Effect of light intensity on photosynthesis
GCSE Biology (8461) · Required practical 6 — method, variables, the marks examiners report students losing.
Investigate the effect of light intensity on the rate of photosynthesis of pondweed by counting the oxygen bubbles released.
Apparatus
- Pondweed (e.g. Cabomba or Elodea) in a boiling tube of sodium hydrogencarbonate solution
- A lamp and a metre ruler to set the distance
- Stopwatch and a beaker of water acting as a heat shield
- Thermometer to check the temperature stays constant
Method
- 1Place the pondweed in sodium hydrogencarbonate solution (a source of carbon dioxide) and set the lamp a measured distance away.
- 2Put a beaker of water (heat shield) between the lamp and the tube so heat from the lamp does not change the temperature.
- 3Leave the plant to acclimatise, then count the number of oxygen bubbles released in one minute.
- 4Repeat and take a mean bubble count at that distance.
- 5Change the distance of the lamp (e.g. 10, 20, 30 cm) and repeat, keeping everything else the same.
Variables
Independent
Light intensity (set by the distance of the lamp)
Dependent
Rate of photosynthesis (bubbles of oxygen per minute)
Control
- Temperature (kept constant with a heat shield)
- Carbon dioxide concentration (the hydrogencarbonate solution)
- The same piece of pondweed and time interval
Results & processing
- Light intensity is proportional to 1 / distance squared (the inverse square law), so halving the distance quadruples the intensity.
- Plot rate (bubbles per minute) against light intensity: the rate rises as intensity increases, then levels off when another factor becomes limiting.
- More bubbles per minute means a faster rate of photosynthesis.
Where students lose marks
Ignoring the heat from the lamp.
Fix: Place a beaker of water between the lamp and the plant so only light intensity changes, not temperature.
Using distance directly as intensity.
Fix: Intensity is proportional to 1 / distance squared, so convert distance to relative intensity before plotting.
Counting bubbles for different lengths of time.
Fix: Count for the same fixed time (e.g. one minute) at every distance for a fair comparison.
Improve the method
- Repeat at each distance and take a mean bubble count.
- Collect and measure the volume of gas with a gas syringe instead of counting bubbles for a more accurate rate.
- Let the plant acclimatise at each new distance before counting.
Try it — exam-style
At 20 cm the plant releases 12 bubbles per minute. Using the inverse square law, explain what happens to the light intensity when the lamp is moved to 10 cm.
On a graph of rate against light intensity, the line levels off at high intensity. Explain why.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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