Catalysts in biological reactions are called enzymes.

Many of the chemical reactions inside the cells of living things would be too slow without enzymes. These reactions occur in the breakdown of chemical molecules, which we see in the digestive system. Enzymes are also involved in the building up of chemical molecules elsewhere in the body.

Enzymes are proteins that have a complex 3D shape. Each enzyme has a region called an active site.

The substrate - the molecule or molecules taking part in the chemical reaction - fits into the active site. Once bound to the active site, the chemical reaction takes place.

Diagram showing how enzymes work in the body

In an organism, the active site of each enzyme is a different shape. It is a perfect match to the shape of the substrate molecule, or molecules. This is essential to the enzyme being able to work. One enzyme is therefore specific to one substrate's chemical reaction, or type of chemical reaction.

This theory for the way in which enzymes work is called the lock and key theory.

Factors affecting enzyme action

Physical factors affect enzyme activity.


At low temperatures, the number of successful collisions between the enzyme and substrate is reduced because their molecular movement decreases. The reaction is slow.

The human body is maintained at 37°C as this is the temperature at which the enzymes in our body work best. This not true of the enzymes in all organisms.

How temperature affects enzyme action

Higher temperatures disrupt the shape of the active site, which will reduce its activity, or prevent it from working. The enzyme will have been denatured.

Enzymes therefore work best at a particular temperature.

Diagram showing how high teperatures alter enzyme structures

A graph to show the effect of temperature on enzyme activity:

Y axis: enzyme activity. X axis: temperature, centigrade.  Plotted line climbs slowly until about half way on x axis. Climbs steeply to optimum temperature then falls steeply to 0.

The effect of pH

Enzymes are also sensitive to pH. Changing the pH of its surroundings will also change the shape of the active site of an enzyme.

Extremes of pH also denature enzymes. The changes are usually, though not always, permanent.

Enzymes work inside and outside cells. Cell pH is kept at 7.0 to 7.4 so cellular enzymes will work best within this pH range. Different parts of the digestive system have different pHs. The enzymes that catalyse reactions in each area have an optimum pH that matches the pH of the area. For example, enzymes that work in the stomach have a very low optimum pH because of the acid present.

A graph to show the effect of pH on enzyme activity:

Graph showing that as the pH increases so does the rate of enzyme activity