Audio Glossary

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  1. NADH: An energy carrier molecule that acts as a reducing agent in the catabolic reactions that produce ATP from the breakdown of sugars into water and carbon dioxide
  2. NADPH: An energy carrier molecule that acts as a reducing agent in photosynthesis.
  3. natural killer (NK) cell: A type of white blood cell that destroys any cell whose plasma membrane is marked with foreign proteins, including viral proteins.
  4. natural selection: An evolutionary mechanism in which those individuals in a population that possess particular inherited characteristics survive and reproduce at a higher rate than other individuals in the population because of those characteristics. Natural selection is the only evolutionary mechanism that consistently improves the survival and reproduction of the organism in its environment.
  5. negative feedback: A mechanism for slowing down or damping the fluctuations in a process. In response to a change in conditions, negative feedback acts to bring conditions back to a set point. Compare positive feedback.
  6. neocortex: A part of the cerebral cortex found only in mammals, where the ability to think complex thoughts and use language resides.
  7. nephron: The basic unit of the vertebrate kidney involved in urine production, consisting of a glomerulus, across which blood is filtered under pressure into a U-shaped tubule, which facilitates reabsorption of water and valuable solutes from the filtrate.
  8. nerve: A major communication pathway in the animal body made up of the axons of many individual neurons bundled together.
  9. nerve cell: See neuron.
  10. nervous system: system A high-speed internal communication system in animals that is made up of specialized cells called neurons.
  11. net primary productivity (NPP): The amount of energy that producers capture by photosynthesis, minus the amount lost as metabolic heat. NPP is usually measured as the amount of new biomass produced by photosynthetic organisms per unit of area during a specified period of time. Compare secondary productivity.
  12. neuromuscular junction: A synapse between a motor neuron and a muscle fiber.
  13. neuron: A type of animal cell that is highly specialized for transmitting action potentials from one part of the body to another.
  14. neurotransmitter: Any of various signaling molecules that transmit signals across the gaps, or synapses, that separate neurons from their target cells.
  15. neutron: A particle found in the nucleus of an atom that has no electrical charge.
  16. neutrophil: A type of white blood cell that destroys invading bacterial cells by phagocytosis.
  17. nicotine: The chemical in tobacco that is responsible for addiction.
  18. nitrogen base: Any of the five nitrogen-rich compounds found in nucleotides. The four nitrogen bases found in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T); in RNA, uracil (U) replaces thymine.
  19. nitrogen fixation: The process by which nitrogen gas (N2), which is readily available in the atmosphere but cannot be used by plants, is converted to ammonium (NH4 +), a form of nitrogen that can be used by plants. Nitrogen fixation is accomplished naturally by bacteria and by lightning, and by humans in industrial processes such as the production of fertilizer.
  20. node of Ranvier: An unmyelinated portion of the axon of a vertebrate neuron. Action potentials jump from node to node, greatly speeding signal transmission.
  21. noncoding DNA: A segment of DNA that does not encode proteins or RNA. Introns and spacer DNA are two common types of noncoding DNA.
  22. noncovalent bond: Any chemical linkage between two atoms that does not involve the sharing of electrons. Hydrogen bonds and ionic bonds are examples of noncovalent bonds.
  23. nonpolar: Of or referring to a molecule or a portion of a molecule that has an equal distribution of electrical charge across all its constituent atoms. Nonpolar molecules do not form hydrogen bonds and therefore tend not to dissolve in water. Compare polar.
  24. nonspecific response: A defensive response found in most animals that leads to the destruction of cells not recognized as belonging to the organism. Compare specific response.
  25. norepinephrine: A hormone produced by the adrenal glands that regulates the availability of glucose in conjunction with epinephrine.
  26. NPP: See net primary productivity.
  27. nuclear envelope: The double membrane that encloses the nucleus of a eukaryotic cell.
  28. nuclear pore: A channel in the nuclear envelope that allows selected molecules to move into and out of the nucleus.
  29. nucleic acid: A polymer made up of nucleotides. There are two kinds of nucleic acids: DNA and RNA.
  30. nucleotide: Any of a class of organic compounds that serve as energy carriers and as the chemical building blocks of nucleic acids such as DNA and RNA. A nucleotide is made up of a phosphate group, a five-carbon sugar, and one of four nitrogen-containing molecules called bases (see nitrogen base). Nucleotides are linked together to form a single strand of DNA or RNA.
  31. nucleus (pl. nuclei): The organelle in a eukaryotic cell that contains the genetic blueprint in the form of DNA.
  32. nucleus accumbens: A part of the human brain that is associated with addictive behavior.
  33. nutrient: In an ecosystem context, an essential element required by a producer. See also macronutrient and micronutrient.
  34. nutrient cycle: The cyclical movement of a nutrient between organisms and the physical environment. There are two main types of nutrient cycles: atmospheric and sedimentary. See also atmospheric cycle and sedimentary cycle.