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Activation energy

The energy required to get a chemical reaction started.


A chemical reaction where more energy is taken in as bonds are broken than is released when bonds form, so the surroundings cool down.


A chemical reaction where more energy is released as bonds form than is taken in to break the bonds, so the surroundings are heated.


The process by which the folding of the long chains of amino acids making up a protein is damaged and destroyed by heat, pH changes etc, changing the structure and function of the entire molecule.


A substance that speeds up the rate of a reaction without being changed or used up itself

Chemical reactions, catalysis and the need for enzymes

In a chemical reaction, chemical bonds are broken and reformed. The breaking of chemical bonds requires energy and the formation of bonds releases energy. If more energy is released as bonds form than is taken in to break the bonds, the reaction is exothermic. If more energy is taken in than is released, the reaction is endothermic. All reactions, whether exothermic or endothermic, require energy to get started. This is known as the activation energy.

One way of supplying the activation energy for a chemical reaction is to heat the reactants. Raising the temperature gives the individual particles more energy so they are more likely to collide with enough energy to react. However many reactions need very high temperatures to overcome the activation energy. This wouldn’t work in living organisms as the molecules that make up the cells and in particular the proteins would denature and the cell would die.


  • A catalyst reduces the activation energy needed for a chemical reaction to take place.
  • Catalysts do not affect or change the reaction that the catalyse – they simply speed it up
  • Catalysts are unchanged themselves at the end of the reaction they have catalysed

Enzymes are biological catalysts.

Energy profile of reaction with and without a catalyst

This diagram shows the energy profile of a reaction with and without a catalyst.

The need for enzymes

Organisms live in many different environments, from deserts to cloud forests, and from frozen waterfalls to temperate woodlands. They may be made of one cell or billions. Wherever they live, they need a way of making sure that the reactions inside them take place at the right speed and without interfering with other reactions. Enzymes make this possible.

Desert Environment Forest Icy Landscape Meadow and Woodland

Enzymes control cell reactions whatever the organism and wherever it is growing