Enzymes and metabolism — IB Diploma Biology SL

Revision notes

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

1. Enzymes as catalysts and their role in metabolism

   Enzymes are biological catalysts: they speed up chemical reactions in cells without being used up. Metabolism is the complex network of interdependent and interacting chemical reactions in an organism. Each enzyme is specific, so many enzymes are needed, and cells control metabolism by controlling enzymes. Anabolic reactions build macromolecules; catabolic reactions break them down.

2. Active sites, induced fit, collisions and denaturation

   Enzymes are globular proteins with an active site, a small region of a few amino acids where the substrate binds and catalysis happens. Binding is by induced fit: the substrate and the enzyme both change shape. Substrate and active site must collide, so molecular motion matters. Specificity depends on active site shape and chemistry, and denaturation destroys both.

3. Effects of temperature, pH and substrate concentration

   Practical

   Enzyme activity rises with temperature up to an optimum, because faster molecules collide more often, then falls as the enzyme denatures. Activity is highest at an optimum pH and falls either side as the active site changes. Rate rises with substrate concentration until all active sites are occupied. Rate is measured as product formed or substrate used per unit time.

4. Effect of enzymes on activation energy

   Activation energy is the energy needed to break bonds within the substrate so that a reaction can start. Enzymes lower the activation energy, so more substrate molecules can react at a given temperature and the rate increases. Energy is released when new bonds form in the products. The overall energy change of the reaction stays the same.