AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Required practical activity 6: investigate the effect of light intensity…
Full specification wording
Rate of photosynthesis: Required practical activity 6: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.
Pondweed releases oxygen bubbles when it photosynthesises.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 7 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Pondweed releases oxygen bubbles when it photosynthesises. The rate can be measured by counting bubbles per minute or by collecting the gas in a capillary tube and measuring the volume. Place a lamp a measured distance from the pondweed, leave it to acclimatise, then count bubbles for one minute. Repeat at several distances, such as 10 cm, 20 cm, 30 cm and 40 cm, and repeat each distance to improve reliability. Control variables include temperature, carbon dioxide concentration, the length of pondweed and the time allowed for acclimatisation. As the lamp moves further away, light intensity falls, so the rate of bubbling usually decreases. The results can be plotted as a graph of rate against distance or against 1 ÷ distance². A control experiment can use a darkened tube to show that bubbles are linked to light.
What earns the marks
- Measure the rate of photosynthesis by counting oxygen bubbles per minute or collecting the gas volume.
- Change light intensity by moving a lamp to different measured distances from the pondweed.
- Control temperature, carbon dioxide concentration, pondweed length and acclimatisation time.
- Repeat readings at each distance and calculate a mean to improve reliability.
- Plot a graph of rate against distance or against 1 ÷ distance² and describe the trend.
- Use a control, such as a darkened tube, to show that light is needed for the bubbles.
Common mistakes and how to put them right
Starting to count bubbles immediately after moving the lamp.
Put it right by allowing time for the pondweed to acclimatise.
Changing more than one variable at a time.
Put it right by keeping all factors except light intensity constant.
Recording a single reading at each distance.
Put it right by repeating and calculating a mean.
Exam tips
- State clearly how the dependent variable is measured, for example bubbles per minute.
- Identify at least two control variables and explain why each is kept constant.
- Describe repeats and a mean when explaining how reliability is improved.
Try the lesson
The start of the Rate of photosynthesis: Required practical activity 6: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed. lesson, made from checked teaching notes for this specification point.
Have a go first
In the pondweed practical, a lamp is placed at different distances from the pondweed. What is counted to measure the rate of photosynthesis?
Listen
Pondweed releases oxygen bubbles when it photosynthesises. The rate can be measured by counting bubbles per minute or by collecting the gas in a capillary tube and measuring the volume. Place a lamp a measured distance from the pondweed, leave it to acclimatise, then count bubbles for one minute. Repeat at several distances, such as 10 cm, 20 cm, 30 cm and 40 cm, and repeat each distance to improve reliability. Control variables include temperature, carbon dioxide concentration, the length of pondweed and the time allowed for acclimatisation. As the lamp moves further away, light intensity falls, so the rate of bubbling usually decreases. The results can be plotted as a graph of rate against distance or against 1 ÷ distance². A control experiment can use a darkened tube to show that bubbles are linked to light. A strong answer earns marks for these points. Measure the rate of photosynthesis by counting oxygen bubbles per minute or collecting the gas volume. Change light intensity by moving a lamp to different measured distances from the pondweed. Control temperature, carbon dioxide concentration, pondweed length and acclimatisation time. Repeat readings at each distance and calculate a mean to improve reliability. A common slip: Starting to count bubbles immediately after moving the lamp. Instead, allowing time for the pondweed to acclimatise. Exam tip: State clearly how the dependent variable is measured, for example bubbles per minute.
Key words
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rate of photosynthesis
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light intensity
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control variables
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reliability
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See every definition
- rate of photosynthesis
- Measured by counting oxygen bubbles per minute or by collecting the gas volume.
- light intensity
- Changed by moving a lamp to different measured distances from the pondweed.
- control variables
- Temperature, carbon dioxide concentration, pondweed length and acclimatisation time are kept constant.
- reliability
- Improved by repeating readings at each distance and calculating a mean.
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