In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate(s). The substrate binds with the enzyme’s active site, and an enzyme-substrate-complex is formed. The substrate is broken down into a product and is released from the active site. The active site is now free to accept another substrate molecule. An example of a substrate would be milk and the enzyme added would be rennin which causes milk to set.