A student investigates the effect of temperature on the rate of enzyme activity using a colorimeter to measure the breakdown of a coloured substrate. The student records absorbance readings every 30 seconds for 5 minutes at each temperature tested. Explain how the student could ensure that the colorimeter readings are valid and comparable between different temperature treatments.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A colorimeter measures the absorbance of light passing through a solution. A higher absorbance value indicates a greater concentration of coloured substrate remaining. The student is testing temperatures of 20 °C, 30 °C, 40 °C, and 50 °C.
Model answer (4 marks)
1. Calibrate the colourimeter with a blank solution before each set of readings to zero the instrument.
2. Use the same wavelength (filter) for all measurements so the absorbance is comparable.
3. Employ identical cuvettes, of the same type and size, and fill them to the same volume each time.
4. Allow the reaction mixture to reach the target temperature before starting the 30‑second readings to keep temperature constant and avoid it being a confounding factor.
2. Use the same wavelength (filter) for all measurements so the absorbance is comparable.
3. Employ identical cuvettes, of the same type and size, and fill them to the same volume each time.
4. Allow the reaction mixture to reach the target temperature before starting the 30‑second readings to keep temperature constant and avoid it being a confounding factor.
Examiner tips
- Mention calibration, wavelength, cuvette consistency and temperature control as separate points.
- Use the exact wording from the mark scheme to secure full marks.
- Keep each point concise and numbered.
Mark scheme (4 marks)
- Calibrate (zero) the colorimeter using a blank solution before each set of readings
- Use the same wavelength of light (filter) throughout the experiment
- Use cuvettes of the same type/size and ensure they are filled to the same volume each time
- Allow the reaction mixture to reach the target temperature before starting readings, to ensure temperature is controlled and not a confounding variable
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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