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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;
Avogadro's c. Avogadro's number
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.
Faraday's c. (F) See faraday.
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).
radioactive c. (LAMBDA) decay c
rate c.'s (k) proportionality c.'s equal to the initial rate of a reaction divided by the concentration of the reactant(s); e.g., in the reaction A -> B + C, the rate of the reaction equals -d[A]/dt = k1[A]. The rate c. k1 is a unimolecular rate c. since there is only one molecular species reacting and has units of reciprocal time (e.g., sec-1). For the reverse reaction, B + C -> A, the rate equals -d[B]/dt = d[A]/dt = k2[B][C]. The rate c. k2 is a bimolecular rate c. and has units of reciprocal concentration-time (e.g., m-1 sec-1).velocity c.'s;
sedimentation c. the c. s in Svedberg's equation for estimating the molecular weight of a protein from the rate of movement in a centrifugal field: where M is the molecular weight, R the gas constant, T the absolute temperature, D the diffusion constant (in square centimeters per second), V the partial specific volume of the protein, rho the density of the solvent. The constant s, with dimensions of time per unit of field force (s = drb/dt / omega2ro where rb is the position at time t, r0 is the position at time 0, and omega is the angular velocity) is usually between 1 x 10-13 and 200 x 10-13 second. The Svedberg unit (S) is arbitrarily set at 1 x 10-13 second and is very often used to describe the sedimentation rate of macromolecules; e.g., 4 S RNA.sedimentation coefficient;
specificity c. ratio of the maximum velocity (Vmax) or kcat to the true Km value for a specific substrate in an enzyme-catalyzed reaction.
time c. that part of a circuit that determines the time interval over which the rate of electrical events will be averaged; in pulmonary physiology, the factors determining rate of flow in the airways.
transformation c. decay c
velocity c.'s (k) rate c.'s


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