Practical work forms an important part of your A-level biology course. Usually, it will involve investigating the effect of an independent variable on a dependent variable. In planning such an investigation, it is important that you try to ensure it is the independent variable, and only the independent variable, that has the effect on the dependent variable that you measure.
Suppose you are planning to investigate the effect of changes in pH (the independent variable) on the rate of an enzyme-catalysed reaction (the dependent variable). You will control the pH by setting up reaction tubes using buffers of different, known pH values. But you know that other factors have an effect on enzyme activity, so you control those too. For example, you set up the reaction tubes in a water bath and continuously check that the bath maintains a constant temperature. You also ensure that you use the same concentration and volumes of enzyme and substrate solution. Your chosen values of pH, temperature, concentration and volume of reactants are controlled variables.
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