LOCK & KEY THEORY                                           


Enzymes (e.g. globular proteins) are biological catalysts which speed up chemical reactions without being use dup in the process. They are vital b/c otherwise reactions would be too slow and the body can’t meet demands => cells die.


Each enzyme only catalyses one reaction/c only a specific shaped substrate will fit into a specific enzyme, and induce a chemical change.

The active site is only a small part of the enzyme.


  1. Substrate diffuses into the active site and binds to it – the substrate must be a specific shape to fit into the enzyme
  2. The substrate forms temporary bonds with the AA’s in the enzyme to produce an ENZYME – SUBSTRATE COMPLEX
  3. The reaction takes place and the bonds in the substrate are broken – in this case, the larger molecule has been broken down into smaller molecules. An ENZYME – PRODUCT COMPLEX has formed
  4. The products unbind from the active site and diffuses out
  5. The enzyme is ready to be used again


INDUCED FIT THEORY- The enzyme holds the substrate in such a way that the enzyme itself slightly changes shape to fit closer around the substrate and his change of shape breaks the bonds in the substrate. It allows for an easier reaction to take place – the shape changes back to normal once the products leave.



Metabolism = chemical processes that occur in living organisms to maintain life


  • Intracellular – occurs within cells
  • Extracellular – occurs in aqueous solution e.g. blood
  • Catabolic – breaks down large substrates to smaller ones (releases energy)
  • g. proteinsàamino acids, proteins => glucose, glycogen => glucose, triglycerides => fatty acids.
  • Anabolic – builds up larger molecules from smaller ones (requires energy)
  • g. amino acids => polypeptides (proteins), glucose => glycogen, fatty acids => triglycerides.