Control of Variables & Uncertainty (AQA A Level Biology): Revision Note

Control of variables & uncertainty

• Enzyme rate experiments are experiments that are carried out to determine the effect of changing a particular factor on the rate of a reaction that is catalysed by an enzyme

• Factors that can be changed include:

  • Temperature

  • pH

  • Enzyme concentration

  • Substrate concentration

• The key thing with enzyme rate experiments is to ensure that only one of these variables is changed during a particular experiment

  • This is known as the independent variable

• All other variables must be controlled (they must stay the same)

  • These are known as the control variables

• For example, if investigating the effect of temperature on the rate of reaction, the pH, enzyme concentration and substrate concentration must be exactly the same (kept constant) each time you run the experiment (at each different temperature you are investigating)

• If these control variables are not kept constant, they could affect the results of the experiment

  • This would make the results unreliable

Calculating uncertainty

• Uncertainty is the amount of error your measurements might contain

• Results from experiments (including enzyme rate experiments) always contain some error (they are never perfect)

• There will always be a small degree of uncertainty in your readings or measurements

  • This is often because the sensitivity of the apparatus being used is limited

  • E.g. If using a gas syringe, measuring results to the nearest 1 cm³, there may be a degree of uncertainty if the real volume produced is 0.5 cm³ smaller or larger. The syringe, therefore, has an uncertainty of ± 0.5 cm³

    • A ‘±’ sign tells you the range in which the true value lies; this range is called the margin of error

• For enzyme rate experiments, you may need to calculate the percentage error of your measurements

  • As long as you know the uncertainty value of your measurements, the percentage error can be calculated using the following formula

percentage error = (uncertainty ÷ measured value) x 100