Practical Skill: Controlling Variables & Calculating Uncertainty (AQA A Level Biology)

Practical Skill: 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

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

• For example, you may want to measure reaction rate by measuring how much of a product is made in a given time period, e.g. using a gas syringe to measure the volume of oxygen produced from the breakdown of hydrogen peroxide by catalase

  • The gas syringe may give readings to the nearest 1 cm³

  • However, the real volume produced could be up to 0.5 cm³ smaller or larger

    • E.g. if the syringe were to collect 4 cm³ of oxygen, then the true value for the volume of oxygen would lie somewhere between 3.5 cm³ and 4.5 cm³

  • The syringe 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

    • E.g. for the example above, the syringe can be said to have measured 4 ± 0.5 cm³

• 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

Percentage error = (uncertainty ÷ measured value) x 100

Percentage error = (0.5 ÷ 50) x 100

Percentage error = 0.01 x 100

Percentage error = 1 %

Step 1: Calculate the uncertainty value

The stopwatch can measure to the nearest millisecond (0.001 second)

This means the actual time taken could be up to 0.0005 seconds shorter or longer than this

This means stopwatch measurements have an uncertainty of ± 0.0005 s

Step 2: Calculate the percentage error of the student’s measurement of 5.245 seconds

Percentage error = (uncertainty ÷ measured value) x 100

Percentage error = (0.0005 ÷ 5.245) x 100

Percentage error = 0.000095 x 100

Percentage error = 0.0095 % or 0.01 %