A student investigates the effect of temperature on the rate of enzyme activity using a colorimeter. They notice that their results show much greater variation at higher temperatures compared with lower temperatures. Explain why repeating measurements and calculating a mean would improve the reliability of the results at higher temperatures, and suggest one additional control variable the student should have kept constant throughout the investigation.

OCR A-Level Biology A (H420) — 1.1 Practical skills assessed in a written examination · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student is measuring the rate of an enzyme-controlled reaction by recording the change in absorbance over time using a colorimeter. The experiment is carried out at five different temperatures: 10 °C, 20 °C, 30 °C, 40 °C and 50 °C. At temperatures of 40 °C and 50 °C, repeat readings differ considerably from one another compared with repeats at lower temperatures.

Model answer (4 marks)

Repeating measurements reduces the effect of random error on the data, so that the observed variation is less likely to be caused by chance.

Calculating a mean of the repeated readings smooths out the variation; the mean represents the central tendency and is a more reliable estimate of the true rate.

The greater variation at 40 °C and 50 °C is because enzyme denaturation is a stochastic process – individual enzyme molecules denature at slightly different times, producing fluctuating reaction rates.

A suitable control variable would be pH; keeping pH constant ensures that any change in activity is due to temperature alone, since pH alters the ionic state of the active site.

Examiner tips

  • Use the command word ‘explain’ to give reasons, not just statements. Show the link between random error, mean and reliability. Mention the stochastic nature of denaturation. Identify a control variable and justify its importance.

Common mistakes

  • Failing to explain why the mean improves reliability. Choosing a control variable that is unrelated to enzyme activity, e.g. light intensity. Not recognising that denaturation is a random process.

Mark scheme (4 marks)

  1. Repeating measurements reduces the effect of random error on the data
  2. Calculating a mean smooths out variation so the central tendency is a more reliable estimate of the true value
  3. Explanation of why variation is greater at higher temperatures, e.g. enzyme denaturation is a stochastic/random process so individual enzyme molecules denature at slightly different times, causing fluctuating rates
  4. A suitable control variable is identified with a valid reason, e.g. pH — changes in pH alter the ionic state of the active site, changing enzyme activity independently of temperature

Key terms in this question

colorimeter · control variable · reliability

Related

More Practical skills assessed in a written examination questions

▶ Try answering this question with AI marking (free) →