1. a basis or foundation, such as the physical medium on which an animal or plant lives or grows.

2. a chemical compound that is acted on by an enzyme. The substrate binds specifically to the enzyme’s active site, thereby lowering the energy required for the reaction, which can therefore proceed much faster. When the process is over, the substrate has been changed into different molecules (reaction products), although the enzyme is unchanged. The released enzyme then repeats the process with another substrate of the same composition.