A student is investigating the effect of temperature on the rate of enzyme activity using a colorimeter. Explain why it is important that the student zeroes (blanks) the colorimeter with a cuvette containing only the buffer solution before taking absorbance readings from the enzyme-substrate mixture.

OCR A-Level Biology A (H420) — 1.2 Practical skills assessed in the practical endorsement · Explain · 4 marks · View as Markdown

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

A student sets up an investigation in which a coloured product is produced when an enzyme acts on a colourless substrate dissolved in a phosphate buffer. The student uses a colorimeter to measure absorbance at regular intervals to track the rate of reaction at different temperatures.

Model answer (4 marks)

Blanking removes the absorbance of the buffer (and cuvette) so that only the coloured product contributes to the reading.

This ensures that the absorbance measured is due solely to the reaction product, not background.

It allows valid comparisons of rates at different temperatures by controlling a variable.

If the colourimeter is not blanked, the buffer’s absorbance would be added to the product signal, giving an over‑estimate of the rate or a non‑zero reading at time zero.

Examiner tips

  • State the purpose of blanking first. Explain how it removes background absorbance. Mention that it allows comparison between temperatures. Show the effect on calculated rate if omitted.

Common mistakes

  • Confusing blanking with calibration. Failing to mention that the blank removes buffer absorbance. Assuming blanking is optional or only for initial reading.

Mark scheme (4 marks)

  1. Blanking removes the absorbance contribution of the buffer solution itself (or the cuvette/solvent)
  2. This ensures that only the absorbance due to the coloured product is measured (not the background)
  3. This allows valid comparisons between readings taken at different temperatures (controls a variable)
  4. Without blanking, any absorbance by the buffer would cause the calculated rate to be an overestimate (or readings would not be zero at time zero)

Key terms in this question

colorimeter · absorbance · buffer solution · cuvette

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