Required practical - Investigating photosynthesis (AQA GCSE Biology Combined Science): Revision Notes
Required practical - Investigating photosynthesis
What this practical is about
This investigation allows you to explore how light intensity affects the rate of photosynthesis in aquatic plants. You'll be measuring how quickly a water plant produces oxygen bubbles when exposed to different amounts of light.
Aim
The main goal is to find out how light intensity affects the speed of photosynthesis in pondweed (or similar water plants like Elodea).
Equipment you need
- Boiling tube and test-tube rack
- Bright lamp (preferably LED), ruler, stopwatch
- Sodium hydrogen carbonate solution
- Freshly cut piece of pondweed
- Beaker of water (if using ordinary lamp)
Why this equipment? The sodium hydrogen carbonate provides CO₂, the boiling tube creates a controlled environment for the plant, and the LED lamp provides consistent light without heating effects.
Method step by step
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Set up your equipment: Place the boiling tube in the test-tube rack.
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Add the solution: Fill the tube with sodium hydrogen carbonate solution. This provides carbon dioxide for photosynthesis.
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Add the plant: Put a fresh piece of pondweed into the solution with the cut end pointing upward.
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Position the light: Place the lamp exactly 10 cm away from the boiling tube. Leave it for 5 minutes to let the plant settle.
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Count bubbles: Count how many oxygen bubbles the plant produces in 1 minute. Do this measurement twice more (3 times total).
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Change the distance: Move the lamp to different distances (like 20 cm, 30 cm, 40 cm) and repeat the bubble counting.
Critical Step: Always wait 5 minutes after moving the lamp before counting bubbles. This allows the plant to adjust to the new light conditions and gives you accurate results.
Recording your results
Create a table to record your data:
| Distance (cm) | Run 1 | Run 2 | Run 3 | Mean |
|---|---|---|---|---|
| 10 | ||||
| 20 | ||||
| 30 | ||||
| 40 |
Calculate the mean by adding the three runs together and dividing by 3.
Worked Example: Calculating the Mean
For 10 cm distance:
- Run 1: 25 bubbles
- Run 2: 23 bubbles
- Run 3: 27 bubbles
Mean = (25 + 23 + 27) ÷ 3 = 75 ÷ 3 = 25 bubbles per minute
Analysing your results
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Work out the average bubble count for each distance.
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Draw a graph with:
- Distance on the horizontal axis
- Mean bubbles per minute on the vertical axis
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Describe the pattern: Explain how the bubble production changes as you move the lamp further away.
Graph Tips: You should see a curve that shows bubble production decreasing as distance increases. This demonstrates the inverse relationship between light intensity and distance.
Important practical tips
Essential Safety and Accuracy Tips
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Use LED lamps to avoid heating up the water. Hot water can affect the results and harm the plant.
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Take multiple measurements at each distance. This makes your results more reliable.
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Use fresh pondweed for the best results. Cut the stem cleanly before starting.
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Keep everything else the same - only change the distance of the lamp.
What the results show
As the lamp moves further away, the light becomes less intense. This means the plant receives less energy for photosynthesis, so it produces fewer oxygen bubbles. The closer the light, the faster photosynthesis happens.
The relationship follows the inverse square law - when you double the distance, the light intensity becomes four times weaker.
Why we add sodium hydrogen carbonate
The Role of Sodium Hydrogen Carbonate
This chemical (NaHCO₃) releases carbon dioxide into the water. Plants need carbon dioxide for photosynthesis, so adding this ensures the plant has enough. Without it, lack of carbon dioxide might limit photosynthesis instead of light intensity.
Key takeaways
Key Points to Remember:
- Light intensity affects photosynthesis rate - more light means faster photosynthesis
- Count oxygen bubbles to measure the rate of photosynthesis
- Take repeat measurements (at least 3) to make results reliable
- Use LED lamps to avoid heating effects
- Sodium hydrogen carbonate provides CO₂ so it's not a limiting factor