Catalysts can be broadly classified into two main categories, homogeneous and heterogeneous. A homogeneous catalyst has the same phase as that of the other substances in the reaction that it aids. Homogeneous catalysts are usually gases or liquids that form a mixture with the reacting substances and there is no boundary of separation between the two. Heterogeneous catalysts on the other hand have a different phase from the substances undergoing a reaction that it is being aided by the catalyst.
Biological catalysts are enzymes that help in allowing certain reactions to take place that in the absence of the catalyst would not occur in the conditions present in biological systems or occur at a rate that is not fast enough to serve its purpose. The substrate forms relatively weak bonds with the enzyme that include hydrogen bonds, weak ionic bonds, and other interactions that are hydrophobic in nature. It is essential that the bond formed between the enzyme and the substrate be a weak one so that after the reaction between the different substrates is complete the resultant can separate from the enzyme easily.