Enzymes are found in both animals and plants, and have important roles in their metabolism.
  1. Describe the general purpose of enzymes.
  2. Explain the difference between the “lock and key” and the “induced fit” models of enzyme activity.
  3. Some of the factors that affect the rate of enzyme activity within cells include the amount of substrate, the enzyme concentration, acidity and temperature. Discuss how any THREE of these factors can change the rate of activity within cells.


Enzymes are proteins that catalyse chemical reactions within the cell. They reduce the activation energy required to break down large, complicated molecules into smaller, more simple molecules that can be used within the cell to help carry out life processes.

There are two models used to describe enzyme activity: the lock and key model and the induced fit model. The lock and key model, the older model, suggests that enzymes are very specific and that substrates fit perfectly into the active sites of the enzyme, hence the name lock and key - one fits perfectly into another. However not all experimental evidence can be adequately explained by using this theory and so biologists came up with the idea of an induced fit. The induced fit model describes the interaction between the substrate and a flexible active site. This model suggests that the active site of an enzyme changes slightly to allow for a better fit between it's active sites and the substrate. Once the reaction has occurred, the active sites regain their natural shape.

If the amount of substrate present increased and the amount of enzymes present during the reaction remained the same, the rate of activity in the cell would decrease as the ratio of substrate to enzyme would decrease. This means that there would be less enzymes present per substrate molecule and it would take a longer time for all of the substrate molecules to react; thus decreased activity within the cells.

If the reverse was to happen and if the concentration of enzymes was to increase and the amount of substrate was to remain constant, the ratio of substrate to enzyme would increase. This means that there would be more enzymes per substrate molecule and since the chemical reactions undergone by substrate molecules occur faster in the presence of enzymes, the activity within in the cells would increase.

Enzymes must be at an optimum temperature and pH to perform at their best. If the temperature is too high or not in their optimum pH, the enzyme will denature and it will not be able to act as a catalyst to speed up metabolic chemical reactions. If the temperature is too low, there will not be enough energy for the enzymes to function at their best and again, they will not be able to act as catalysts to speed up metabolic chemical reactions.