A student investigates the effect of temperature on the rate of enzyme activity using a colorimeter to measure the breakdown of a coloured substrate. Explain why the student should use a control cuvette containing only buffer solution when taking colorimeter readings.

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 enzyme activity at five different temperatures (10 °C, 20 °C, 30 °C, 40 °C, 50 °C). At each temperature, the student uses a colorimeter set to an appropriate wavelength to record the change in absorbance of the substrate solution over two minutes. The student notices that the buffer solution used has a very faint yellow tint.

Model answer (4 marks)

The student should use a control cuvette containing only buffer to zero the colourimeter.
1. Zeroing removes the background absorbance of the buffer and cuvette.
2. It ensures that subsequent readings reflect only the substrate’s absorbance.
3. Without this correction the data would include the buffer’s faint yellow tint, giving inaccurate rates.
4. Because temperature can change the buffer’s absorbance, the colourimeter must be re‑zeroed at each temperature.

Examiner tips

  • Use the word ‘zero’ or ‘calibrate’ to show understanding of the instrument’s function.
  • Explain that background absorbance is subtracted, not just ignored.
  • Mention that temperature can alter background absorbance, justifying re‑zeroing.
  • Keep the answer concise and point‑wise to match the 4‑mark structure.

Common mistakes

  • Failing to mention that the buffer’s absorbance must be subtracted, not merely noted.
  • Assuming the colourimeter automatically corrects for background without a control cuvette.
  • Not recognising that temperature changes the buffer’s absorbance and therefore re‑zeroing is required.

Mark scheme (4 marks)

  1. The control cuvette is used to zero/calibrate the colorimeter
  2. This removes/accounts for any absorbance due to the buffer solution itself (or the cuvette)
  3. Without zeroing, the absorbance values recorded would include the background absorbance of the buffer, making results inaccurate/unreliable
  4. The colorimeter must be re-zeroed with the control cuvette at each new temperature because temperature may alter the absorbance properties of the buffer/cuvette

Key terms in this question

colorimeter · control cuvette · buffer solution

Related

More Practical skills assessed in a written examination questions

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