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Science

DNA and enzymes

Enzymes

Enzymes are soluble protein molecules that can speed up chemical reactions in cells. These reactions include respiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration ], photosynthesis [photosynthesis: The chemical change that occurs in the leaves of green plants. It uses light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product of photosynthesis. ] and making new proteins. For this reason enzymes are sometimes called biological catalysts.

Each enzyme will only speed up one reaction as the shape of the enzyme molecule needs to match the shape of the molecule it reacts with (the substrate molecule). The part of the enzyme molecule that matches the substrate is called the active site.

In this animation an enzyme is shown joining two smaller substrate molecules together to make a larger molecule.

Enzymes and temperature

At low temperatures, enzyme reactions are slow. They speed up as the temperature rises until an optimum temperature is reached. After this point the reaction will slow down and eventually stop.

Graph showing pH and enzyme activity. Between pH 4.5 and pH 8, enzyme activity increases steadily. It peaks at pH 8, then decreases fairly rapidly

Temperature and enzyme activity

Enzymes and pH

Most enzymes work fastest in neutral conditions. Making the solution more acid or alkaline will slow the reaction down. At extremes of pH the reaction will stop altogether. Some enzymes, such as those used in digestion, are adapted to work faster in unusual pH conditions and may have an optimum pH of 2 (very acidic) if they act in the stomach.

Graph showing temperature and enzyme activity. Between 0 and 40ºC, enzyme activity increases steadily. It peaks at 40°C (optimum temperature), then decreases rapidly

pH and enzyme activity

Read on if you're taking the higher paper.

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