Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrate in a liquid or gaseous reaction mixture. Homogeneous catalysts function in the same phase as the reactants, typically homogeneous catalysts are dissolved in a solvent with the substrates. In acid catalysis and base catalysis a chemical reaction is catalyzed by an acid or a base. The acid is the proton donor and the base is the proton acceptor. Typical reactions catalyzed by electron transfer are esterification and aldol reactions. Enzyme catalysis is the increase in the speed of a chemical reaction by the active site of a protein.