In biochemistry, a<unk> is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the<unk> s). The<unk> binds with the enzyme's active site, and an enzyme-substrate complex is formed. The<unk> is broken down into a product and is released from the active site. The active site is now free to accept another<unk> molecule.   An example of a<unk> would be milk and the enzyme added would be rennin which causes milk to set.  Another example would be  the reaction of catalase in the decomposition of hydrogen peroxide.
A general equation is as follows:
E + S ⇌ ES → EP ⇌ E + P
where E = enzyme, S =<unk> s), P = product(s)
Note that only the middle step is irreversible.
By increasing the<unk> concentration, the rate of reaction will increase due to the increase in likelihood of enzyme-substrate complexes forming, this occurs until the enzyme becomes the limiting factor.
Substrate 
Carbide
Coenzyme
Carbonyl
Antigen
Phytochemical
Electrostatics
Biomolecule