10Th Chem N

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NAD+ nicotinamide adenine dinucleotide, an organic molecule that serves as an electron carrier by being oxidized to NAD+ and reduced to NADH

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NAD+

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NADP+ nicotinamide adenine dinucleotide phosphate, an organic molecule that is composed of NAD plus an extra phosphate group and that acts as a hydrogen acceptor by being reduced to NADPH

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NADP+

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narcotic a drug that is derived from opium and that relieves pain and induces sleep; examples include heroine, morphine, and codeine

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NASA National Aeronautics and Space Administration, the agency of the U.S. federal government that was established in 1958 to conduct all nonmilitary space activities

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NASA

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nastic movement a type of plant response that is independent of the direction of a stimulus

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nastic movement

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natural bridge an arch of rock that forms by water erosion

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natural bridge

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natural gas a mixture of gaseous hydrocarbons located under the surface of the Earth, often near petroleum deposits; used as a fuel

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natural gas

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natural killer cell a type of white blood cell that is present in individuals who have not been immunized and that kills a variety of cells

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natural killer cell

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natural levee a raised riverbank that results when a river deposits its load at the river's edges

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natural levee

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natural resource any natural material that is used by humans, such as water, petroleum, minerals, forests, and animals

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natural resource

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natural selection the process by which individuals that are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do; a theory to explain the mechanism of evolution

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natural selection

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nauplius the free-swimming larva of most crustaceans

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nauplius

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Neanderthal a species of hominids that lived in Europe, Africa, and western Asia from 125,000 years ago to about 35,000 years ago

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Neanderthal

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neap tide a tide of minimum range that occurs during the first and third quarters of the moon

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neap tide

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nebula a large cloud of gas and dust in interstellar space; a region in space where stars are born

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nebula

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nebular hypothesis a model for the formation of the solar system in which the sun and planets condense from a cloud (or nebula) of gas and dust

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nebular hypothesis

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nectar a sugar-containing fluid secreted by flowers to attract birds or insects for pollination

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nectar

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nekton all organisms that swim actively in open water, independent of currents

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nekton

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nematocyst in cnidarians, a stinging cell that is used to inject a toxin into prey

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nematocyst

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neon light light emitted by atoms of certain gases, such as neon, when the atoms are ionized by an electric current

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neon light

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nephridium a tubule through which some invertebrates eliminate wastes

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nephridium

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nephron the functional unit of the kidney

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nephron

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neritic zone a shallow marine environment that is near the shore or over the continental shelf and that is rich in minerals and nutrients produced by biotic activity

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neritic zone

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nerve a collection of nerve fibers through which impulses travel between the central nervous system and other parts of the body

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nerve net in cnidarians, a network of nerve cells that lacks a central control; impulses pass in any or all directions to produce a generalized response

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nerve net

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nervous system the structures that control the actions and reactions of the body in response to stimuli from the environment; it is formed by billions of specialized nerve cells, called neurons

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nervous system

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nervous tissue the tissue of the nervous system, which consists of neurons, their supporting cells, and connective tissue

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nervous tissue

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net force a single force whose external effects on a rigid body are the same as the effects of several actual forces acting on the body

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net force

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net primar productivity the rate at which biomass accumulates in an ecosystem

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net primary productivity

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neuron a nerve cell that is specialized to receive and conduct electrical impulses

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neuron

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nerve cells in the hypothalamus that synthesize and secrete hormones

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neurosecretory cells

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neurotransmitter a chemical substance that transmits nerve impulses across a synapse

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neurotransmitter

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neutralization reactionthe reaction of the ions that characterize acids (hydronium ions) and the ions that characterize bases (hydroxide ions) to form water molecules and a salt

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neutralization reaction

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neutron a subatomic particle that has no charge and that is located in the nucleus of an atom

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neutron

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neutron star a star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons

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neutron star

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neutrophil a large leukocyte that contains a lobed nucleus and many cytoplasmic granules

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neutrophil

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new moon the phase of the moon during which the side of the moon facing the Earth is unlit by the sun

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new moon

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newton the SI unit for force; the force that will increase the speed of a 1 kg mass by 1 m/s each second that the force is applied (symbol, N)

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newton

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niche the unique position occupied by a species, both in terms of its physical use of its habitat and its function within an ecological community

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niche

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nicotine a toxic, addictive alkaloid that is derived from tobacco and that is one of the major contributors to the harmful effects of smoking

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nicotine

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nictitating membrane a third eyelid found under the lower eyelid of many vertebrates, including birds, reptiles, and amphibians

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nictitating membrane

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nitrification the process by which nitrites and nitrates are produced by bacteria in the soil

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nitrogen cycle the process in which nitrogen circulates among the air, soil, water, plants, and animals in an ecosystem

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nitrogen cycle

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nitrogen fixation the process by which gaseous nitrogen is converted into ammonia, a compound that organisms can use to make amino acids and other nitrogen-containing organic molecules

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nitrogen fixation

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nitrogen-fixing bacteria bacteria that convert atmospheric nitrogen into ammonia

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nitrogen-fixing bacteria

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nitrogenous base an organic base that contains nitrogen, such as a purine or pyrimidine; a subunit of a nucleotide in DNA and RNA

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nitrogenous base

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noble gas one of the elements of Group 18 of the periodic table (helium, neon, argon, krypton, xenon, and radon); noble gases are unreactive

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noble gas

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node in biology, a joint between two adjacent sections in the stem of a plant where buds form and leaves or branches start to grow; usually marked by a knot or swelling

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node

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node in physics, a point in a standing wave that maintains zero amplitude

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node

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nodule a lump of minerals whose composition differs from the composition of the surrounding sediment or rock; also a lump of minerals that is made of oxides of manganese, iron, copper, or nickel and that is found in scattered groups on the ocean floor

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nodule

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noise a sound that consists of a random mix of frequencies

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noise

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nomenclature a naming system

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nomenclature

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nonconformity an unconformity in which younger sedimentary rocks rest on top of older igneous or metamorphic rocks

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nonconformity

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nondisjunction the failure of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during mitosis or meiosis II

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nondisjunction

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nonelectrolyte a liquid or solid substance or mixture that does not allow an electric current

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nonelectrolyte

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nonfoliated the metamorphic rock texture in which mineral grains are not arranged in planes or bands

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nonfoliated

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noninfectious disease a disease that cannot spread from one individual to another

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noninfectious disease

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nonmetal an element that conducts heat and electricity poorly and that does not form positive ions in an electrolytic solution

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nonmetal

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pollution that comes from many sources rather than from a single specific site; an example is pollution that reaches a body of water from streets and storm sewers

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nonpoint-sourcepollution

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nonpolar compound a compound whose electrons are equally distributed among its atoms

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nonpolar compound

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nonpolar covalent bond a covalent bond in which the bonding electrons are equally attracted to both bonded atoms

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nonpolar covalent bond

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a resource that forms at a rate that is much slower than the rate at which the resource is consumed

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nonrenewable resource

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nonsilicate mineral a mineral that does not contain compounds of silicon and oxygen

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nonsilicate mineral

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nonvascular plant a plant that belongs to one of the three groups of plants (liverworts, hornworts, and mosses) that lack specialized conducting tissues and true roots, stems, and leaves

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nonvascular plant

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norepinephrine a chemical that is both a neurotransmitter produced by the sympathetic nerve endings in the autonomic nervous system and a hormone secreted by the adrenal medulla to stimulate the functions of the circulatory and respiratory systems especially (abbreviation, NE)

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norepinephrine

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normal distribution a distribution of numerical data whose graph forms a bell-shaped curve that is symmetrical about the mean

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normal distribution

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normal fault a fault in which the hanging wall moves down relative to the footwall

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normal fault

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normal force a force that acts on a surface in a direction perpendicular to the surface

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normal force

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no-till farming a technique in which little or no plowing occurs before crops are planted; used to prevent soil erosion

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no-till farming

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notochord the rod-shaped supporting axis found in the dorsal part of the embryos of all chordates, including vertebrates

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notochord

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nova a star that suddenly becomes brighter

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nova

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nuclear chain reaction a continuous series of nuclear fission reactions

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nuclear chain reaction

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nuclear energy the energy released by a fission or fusion reaction; the binding energy of the atomic nucleus

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nuclear energy

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nuclear envelope the double membrane that surrounds the nucleus of a eukaryotic cell

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nuclear envelope

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nuclear fission the process by which the nucleus of a heavy atom splits into two or more fragments; the process releases neutrons and energy

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nuclear fission

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nuclear forces the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus

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nuclear forces

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nuclear fusion the process by which nuclei of small atoms combine to form a new, more massive nucleus; the process releases energy

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nuclear fusion

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nuclear matrix the fibrous skeletal structure found in the nucleus of a eukaryotic cell

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nuclear matrix

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nuclear pore one of the small holes in the nuclear envelope through which substances pass between the nucleus and the cytoplasm

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nuclear pore

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nuclear power plant a facility that uses heat from nuclear reactors to produce electrical energy

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nuclear power plant

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nuclear radiation the particles that are released from the nucleus during radioactive decay, such as neutrons, electrons, and photons

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nuclear radiation

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nuclear reaction a reaction that affects the nucleus of an atom

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nuclear reaction

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nuclear reactor a device that uses controlled nuclear reactions to produce energy or nuclides

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nuclear reactor

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waste that contains radioisotopes

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nuclear waste

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nucleic acid an organic compound, either RNA or DNA, whose molecules are made up of one or two chains of nucleotides and carry genetic information

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nucleic acid

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nucleolus the part of the eukaryotic nucleus where ribosomal RNA is synthesized

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nucleolus

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nucleon a proton or neutron

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nucleon

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nucleotide in a nucleic-acid chain, a subunit that consists of a sugar, a phosphate, and a nitrogenous base

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nucleotide

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nucleus in physical science, an atom's central region, which is made up of protons and neutrons

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nucleus

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nucleus in a eukaryotic cell, a membrane-bound organelle that contains the cell’s DNA and that has a role in processes such as growth, metabolism, and reproduction

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nucleus

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nuclide an atom that is identified by the number of protons and neutrons in its nucleus

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nuclide

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nut a dry fruit that has one seed and a hard shell; cultivated as a food crop

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nut

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nutrient a substance or compound that provides nourishment (or food) or raw materials needed for life processes

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nutrient

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nymph an immature stage of some insects that is similar in function and structure to the adult

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nymph