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Chemical Reactions and Chemical Equilibrium
A chemical reaction is at equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction, and thus there is no net change in the concentration of the reactants or products.
The equilibrium constant Keq of a reaction reflects the ratio of products to reactants at equilibrium and thus is a measure of the extent of the reaction and the relative stabilities of the reactants and products.
The Keq depends on the temperature, pressure, and chemical properties of the reactants and products but is independent of the reaction rate and of the initial concentrations of reactants and products.
For any reaction, the equilibrium constant Keq equals the ratio of the forward rate constant to the reverse rate constant (kf/kr). The rates of conversion of reactants to products and vice versa depend on the rate constants and the concentrations of the reactants or products.
Within cells, the linked reactions in metabolic pathways generally are not at equilibrium, but rather at steady state, at which the rate of formation of the intermediates equals their rate of consumption (see Figure 2-23) and thus the concentrations of the intermediates are not changing.
The dissociation constant Kd for the noncovalent binding of two molecules is a measure of the stability of the complex formed between the molecules (e.g., ligand-
The pH is the negative logarithm of the concentration of hydrogen ions (–log [H+]). The pH of the cytoplasm is normally about 7.2–
Acids release protons (H+), and bases bind them.
Buffers are mixtures of a weak acid (HA) and its corresponding base form (A–), which minimize the change in pH of a solution when an acid or base is added. Biological systems use various buffers to maintain their pH within a very narrow range.