of one amino acid and the carboxyl group of another that links amino acids together.
  18. perennial: A plant that lives for three or more years.
  19. peripheral nervous system (PNS): The portion of the nervous system that carries signals to and from the central nervous system. The peripheral nervous system consists of the sensory organs and all the nerves that are not part of the central nervous system. Compare central nervous system.
  20. Permian extinction: The largest mass extinction in the history of life on Earth; it occurred 250 million years ago, driving up to 95 percent of the species in some groups to extinction.
  21. petal: A part of a flower that often serves to attract pollinators to the flower. Petals are the second (from the outside) of the four whorls of modified leaves that make up a flower. Compare sepal, stamen, carpel.
  22. pH: The concentration of hydrogen ions in a solution. The pH scale runs from 1 to 14. A pH of 7 is neutral; values below 7 indicate acids, and values above 7 indicate bases.
  23. phagocytosis: A form of endocytosis by which a cell engulfs a large particle, such as another cell; “cell eating.”
  24. pharynx: The portion of the respiratory system where the mouth and the nasal cavity join to form a single passageway.
  25. phenotype: The observable physical characteristics of an organism. Compare genotype.
  26. pheromone: A chemical signal produced by one individual to communicate its identity and location to another individual.
  27. phloem: A tissue composed of living cells through which a plant transports the products of photosynthesis. Compare xylem.
  28. phospholipid: A lipid molecule with an attached phosphate group. Phospholipids are the major components of all biological membranes.
  29. phospholipid bilayer: A double layer of phospholipid molecules arranged so that their hydrophobic "tails" lie sandwiched between their hydrophilic "heads." A phospholipid bilayer forms the basic structure of all biological membranes.
  30. phosphorylation: The addition of a phosphate group to an organic molecule.
  31. photoperiodism: A physiological response by a plant to day length; examples include the timing of flowering, seed germination, and bud dormancy (during winter).
  32. photoreceptor: A sensory cell that initiates an action potential in response to light.
  33. photosynthesis: A process by which organisms capture energy from sunlight and use it to synthesize sugars from carbon dioxide and water.
  34. photosystem: A large complex of proteins and chlorophyll that captures energy from sunlight. Two distinct photosystems (I and II) are present in the thylakoid membranes of chloroplasts.
  35. photosystem I: The photosystem that is primarily responsible for the production of NADPH.
  36. photosystem II: The photosystem in which light energy is used to initiate an electron flow along the electron transport chain that leads to the production of ATP.
  37. phylum: The level in the Linnaean hierarchy that is above classes and therefore comprises classes, but that is below kingdoms and so composes kingdoms.
  38. pinna (pl. pinnae): The curled, cartilaginous part of the ear that is visible externally. The pinna funnels pressure changes from the environment into the auditory canal.
  39. pinocytosis: A form of nonspecific endocytosis by which cells take in fluid; "cell drinking."
  40. pituitary gland: A gland associated with the vertebrate brain that releases hormones that control the release of hormones by other glands. Along with the hypothalamus, the pituitary helps regulate interactions between the ner vous and endocrine systems.
  41. placenta: A structure found in mammals that transfers nutrients and gases from the blood of the mother to the blood of the developing fetus in her uterus.
  42. plant breeding: A traditional breeding technique used to create new crop varieties by allowing only plants with particular phenotypic characteristics to produce offspring.
  43. plantae: The kingdom that encompasses plants.
  44. plaque: A fatty growth that develops on the walls of arteries as a result of excess cholesterol in the human diet.
  45. plasma membrane: The phospholipid bilayer that surrounds the cell.
  46. plasmid: A small circular segment of DNA found naturally in bacteria. Plasmids are involved in natural gene transfers among bacteria and can be used as vectors in genetic engineering.
  47. plasmodesma (pl. plasmodesmata): A tunnel-like channel between two plant cells that provides a cytoplasmic connection allowing the flow of small molecules and water between them.
  48. platelet: A type of sticky cell fragment that circulates in the blood and that helps to form a blood clot by combining with proteins that also circulate in the blood to form a gel-like mesh that traps blood cells.
  49. polar: Of or referring to a molecule or a portion of a molecule that has an uneven distribution of electrical charge. Polar molecules can easily interact with water molecules and are therefore soluble. Compare nonpolar.
  50. polar body: The smaller of the two haploid cells that result from the first meiotic division of the primary oocyte. The polar body plays no role in development
  51. polarization: The state of an axon in which there is a difference in electrical charge between the inside and the outside of the plasma membrane. Compare depolarization.
  52. pollen grain: The small, mobile, multicellular haploid stage of a seed plant's life cycle in which sperm are produced. Compare embryo sac.
  53. pollinator: An animal that carries pollen grains from the stamens of one flower to the stigmas of other flowers of the same species.
  54. pollinator mutualism: A mutualism in which an animal transfers pollen grains from one flower to the female reproductive organs of another flower of the same species and receives food as a reward for this service.
  55. polygenic: Of or referring to inherited traits that are determined by the action of more than one gene.
  56. polymer: A large organic molecule composed of many monomers linked together.
  57. polymerase chain reaction (PCR): A method of DNA technology that uses the DNA polymerase enzyme to make multiple copies of a targeted sequence of DNA.
  58. polyploidy: condition in which an organism has three or more complete sets of chromosomes (rather than the usual two complete sets). Polyploidy can cause new species to form rapidly without geographic isolation.
  59. polysaccharide: A polymer composed of many linked monosaccharides. Starch and cellulose are examples of polysaccharides.
  60. population: A group of interacting individuals of a single species located within a particular area.
  61. population cycle: A pattern in which the population sizes of two species increase and decrease together in a tightly linked cycle; this pattern can occur when at least one of the two species involved is very strongly influenced by the other.
  62. population density: The number of individuals in a population, divided by the area covered by the population.
  63. population size: The total number of individuals in a population.
  64. positive feedback: A mechanism for speeding up or amplifying a process. Compare negative feedback.
  65. positron emission tomography (PET): An imaging technology that can detect which parts of the brain are active at any particular time by following the location of radioactively labeled chemicals.
  66. power arm: The section of a lever that extends from the fulcrum to the point at which force is applied. Compare load arm.
  67. precision farming: A method of farming that uses intensive data collection, computers, and in some cases, satellite data to inform farmers when to fertilize or use pesticides.
  68. predator: An organism that kills other organisms for food.
  69. predictions: With respect to the scientific method, statements about logical consequences that should be observed if a hypothesis is correct.
  70. preimplantation genetic diagnosis (PGD): A procedure used in in vitro fertilization in which one or two cells are removed from a developing embryo and tested for genetic disorders; embryos that are free of genetic disorders are then implanted into the mother's uterus.
  71. pressure drag: Drag that results from a difference in the pressures in front of and behind a moving object. Compare friction drag.
  72. prey: Animals that predators kill and eat.
  73. primary consumer: An organism that eats a producer. Compare secondary consumer.
  74. primary growth: An increase in the length (or height) of a plant that results from divisions of apical meristem cells.
  75. primary immune response: The relatively slow mobilization of B cells and T cells following the first exposure to a pathogen. Compare secondary immune response.
  76. primary oocyte: A diploid animal cell that can undergo meiosis to form mature ova.
  77. primary spermatocyte: A diploid animal cell that can undergo meiosis to form mature sperm.
  78. primary structure: The sequence of amino acids in a protein.
  79. primary succession: Ecological succession that occurs in newly created habitat, as when an island rises from the sea or a glacier retreats, exposing newly available bare ground. Compare secondary succession.
  80. primate: An order of mammals whose living members include lemurs, tarsiers, monkeys, humans, and other apes. Primates share characteristics such as flexible shoulder and elbow joints, opposable thumbs or big toes, forward-facing eyes, and brains that are large relative to body size.
  81. producer: An organism that uses energy from an external source, such as sunlight, to produce its own food without having to eat other organisms or their remains. Compare consumer.
  82. productivity: The mass of plant matter that can be produced in a given area from the available nutrients and sunlight.
  83. progesterone: A steroid hormone that helps maintain pregnancy in female mammals.
  84. prokaryote: A single-celled organism that does not have a nucleus. All prokaryotes are members of the domains Bacteria or Archaea. Compare eukaryote.
  85. prometaphase: The stage of mitosis during which chromosomes become attached to the mitotic spindle.
  86. promoter: The DNA sequence in a gene to which RNA polymerase binds to begin transcription.
  87. prophase: The stage of mitosis during which chromosomes first become visible under the microscope.
  88. proprioceptor: A pressure-sensitive sensory receptor that provides information on the position of an animal's body relative to its surroundings and of the animal's body parts relative to one another.
  89. prostaglandin: One of a class of chemical messages released by damaged cells in response to histamines to stimulate increased blood flow to the area.
  90. protein: A linear polymer of amino acids linked together in a specific sequence. Most proteins are folded into complex three-dimensional shapes.
  91. protista: The oldest eukaryotic kingdom, consisting of a diverse collection of mostly single-celled but some multicellular organisms
  92. proton: A positively charged particle found in atoms. Each atom contains a characteristic number of protons. Compare electron.
  93. proton gradient: A gene that promotes cell division in response to normal growth signals.
  94. proto-oncogene: A gene that promotes cell division in response to normal growth signals.
  95. protostome: Any of a group of animals, including insects, worms, and snails, in which the first opening to develop in the early embryo becomes the mouth. Compare deuterostome.
  96. pseudopodium (pl. pseudopodia): A dynamic protrusion of the plasma membrane that enables some cells to move. The extension of pseudopodia depends on actin filaments inside the cell.
  97. puberty: In humans, the transition from childhood to reproductive maturity.
  98. pulmonary: The loop of the circulatory system through which blood travels from the heart to the lungs and back.
  99. Punnett square: A diagram in which the possible types of male and female gametes are listed on two sides of a square, providing a graphic way to predict the genotypes of the offspring produced in a genetic cross.
  100. pupil: The opening through which light enters the eye.
  101. pyruvate: A three-carbon molecule produced by glycolysis that is processed in the mitochondria to generate ATP.