Enzymes as biological catalysts, the lock and key model, the effect of temperature, pH and substrate concentration on enzyme activity, and movement of substances by diffusion, osmosis and active transport.

What this dot point is asking

Edexcel wants you to describe enzymes as biological catalysts using the lock and key model, explain how temperature, pH and substrate concentration affect enzyme activity, and describe how substances move into and out of cells by diffusion, osmosis and active transport.

Enzymes as biological catalysts

Enzymes build large molecules from small ones, break large molecules down, and change one molecule into another. Digestive enzymes such as amylase (carbohydrase), protease and lipase break down starch, proteins and fats into small soluble molecules that can be absorbed.

Factors affecting enzyme activity

• Temperature. Rate rises with temperature as molecules collide more often, up to an optimum (around $37\,^{\circ}C$ for human enzymes). Above this, the enzyme denatures: the active site changes shape and the substrate no longer fits.
• pH. Each enzyme has an optimum pH. A pH that is too high or too low also denatures the enzyme. Stomach protease (pepsin) works best in acidic conditions, while enzymes in the small intestine prefer slightly alkaline conditions.
• Substrate concentration. Rate rises as substrate concentration increases, because successful collisions become more likely, until all the active sites are working as fast as they can and the rate levels off.

Movement of substances

Three processes move substances across the cell membrane:

• Diffusion is the passive net movement of particles from a higher to a lower concentration, down a concentration gradient. The rate increases with a steeper gradient, higher temperature, a larger surface area and a shorter distance.
• Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution (high water concentration) to a more concentrated solution (low water concentration).
• Active transport moves substances against the concentration gradient, from low to high concentration, and needs energy from respiration. Root hair cells use it to absorb mineral ions, and the gut uses it to absorb glucose.

A larger surface area to volume ratio speeds up exchange, which is why exchange surfaces such as the small intestine and the lungs are large, thin and well supplied with blood.

Try this

Q1. Name the model used to explain enzyme specificity. [1 mark]

• Cue. The lock and key model.

Q2. State why active transport requires energy. [2 marks]

• Cue. It moves substances against the concentration gradient, from low to high concentration, which cannot happen passively.