Enzymes — Cambridge IGCSE Combined Science

Revision notes

4 short notes, in the order of the specification. Each one in short:

1. Enzymes as biological catalysts

   Enzymes are proteins that act as biological catalysts, speeding up metabolic reactions without being changed or used up. Cambridge IGCSE Biology 5.1.

2. How enzymes work

   Supplement

   An enzyme works because its active site has a shape complementary to its substrate, so the substrate fits into it. The substrate binds to the active site to form an enzyme-substrate complex, is changed into products, and the products leave. Enzymes are specific: only a substrate with a complementary shape can fit the active site.

3. Temperature, pH and enzyme activity

   Supplement

   Enzyme activity is highest at an optimum temperature and an optimum pH. Below the optimum temperature, molecules have less kinetic energy, so there are fewer effective collisions. Above it, the bonds holding the enzyme in shape break, the active site changes shape, and the enzyme is denatured. At extreme pH the enzyme is also denatured.

4. Investigating temperature and pH

   Practical

   The effect of temperature or pH on enzyme activity is investigated by timing how long amylase takes to digest starch. Drops of the mixture are added to iodine every 30 seconds; when it stays orange-brown, no starch is left. Activity rises to an optimum, then falls as the enzyme is denatured. Rate of reaction = 1 ÷ time taken.