Nuclear membrane See nucleus

nuclear pore See nucleus.

nuclear power Electricity produced by power plants in which nuclear energy is converted into heat, which is used to produce steam to drive turbines, which power electricity generators.

nuclear reaction A reaction that involves a change in the nucleus of an atom, for example radioactive decay, nuclear fission or fusion, or neutron capture. It differs from chemical reactions, which are limited to changes in the number and/or distribution of electrons surrounding the nucleus. See nuclear energy; radioactive decay.

nuclear reactor A device that initiates and controls nuclear fission reactions to produce heat. See nuclear energy.

nuclear winter The significant cooling of the global climate suggested to be a possible consequence of nuclear war, due to the resulting high-altitude dust clouds caused by the nuclear explosions and the burning of forests, fossil fuels, and plastics set alight by firestorms triggered by exploding nuclear warheads. Such a large accumulation of dust would block sunlight for weeks afterward, perhaps reducing surface temperatures by as much as 22°C (40°F). The low temperatures, frosts, and dim light, as well as the radioactive fallout, would destroy plant and animals life.

nucleoplasm See nucleus.

nucleotide A compound formed by condensation of a nitrogenous base (a purine, pyrimidine, or pyridine) with a sugar (ri-bose or deoxyribose) and phosphoric acid. ATP is a mononucleotide (consisting of a single nucleotide), the coenzymes NAD and FAD are dinucleotides (consisting of two linked nucleotides), and the nucleic acids (see DNA: RNA) are polynucleotides (consisting of chains of many linked nu-cleotides).

nucleus (pl. nuclei) 1. An organelle of eukaryote cells containing the genetic information (dna) and hence controlling the cell's activities. It is the largest organelle, typically spherical and consisting of nu-cleoplasm bounded by a double membrane, the nuclear membrane, which is perforated by many pores (nuclear pores) that allow exchange of materials with the cytoplasm. The outer nuclear membrane is an extension of the endoplasmic reticulum, a network of membrane compartments distributed throughout the cytoplasm, in which glycoproteins and other complex compounds are synthesized. In the nondi-viding (interphase) nucleus the genetic material is irregularly dispersed as chromatin; during nuclear division (mitosis or meiosis) this condenses into densely staining chromosomes, and the nuclear envelope. 2. The central positively charged 'core' of an atom.

NUE See nutrient use efficiency.

null hypothesis A statistical hypothesis that can be tested experimentally. It proposes that any observed difference (for example, between the means of two samples) is due to chance alone and not to the effect of an internal or external factor. To test how significant the difference between the expected result and that proposed by the null hypothesis, a statistical test such as Student's t-test may be used.

numerical response A change in the size of a predator population as a result of

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