What is the main function of digestion? breaking down food into forms that the body can use transporting oxygen to the different parts of the body absorbing salts through the villi pumping blood to different parts of the body

It would definitely be the breaking down of food for energy based purposes.

an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme