| kinetic molecular theory | <chemistry> This theory assumes that molecules must collide in order to react. The more collisions the more likely it is for a reaction to occur. However, depending on the conditions, only a small fraction of the collisions are effective in producing a reaction. There are several constraints. In order for a reaction to occur, bonds initially are broken, which requires energy. This energy depends on the type of the reaction and comes from the kinetic energies that the molecules possess before the collision. It is called the activation energy. Increasing the temperature increases the kinetic energies and more collisions will occur. In adition, at a higher temperature a greater number of the reacting molecules might possess an energy equal to or greater than the activation energy. However the molecules must also collide in a specific orientation, called the steric factor in order for a reaction to occur. A reaction will only be successful, if the collision has enough energy to be either equal to or greater than the activation energy and if the orientation of the collision allows for correct bond formation. These factors are in the Arrhenius equation: k = zp The rate constant k is proportional to the Arrhenius factor A. A is the product of the collision frequency z, and the steric factor p. The fraction of collisions with sufficient energy to produce a reaction are in the term of the equation. (09 Jan 1998) |
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| unineme theory | <cell biology> Theory that proposes that each chromosome (before S phase) consists of a single strand of DNA. Now generally accepted and being noncontroversial the term has fallen into disuse. (18 Nov 1997) |
| Knoop's theory | That the catabolism of fatty acids occurs in stages in each of which there is a loss of two carbon atoms as a result of oxidation at the beta-carbon atom, e.g. (05 Mar 2000) |
| Flourens' theory | That thought is a process depending upon the action of the entire cerebrum. (05 Mar 2000) |
| Ladd-Franklin theory | A theory, pertaining to colour vision, that gray is the earliest of colour sensations, from which are derived, by molecular change, two paired substances that, respectively, detect yellow and blue, and that the yellow gives rise to paired substances for detection of red and green. Synonym: Ladd-Franklin theory. (05 Mar 2000) |
| lamarckian theory | That acquired characteristics may be transmitted to the descendants and that experience, and not biology alone, can change and thereby influence genetic transmission. (05 Mar 2000) |
| Frerichs' theory | That uraemia represents a toxic condition caused by ammonium carbonate, which is formed as the result of the action of a plasma enzyme on the increased amounts of urea. (05 Mar 2000) |
| learning theory | Any of several prominent theories designed to explain learning, especially those promulgated by Pavlov, Thorndike, Guthrie, Hull, Kohler, Spence, Miller, Skinner, and their modern followers. See: conditioning. (05 Mar 2000) |
| freudian theory | <psychology> Philosophic formulations which are basic to psychoanalysis. Some of the conceptual theories developed were of the libido, repression, regression, transference, id, ego, superego, oedipus complex, etc. (12 Dec 1998) |
| Freud's theory | A comprehensive theory of how personality is formed and develops in normal and emotionally disturbed individuals; e.g., that an attack of conversion hysteria is due to a psychic trauma which was not adequately reacted to at the time it was received, and persists as an affect memory. See: psychoanalysis. (05 Mar 2000) |
| frontal zone contraction theory | Model proposed to account for the movement of giant amoebae in which cytoplasmic contraction at the front of the leading pseudopod (fountain zone) pulls viscoelastic cytoplasm forward in the centre of the cell and forms a tube of more rigid cytoplasm immediately below the plasma membrane behind the active region. The peripheral contracted cytoplasm relaxes into a weaker gel at the rear and is pulled forward in its turn. Contrasts with the ectoplasmic tube contraction model. (18 Nov 1997) |
| libido theory | Freud's theory that humans psychic life results mainly from instinctual or libidinal needs and the attempts to satisfy them. (05 Mar 2000) |
| Liebig's theory | That the hydrocarbons that oxidise readily and burn are aliments that produce the greatest quantity of animal heat. (05 Mar 2000) |
| lipoid theory of narcosis | That narcotic efficiency parallels the coefficient of partition between oil and water, and that lipoids in the cell and on the cell membrane absorb the drug because of this affinity. Synonym: Meyer-Overton theory of narcosis. (05 Mar 2000) |
| local circuit theory | <physiology> A generally accepted model for neuronal conduction, by which depolarisation of a small region of a neuronal plasma membrane produces transmembrane currents in the neighbouring regions, tending to depolarise them. As the sodium channels are voltage gated, the depolarisation causes further channels to open, thus propagating the action potential. (18 Nov 1997) |