4.3 Required practical: light intensity and photosynthesis
6 exam questions worth 10 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Give the independent variable and the dependent variable in this investigation.
Mark scheme
- independent: light intensity / distance from the lamp (1)
- dependent: rate of photosynthesis / number of bubbles per minute / volume of oxygen (1)
Model answer
The independent variable is the light intensity (distance from the lamp), and the dependent variable is the number of oxygen bubbles produced per minute.
Why is the pondweed placed in sodium hydrogencarbonate solution?
Mark scheme
it releases / supplies carbon dioxide (so CO2 is not limiting)
Model answer
It supplies carbon dioxide, so carbon dioxide does not limit the rate.
A beaker of water is placed between the lamp and the boiling tube.
Explain why.
Mark scheme
- absorbs heat from the lamp / acts as a heat filter (1)
- keeps the temperature constant, which would otherwise affect the rate / enzymes (1)
Model answer
The water absorbs heat from the lamp, so the temperature of the pondweed stays the same and does not affect the rate of photosynthesis.
Counting bubbles is not very accurate. Suggest a better way to measure the rate, and explain why it is better.
Mark scheme
- collect the gas in a gas syringe / measuring cylinder and measure its volume (1)
- bubbles can be different sizes / bubbles may be missed (1)
Model answer
Collect the oxygen in a gas syringe and measure its volume, because bubbles can be different sizes and are easy to miscount.
The lamp is moved from 20 cm to 10 cm from the pondweed.
Use the inverse square law to work out how many times greater the light intensity becomes.
Mark scheme
- light intensity ∝ 1/d2 / distance halved (1)
- 4 (times greater) (1)
Model answer
The distance is halved, and intensity ∝ 1/d2, so the light intensity becomes 22 = 4 times greater.
When the lamp was very close, moving it closer did not increase the number of bubbles.
Suggest why.
Mark scheme
another factor is limiting / carbon dioxide or temperature is limiting
Model answer
Light was no longer the limiting factor; another factor, such as carbon dioxide concentration, was limiting the rate.
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