Cambridge IGCSE Biology Notes 5: Enzymes

Topic 5 of Cambridge IGCSE Biology 0610 and 0970 develops enzymes as protein catalysts needed throughout metabolism. Core candidates describe catalyst behaviour, complementary active sites, product formation and the effects of temperature and pH. Supplement candidates explain specificity, enzyme-substrate complexes, effective collisions, shape and fit, and denaturation.

Catalysts and metabolic reactions

A catalyst increases the rate of a chemical reaction and is not changed by the reaction. It participates in the reaction pathway but is available again after products form. It does not become one of the products and is not steadily consumed.

Enzymes are proteins that function as biological catalysts. They are involved in all metabolic reactions. Metabolism means the chemical reactions taking place in cells and organisms, including reactions that build larger molecules and reactions that break molecules down.

Enzymes are important because uncatalysed reactions would often proceed too slowly at the temperatures compatible with life. Enzymes provide reaction rates that can sustain processes such as respiration, digestion, growth, repair and synthesis without requiring temperatures that would damage cells.

An enzyme changes reaction rate, not the identity of the reactants or products. It also does not provide the raw material for a reaction. Cells still need the correct substrate and suitable conditions.

Active-site action

The active site is a region of an enzyme with a particular three-dimensional shape. The substrate is the molecule on which the enzyme acts. For a reaction to occur, the substrate must fit the active site.

At Core level, describe the active-site shape as complementary to the substrate. Complementary means that the shapes match in a way that permits binding. The enzyme converts the bound substrate into product or products, which leave the active site. The enzyme can then act again.

At Supplement level, name the temporary enzyme-substrate complex. A complete sequence is:

1. Substrate molecules move and collide with the enzyme.
2. A substrate with the correct complementary shape fits the active site.
3. An enzyme-substrate complex forms.
4. The reaction takes place and product forms.
5. Product leaves because it no longer fits in the same way.
6. The unchanged enzyme is available for another substrate.

The active-site model is a molecular explanation, not a claim that enzymes and substrates are rigid household objects. What matters for this syllabus is complementary shape and fit.