Skills applied: CP10 photosynthesis, CP12 temperature and development

CP10 analysis

Method recap (see Core practical 10, Unit 4): pondweed in sodium hydrogencarbonate solution, gas measured in a photosynthometer at different light intensities, wavelengths, temperatures or CO₂ concentrations.

• Light intensity ∝ 1 / d² (point source).
• Volume in a capillary: V = πr²l.
• Limiting factors: a plateau means another factor limits.
• Control temperature with a water bath or a heat shield (LED lamps give out little heat); acclimatise after each change.
• Limitations: gas is not pure oxygen; oxygen is used in respiration and dissolves; small volumes are hard to measure; leaf area varies.

CP12 analysis

Method recap (see Climate change, temperature and Q10, Unit 4): eggs or larvae (for example brine shrimp, Drosophila or seeds) kept at several temperatures; measure time to hatch or reach a stage, or percentage hatched.

• Convert time to rate (1 / time) before calculating Q10.
• Controlled: same species and batch, same volume and salinity of water, same light.
• Use thermostatically controlled incubators or water baths; check temperature with a thermometer.
• Ethics: use invertebrates; return or humanely dispose of organisms.
• Analysis: scatter graph of rate against temperature and Spearman's rank, or a t-test for two temperatures.

Exam angles

• Explain a plateau in terms of a limiting factor.
• Explain why the lamp distance alone may not give accurate light intensity (background light; a light meter is better).
• Explain why development rate falls at very high temperatures (enzymes denature).

Worked example

A lamp is moved from 40 cm to 20 cm from pondweed. By what factor does the light intensity change, and why might the rate not increase by the same factor?

1. Intensity ∝ 1/d²: (40 / 20)² = 4
2. Rate rises by less if another factor (CO₂, temperature) becomes limiting, or the lamp heats the water

Answer: Light intensity increases 4 times; the rate may rise less because another factor becomes limiting.

Check yourself

1. A lamp is moved from 15 cm to 30 cm from a plant. What fraction of the original light intensity is received? Give a decimal.
   Show answer

   0.25. (15/30)² = 0.25.

2. Gas in a capillary of radius 0.6 mm forms a bubble 15 mm long after 5 minutes. Calculate the rate in mm³ min⁻¹ to 3 significant figures.
   Show answer

   3.39 mm³ min⁻¹. V = π x 0.36 x 15 = 16.96 mm³; ÷ 5 = 3.39 mm³ min⁻¹.

3. Brine shrimp eggs hatch in 48 hours at 20 °C and 20 hours at 30 °C. Calculate Q10.
   Show answer

   2.4. Rates 1/48 and 1/20; Q10 = 48 / 20 = 2.4.

4. The rate of photosynthesis stops rising as light intensity increases further. Why?
   • Another factor such as CO₂ concentration or temperature has become limiting
   • The plant has died
   • Light no longer reaches the chloroplasts
   • Oxygen is limiting
   Show answer

   Another factor such as CO₂ concentration or temperature has become limiting. A plateau means light is no longer the limiting factor.

5. Why is a heat shield or LED lamp used in CP10?
   • To increase light intensity
   • To provide CO₂
   • To stop the lamp changing the water temperature
   • To change the wavelength
   Show answer

   To stop the lamp changing the water temperature. Temperature is a controlled variable.

6. Why are eggs for CP12 taken from the same batch?
   • To reduce genetic and age variation between groups
   • So they hatch at the same time regardless of temperature
   • To make the experiment ethical
   • Because other batches will not hatch
   Show answer

   To reduce genetic and age variation between groups. Standardising the organisms.

7. Why does the development rate of larvae fall above about 35 °C?
   • Kinetic energy falls
   • Oxygen becomes more soluble
   • Enzymes begin to denature
   • The larvae stop needing energy
   Show answer

   Enzymes begin to denature. Above the optimum, enzyme shape changes.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills
• Pearson mark scheme WBI16/01, October 2023 (via Physics & Maths Tutor)

Last updated 8 November 2026. Spotted a mistake? Tell us.