Edexcel IGCSE Biology 2 44b Light Intensity and Net Gas Exchange Questions

Practise respiration practicals using indicator colour, CO2 release, light intensity and the balance between respiration and photosynthesis.

Exam points

Explain that respiration continues in light and darkness, while photosynthesis changes the net gas exchange in light.
Predict net oxygen or carbon dioxide exchange at different light intensities by comparing photosynthesis and respiration, including the compensation point.
Explain why hydrogen-carbonate indicator stays unchanged when carbon dioxide released by respiration equals carbon dioxide absorbed by photosynthesis.
Interpret indicator results for leaves and respiring animals in light and darkness, using the empty tube as a control.

[Maximum number: 2]

The diagram shows a single-celled organism called Chlorella that lives in fresh water. Chlorella has a chloroplast and can photosynthesise.

The graph shows the effect of light intensity on gas exchange by Chlorella.
oxygen released by Chlorella
oxygen taken in by Chlorella

Explain why Chlorella takes in oxygen at light intensities below 10 arbitrary units.

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