Enzymes

ENZYME: biological catalyst made of protein which alter the rate of chemical reactions, remaining unchanged at the end of reaction.
SUBSTRATE: substances upon which enzymes act.

WHY BIOLOGICAL?
Produced by living cells
Protein in nature (and hence can be denatured by extreme pH and heat)

WHY CATALYSTS?
Alter rate of chemical reactions
But are not changed at the end of the reaction
Hence required in minute (ie small) amounts as they can be REUSED

Lock & Key Hypothesis
To explain why enzymes are SPECIFIC in action
Enzyme = Lock
Substrate = Key
For key to fit into lock, it has to have a shape that is COMPLEMENTARY to the shape of the lock. Similarly, for the enzyme to act on the substrate, the substrate must have a shape that is COMPLEMENTARY to the ACTIVE SITE of the enzyme.