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nanometer -- n. A unit of measure; one millionth (10-9) of a meter.
nematocyst -- Older name for a cnidocyst.
nerve -- A bundle of neurons, or nerve cells. More properly, it is a bundle of axons.
nerve cord -- Primary bundle of nerves in chordates, which connects the brain to the major muscles and organs of the body.
neuron -- A specialized cell that can react to stimuli and transmit impulses. A neuron consists of a body which contains the nucleus; dendrites, which are short branches off the body that receive incoming impulses; and a long axon which carries impulses away from the body and to the next neuron.
neurotoxin -- poison which interferes with nerve function, usually by affecting the flow of ions through the cell membrane.
niche -- n. The portion of the environment that a species occupies, defined in terms of the conditions under which an organism can survive, and the presence of other competing organisms.
nitrogen fixation -- The conversion of gaseous nitrogen into a form usable by plants. Ususally by bacteria.
nocturnal -- Active only at night.
node -- The region of a stem between two internodes, where there is branching of the vascular tissue into leaves or other appendages.
notochord -- Characteristic of chordates, the notochord is a stiff rod of tissue along the back of the body. In vertebrates, the backbone is deposited around the notochord and nerve cord.
nuclear membrane -- The double membrane which surrounds the eukaryotic nucleus. It has many pores in its surface which regulate the flow of large compounds into and out of the nucleus.
nucleic acid -- class of biochemical compounds which includes DNA and RNA. They are among the largest molecules known. MORE?
nucleoid -- Region in prokaryotes where the DNA is concentrated. Unlike a nucleus, it is not bound by a membrane.
nucleotide -- unit from which nucleic acids are constructed by polymerization. It contains a sugar, a phosphate group, and an organic base. ATP is a nucleotide.
nucleus -- Membrane-bound organelle which contains the DNA in the form of chromosomes. It is the site of DNA replication, and the site of RNA synthesis.
nutrient -- Any element or simple compound necessary for the health and survival of an organism. This includes air and water, as well as food.
nutrient cycling -- All the processes by which nutrients are transferred from one organism to another. For instance, the carbon cycle includes uptake of carbon dioxide by plants, ingestion by animals, and respiration and decay of the animal.
nymph -- In aquatic insects, the larval stage.