Distinguish between prosthetic group and co-factors.
Certain enzymes require non-protein components such as vitamins, metal ions etc. tightly bound to them in order to perform their function. These components are called the prosthetic groups. A covalent bond usually exists between the prosthetic group and the enzyme. A co-factor is also equally essential for the activity of certain enzymes, however the co-factor is not chemically bound to the enzyme, it merely participates in the reaction.