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Enzymes are proteins that act as catalysts. They are globular protein whose shape is maintained by hydrogen bonds, disulphide bridges and ionic attractions.

Induced Fit

Suggests that the enzyme active site changes shape to suit to the shape of the enzyme. They bind together which results in the new shape of the active site being catalytically active so it lowers the activation energy.

Lock and Key

The substrate and the enzyme active site have complementary shapes

Activation Energy

Chemical reactions require energy to start them off this is the activation energy.

Definition of activation energy - the energy needed to break the existing bonds inside molecules.

Enzymes lower the activation energy thereby reducing the input of energy needed so reactions can occur at lower temperatures.


The rate at which enzymes work

There are a number of different things that can affect the rate at which enzymes work.

  • pH (the measure of hydrogen ions present) - a change in the pH can break hydrogen bonds within the enzyme thus changing its shape and making it less efficient. Each enzyme has an optimum pH


  • Enzyme concentration - if the other conditions are suitable and there is an excess of substrate the rate of reaction is directly proportional to enzyme concentration ( resulting in a straight line graph). However if the amount of substrate is limited it can limit the rate of reaction.


  • Substrate concentration - at low substrate concentrations not all of the enzyme active sites are occupied…


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