1.12 - Building and breaking molecules
Follow enzyme-catalysed reactions that build carbohydrates, proteins and lipids or break them into smaller molecules. Explain why these reactions are essential for digestion, growth and repair.
1.12 — Building and breaking biological molecules
An enzyme is a biological catalyst: it increases reaction rate without being used up. Different enzymes have active sites suited to particular substrates, the molecules on which they act.
Enzymes are not only digestive agents. They catalyse reactions throughout living organisms, including reactions that synthesise larger biological molecules and reactions that break them down.
| Molecule group | Enzyme-catalysed synthesis | Enzyme-catalysed breakdown |
|---|---|---|
| Carbohydrates | sugars -> carbohydrates | carbohydrates -> sugars |
| Proteins | amino acids -> proteins | proteins -> amino acids |
| Lipids | fatty acids + glycerol -> lipids | lipids -> fatty acids + glycerol |
In a synthesis reaction, suitable smaller substrates bind and are joined into a larger product. In a breakdown reaction, a larger substrate binds and smaller products are released. Different enzymes catalyse particular reactions; the table does not imply that one enzyme carries out every arrow or that the same enzyme must catalyse both directions.
This is why enzymes matter as biological catalysts. Their active sites make reactions specific, they increase reaction rates so essential building and breakdown can occur quickly enough under biological conditions, and they are not used up after one reaction. Amylase provides one example of the breakdown side: amylase catalyses the conversion of starch, a carbohydrate, into smaller sugars.
1.12 — Using the small molecules again
Breakdown and synthesis have different outcomes. Breaking a large food molecule into smaller molecules makes those smaller molecules available for absorption and use. Inside living cells, synthesis can join suitable small molecules into the larger molecules needed for growth and repair. Neither process means that an enzyme becomes part of the product.
Worked example: from food protein to a new muscle protein
- Enzymes break food proteins down into amino acids. The amino acids are the breakdown products.
- After absorption, amino acids reach body cells. They can become starting materials for protein synthesis.
- Enzyme-catalysed reactions join amino acids to build proteins needed by those cells.
The building materials are reused, but the original food protein does not pass unchanged into a muscle. Its amino acids can be rearranged into different proteins. Likewise, sugars can be joined into carbohydrates, and fatty acids plus glycerol can be used to make lipids. Each route needs suitable substrates and enzymes; one enzyme does not catalyse every route.