A homogeneous catalyst is one whose molecules are dispersed in the same phase (usually gaseous or liquid) as the reactant's molecules. A heterogeneous catalyst is one whose molecules are not in the same phase as the reactants, which are typically gases or liquids that are adsorbed onto the surface of the solid catalyst. Enzymes and other biocatalysts are often considered as a third category. In the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations. The effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons (which reduce the catalytic activity) or promoters (which increase the activity and also affect the temperature of the reaction)