A quantity that, under stated conditions, does not vary with changes in the environment.
- 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.. SYN: binding c..
- Avogadro c. SYN: Avogadro number.
- binding c. SYN: association c..
- decay c. the fractional change in the number of atoms of a radionuclide that occurs in unit time; the c. λ in the equation for the fraction (dN/N) of the number of atoms (N) of a radionuclide disintegrating in time dt, dN/N = −λdt. SYN: disintegration c., radioactive c., transformation c..
- diffusion c. SYN: diffusion coefficient.
- disintegration c. SYN: 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 the 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 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]; 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.
- [b]Faraday c. (F) 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. SYN: negative S, Svedberg of flotation.
- gas c. (R) R = 8.314 × 107 ergs K−1 mol−1 = 8.314 J K−1 mol−1.
- Hill c. SYN: Hill coefficient.
- Michaelis c. 1. the true dissociation c. 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. SYN: Michaelis-Menten c..
- Michaelis-Menten c. (Km) SYN: 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 × 10−8 dyne cm2 g−2 = 6.67259 × 10−11 m3 kg−1 s−2 in SI units.
- permeability c. a measure of the ease with which an ion can cross a unit area of membrane driven by a 1.0 mol/L difference in concentration; usually expressed in centimeters per second. Cf.:permeability coefficient.
- Planck c. (h) a c., 6.6260755 × 10−34 J s or 6.6260755 × 10−27 erg-seconds = 6.6260755 × 10−34 J Hz−1.
- radioactive c. (Λ) SYN: decay c..
- rate constants (k) proportionality constants 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., s−1). For the reverse reaction, B + C → A, the rate equals −d/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 s−1). SYN: velocity constants.
- [b]sedimentation c. the c. s in Svedberg 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 c., T the absolute temperature, D the diffusion c. (in square centimeters per second), V the partial specific volume of the protein, ρ the density of the solvent. The c. s, with dimensions of time per unit of field force (s = drb/dt ω2ro where rb is the position at time t, r0 is the position at time 0, and ω is the angular velocity) is usually between 1 × 10−13 and 200 × 10−13 s. The Svedberg unit (S) is arbitrarily set at 1 × 10−13 s and is very often used to describe the sedimentation rate of macromolecules; e.g., 4S RNA. SYN: 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. SYN: decay c..
- velocity constants (k) SYN: rate constants.

* * *

con·stant 'kän(t)-stənt adj remaining unchanged
con·stant·ly adv
constant n something invariable or unchanging esp a number that has a fixed value in a given situation or universally or that is characteristic of some substance or instrument

* * *

con·stant (konґstənt) [L. constans standing together] 1. not failing; remaining unaltered. 2. a datum, fact, or principle that is not subject to change.

Medical dictionary. 2011.


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