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# constant

constant (kon´stant)

A quantity that, under stated conditions, does not vary with changes in the environment.
Ambard's c. See Ambard's laws, under law.
association c. 1. in experimental immunology, a mathematical expression of hapten-antibody interaction: average association c., K = [hapten-bound antibody]/[free antibody][free hapten]; 2. (Ka) the equilibrium c. involved in the association of two or more compounds or ions into a new compound; the reciprocal of the dissociation c.binding c;
binding c. association c
decay c. the fractional change in the number of atoms of a radionuclide which occurs in unit time; the constant l in the equation for the fraction (DN/N) of the number of atoms (N) of a radionuclide disintegrating in time Dt, DN/N = - lDt.disintegration c., radioactive c., transformation c;
diffusion c. diffusion coefficient
disintegration c. decay c
dissociation c. (Kd, K) the equilibrium c. involved in the dissociation of a compound into two or more compounds or ions. The reciprocal of the association c. (2).
dissociation c. of an acid (Kd, Ka) expressed by general equation [H+][A-]/[HA] = Ka, where HA is the undissociated acid.
dissociation c. of a base (Kb) expressed by the general equation [B+][OH-]/[BOH] = Kb, where BOH is the undissociated base.
dissociation c. of water (Kw) expressed by the equation [H+][OH-] = Kw = 10-14 at 25°C.
equilibrium c. (Keq) in the reaction A + B <-> C + D at equilibrium (i.e., no net change in concentrations of A, B, C, or D), the concentrations of the four components are related by the equation Keq = [C][D]/[A][B]; Keq is the equilibrium c. If any component in the reaction has a multiplier (e.g., H2 <-> 2H), that multiplier appears as an exponent in the calculation of K (e.g., Keq = [H]2/[H2]). When this equation is applied to the ionization of a substance in solution, Keq is called the dissociation c. (Kd) and its negative logarithm (base 10) is the pKd. See also Henderson-Hasselbalch equation, mass-action ratio.
flotation c. (Sf) characteristic sedimentation behavior of a lipoprotein fraction of plasma in a centrifugal field in a medium of appropriate density, achieved by adding a salt or D2O to the plasma.negative S, Svedberg of flotation;
gas c. (R) R (symbol for the constant) = 8.314 x 107 ergs per degree Celsius per mole = 8.314 J K-1 mol-1 (joules per kelvin mole).
Hill c. Hill coefficient
Michaelis c. 1. the true dissociation constant for the enzyme-substrate binary complex in a single-substrate rapid equilibrium enzyme-catalyzed reaction (usually symbolized by Ks); 2. 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 <-> ES <-> E + products where E represents the free enzyme, S is the substrate, and ES is the central binary complex. The expression for the Michaelis c. will be more complex for multisubstrate reactions. An apparent Michaelis c. is a c. 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.Michaelis-Menten c;
Michaelis-Menten c. (Km) Michaelis c
Newtonian c. of gravitation (G) a universal c. relating the gravitational force, f., attracting two masses, m1 and m2, toward each other when they are separated by a distance, r, in the equation: f = G(m1m2/r2); it has the value of 6.67259 x 10-8 dyne cm2 g-2 = 6.67259 x 10-11 m3 kg-1 s-2 in SI units.
permeability c. (P with a subscript for the ion, P) 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. Cf. permeability coefficient.
Planck's c. (h) a c., 6.6260755 x 10-34 J. s (joule-seconds) or 6.6260755 x 10-27 erg-seconds = 6.6260755 x 10-34 J Hz-1 (joule per hertz).