nares Nostrils; the openings in the nose through which air enters.
nastic movement A plant's response to a stimulus in which the direction of the response is independent of the direction of the stimulus. Non-directional plant movements.
natural selection The process of differential survival and reproduction of Þtter genotypes; can be stabilizing, directional, or disruptive. Better adapted individuals are more likely to survive to reproductive age and thus leave more offspring and make a larger contribution to the gene pool than do less Þt individuals. The differential survival and reproductive successes of individuals in a variable population that powers the evolutionary process. When all individuals survive and reproduce (except for chance occurrences) natural selection works at a lower rate, if at all. PICTURE
nectaries Nectar-secreting organs in þowering plants that serve as insect feeding stations and thus attract insects, which then assist in the transfer of pollen.
negative feedback The stopping of the synthesis of an enzyme by the accumulation of the products of the enzyme-mediated reaction.
negative feedback control Occurs when information produced by the feedback reverses the direction of the response; regulates the secretion of most hormones.
negative feedback loop A biochemical pathway where the products of the reaction inhibit production of the enzyme that controlled their formation.
nektonic organisms "Swimmers"; one of the two main types of organisms in the pelagic zone of the marine biome.
nephridium The excretory organ in þatworms and other invertebrates; a blind-ended tubule that expels waste through an excretory pore.
nephron A tubular structure that is the Þltering unit of the kidney; consists of a glomerulus and renal tubule. PICTURE
nerve cord A dorsal tubular cord of nervous tissue above the notochord of a chordate.
nerve net An interconnected mesh of neurons that sends signals in all directions; found in radially symmetrical marine invertebrates, such as jellyÞsh and sea anemones, that have no head region or brain. PICTURE
nerves Bundles of neuronal processes enclosed in connective tissue that carry signals to and from the central nervous system. PICTURE
nervous system One of eleven major body organ systems in animals; coordinates and controls actions of internal organs and body systems, receives and processes sensory information from the external environment, and coordinates short-term reactions to these stimuli. PICTURE 1 | PICTURE 2 | PICTURE 3
net primary productivity (NPP) The rate at which producer (usually plants) biomass is created in a community.
net secondary productivity (NSP) The rate at which consumer and decomposer biomass is produced in a community.
neural tube A tube of ectoderm in the embryo that will form the spinal cord.
neuromuscular junction The point where a motor neuron attaches to a muscle cell.
neurons Highly specialized cells that generate and transmit bioelectric impulses from one part of the body to another; the functional unit of the nervous system. A cell of the nerve tissue having a cell body input zone of dendrites and an output zone of an axon (of varying length). The electrochemical nerve impulse/message is transmitted by neurons. PICTURE 1 | PICTURE 2
neurotoxin Chemical that paralyzes nerves. Neurotoxins are produced by a variety of organisms, most notably some of the heterotrophic dinoflagellates.
neurotransmitters Chemicals released from the tip of an axon into the synaptic cleft when a nerve impulse arrives; may stimulate or inhibit the next neuron. The chemical that crosses the synaptic cleft and causes the transmission of the nerve message in an adjacent neuron or the stimulation of an effector cell (muscle or gland). PICTURE
neutron An uncharged subatomic particle in the nucleus of an atom. The large (mass approximately equal to 1 atomic mass unit), electrically neutral particle that may occur in the atomic nucleus.
niche The biological role played by a species.
niche overlap The extent to which two species require similar resources; speciÞes the strength of the competition between the two species.
nicotine adenine dinucleotide phosphate (NADP+) A substance to which electrons are transferred from photosystem I during photosynthesis; the addition of the electrons reduces NADP, which acquires a hydrogen ion to form NADPH, which is a storage form of energy that can be transferred to the Calvin Cycle for the production of carbohydrate.
node The stem region of a plant where one or more leaves attach. Where leaves are attached to stems.
node of Ranvier A gap between two of the Schwann cells that make up an axon's myelin sheath; serves as a point for generating a nerve impulse.
nondisjunction The failure of chromosomes to separate properly during cell division. The unequal segregation of chromosomes during meiosis. This forms cells with either too many (possibly one or more single or sets of chromosomes too many) or too few chromosomes. Thought to be a common cause for Down Syndrome, where sufferers often have an extra copy of chromosome 21.
nonvascular plants Plants lacking lignified vascular tissue (xylem), vascularized leaves, and having a free-living, photosynthetic gametophyte stage that dominates the life cycle. Common examples are the mosses and liverworts.
norepinephrine A hormone produced in the adrenal medulla and secreted under stress; contributes to the "Þght or þight" response.
notochord In chordates, a cellular rod that runs the length of the body and provides dorsal support. Also, a structure of mesoderm in the embryo that will become the vertebrae of the spinal column. The stiff rod-like structure that all members of the Phylum Chordata develop at some stage during their life.
nuclear area In prokaryotic cells, a region containing the cell's genetic information. Unlike the nucleus in eukaryotic cells, it is not surrounded by a membrane.
nuclear pores Openings in the membrane of a cell's nuclear envelope that allow the exchange of materials between the nucleus and the cytoplasm. PICTURE
nucleic acids Polymers composed of nucleotides; e.g., DNA and RNA.
nucleoid The area of the prokaryotic cytoplasm where the chromatin is localized.
nucleolus A round or oval body in the nucleus of a eukaryotic cell; consists of DNA and RNA and produces ribosomal RNA (pl.: nucleoli). PICTURE
nucleosomes Spherical bodies formed by coils of chromatin. The nucleosomes in turn are coiled to form the Þbers that make up the chromosomes.
nucleotide sequences The genetic code encrypted in the sequence of bases along a nucleic acid.
nucleotides The subunits of nucleic acids; composed of a phosphate, a sugar, and a nitrogen-containing base. The fundamental structural unit of the nucleic acid group of organic macromolecules. Some nucleotides are involved in information storage (as nucleotides in DNA), protein synthesis (as nucleotides in RNA), and energy transfers (as single nucleotide ATP, GTP, and double nucleotide NADH and NADPH).
nucleus (atom) An atom's core; contains protons and one or more neutrons (except hydrogen, which has no neutrons).
nucleus (cell) The largest, most prominent organelle in eukaryotic cells; a round or oval body that is surrounded by the nuclear envelope and contains the genetic information necessary for control of cell structure and function. PICTURE
nyctinasty A nastic movement in a plant that is caused by light and dark.
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