| Avogadro's constant | <physics> The number of molecules in a mole (gram molecular weight) of a substance, equals 6.02 x 1023 molecules. (09 Oct 1997) |
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| base dissociation constant | <chemistry> This is the equilibrium constant for the reaction in which a weak base breaks apart in water to form its conjugate acid and hydroxide ion. (09 Oct 1997) |
| binding constant | <chemistry> Reciprocal of dissociation constant. A measure of the extent of a reversible association between two molecular species at equilibrium. (18 Nov 1997) |
| boltzmann constant | <radiobiology> K = 1.38 x 10^-16 erg/degree. This is the ratio of the universal gas constant to Avogadro's number. It is also used to relate temperatures (Kelvin) to energies (ergs or Joules) via E = (constant of order unity) kT. (09 Oct 1997) |
| radioactive constant | <physics, radiobiology> The fraction of the amount of a radionuclide that undergoes transition per unit time. Formally: Lamda=dP/dt Where dP is the probability of a given nucleus undergoing spontaneous nuclear transition in the time interval dt. (16 Dec 1997) |
| permeability constant | A measure of the ease with which an ion can cross a unit area of membrane driven by a 1.0 m difference in concentration; usually expressed in centimeters per second. Compare: permeability coefficient. (05 Mar 2000) |
| Michaelis constant | <chemistry> The true dissociation constant for the enzyme-substrate binary complex in a single-substrate rapid equilibrium enzyme-catalyzed reaction (usually symbolised by Ks), the concentration of the substrate at which half the true maximum velocity of an enzyme-catalyzed reaction is achieved (when velocities are measured under initial rate and steady state conditions). The ratio of rate constants (k2 + k3)/k1 in the single-substrate enzyme-catalyzed reaction: E + S &dblarr; ES &dblarr; E + products where E represents the free enzyme, S is the substrate, and ES is the central binary complex. The expression for the Michaelis constant will be more complex for multisubstrate reactions. An apparent Michaelis constant is a constant determined either under conditions that are not strictly steady state and initial rate or one that varies with the concentration of one or more cosubstrates. See: Michaelis-Menten equation. Synonym: Michaelis-Menten constant. (05 Mar 2000) |
| Michaelis-Menten constant | <chemistry> The true dissociation constant for the enzyme-substrate binary complex in a single-substrate rapid equilibrium enzyme-catalyzed reaction (usually symbolised by Ks), the concentration of the substrate at which half the true maximum velocity of an enzyme-catalyzed reaction is achieved (when velocities are measured under initial rate and steady state conditions). The ratio of rate constants (k2 + k3)/k1 in the single-substrate enzyme-catalyzed reaction: E + S &dblarr; ES &dblarr; E + products where E represents the free enzyme, S is the substrate, and ES is the central binary complex. The expression for the Michaelis constant will be more complex for multisubstrate reactions. An apparent Michaelis constant is a constant determined either under conditions that are not strictly steady state and initial rate or one that varies with the concentration of one or more cosubstrates. See: Michaelis-Menten equation. Synonym: Michaelis-Menten constant. (05 Mar 2000) |
| Planck's constant | A constant, 6.6260755 × 10-34 J - s (joule-seconds) or 6.6260755 × 10-27 erg-seconds = 6.6260755 × 10-34 J Hz-1 (joule per hertz). (05 Mar 2000) |
| constant | A quantity that, under stated conditions, does not vary with changes in the environment. (05 Mar 2000) |
| constant coupling | Where several premature beats are seen, the interval between each of them and the preceding normal beat is constant. Synonym: constant coupling. Variable coupling, where several extrasystoles are seen, the interval between each of them and the preceding sinus beat varies. (05 Mar 2000) |
| constant field equation | An equation derived to predict membrane potentials in terms of the membrane's permeability to ions and their concentrations on either side. Synonym: constant field equation, Goldman-Hodgkin-Katz equation, GHK equation. (05 Mar 2000) |
| constant infusion pump | An electrically driven device for delivery from a reservoir of a constant, often very small, volume of solution over a prolonged period of time. (05 Mar 2000) |
| constant region | The stem and forking part of the Y-shaped antibody protein, consisting of amino acid chains, that is exactly the same in all antibody molecules within the same individual. (The ends of the Y-shaped molecule will vary widely between different antibodies). (09 Oct 1997) |
| haemoglobin Constant Spring | An abnormal haemoglobin having an extended polypeptide chain (31 additional amino acid residues) on the a chain (thus, the a chain is 172 amino acids long); approximately 20% of the individuals with Hb H disease also have this defect. (05 Mar 2000) |