Ever wondered how plants make their own food using just... Show more
Biology104 views·Updated May 14, 2026·3 pagesGCSE AQA Combined Science: Photosynthesis Practical Guide
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Poppy@poppy_s
1 / 3
1
of 3
Setting Up Your Photosynthesis Experiment
You'll be investigating how light intensity affects the rate of photosynthesis by counting oxygen bubbles produced by pondweed. The chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Your independent variable is light intensity (what you change), whilst your dependent variable is the number of oxygen bubbles produced (what you measure). Keep everything else constant - the size of pondweed, volume of water, and temperature.
The key relationship you'll discover is the inverse square law: light intensity = 1/distance². This means if you double the distance from the light, the intensity becomes four times weaker.
Quick Tip: Always use an LED light source rather than a normal bulb - it produces less heat, which could mess up your results by changing the water temperature.
2
of 3
Running the Practical
Start by placing your boiling tube 10cm away from an LED light source. Fill it with sodium hydrogen carbonate solution - this releases the carbon dioxide your pondweed needs for photosynthesis.
Pop a piece of pondweed into the tube with the cut end pointing upwards - this is where the oxygen bubbles will emerge. Give it a few minutes to settle into its new environment before you start timing.
You should see oxygen bubbles streaming from the cut end - that's photosynthesis in action! Count the bubbles produced in one minute, then repeat this four more times to calculate a reliable mean average.
Remember: Repeat the entire experiment at distances of 20cm, 30cm, 40cm, and 50cm to see how light intensity affects the rate.
3
of 3
Improving Your Results and What You'll Find
Counting bubbles can be tricky - they might come too fast or vary in size. A better method is to measure the volume of oxygen produced instead of counting individual bubbles.
Place the pondweed under a funnel and catch the gas in a measuring cylinder filled with water. This gives you more accurate measurements of the actual volume of oxygen produced.
Your results will show that doubling the distance makes the light intensity four times weaker, causing oxygen production to drop by four times too. This demonstrates the inverse square law perfectly.
Pro Tip: Always calculate mean averages from multiple readings - this makes your data much more reliable and helps you spot any anomalous results.
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Biology104 views·Updated May 14, 2026·3 pagesGCSE AQA Combined Science: Photosynthesis Practical Guide
P
Poppy@poppy_s
Ever wondered how plants make their own food using just sunlight, water, and carbon dioxide? This photosynthesis practical helps you measure how light intensity affects the rate at which plants produce oxygen - and it's easier than you might think!
1
of 3Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Setting Up Your Photosynthesis Experiment
You'll be investigating how light intensity affects the rate of photosynthesis by counting oxygen bubbles produced by pondweed. The chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Your independent variable is light intensity (what you change), whilst your dependent variable is the number of oxygen bubbles produced (what you measure). Keep everything else constant - the size of pondweed, volume of water, and temperature.
The key relationship you'll discover is the inverse square law: light intensity = 1/distance². This means if you double the distance from the light, the intensity becomes four times weaker.
Quick Tip: Always use an LED light source rather than a normal bulb - it produces less heat, which could mess up your results by changing the water temperature.
2
of 3Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Running the Practical
Start by placing your boiling tube 10cm away from an LED light source. Fill it with sodium hydrogen carbonate solution - this releases the carbon dioxide your pondweed needs for photosynthesis.
Pop a piece of pondweed into the tube with the cut end pointing upwards - this is where the oxygen bubbles will emerge. Give it a few minutes to settle into its new environment before you start timing.
You should see oxygen bubbles streaming from the cut end - that's photosynthesis in action! Count the bubbles produced in one minute, then repeat this four more times to calculate a reliable mean average.
Remember: Repeat the entire experiment at distances of 20cm, 30cm, 40cm, and 50cm to see how light intensity affects the rate.
3
of 3Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Improving Your Results and What You'll Find
Counting bubbles can be tricky - they might come too fast or vary in size. A better method is to measure the volume of oxygen produced instead of counting individual bubbles.
Place the pondweed under a funnel and catch the gas in a measuring cylinder filled with water. This gives you more accurate measurements of the actual volume of oxygen produced.
Your results will show that doubling the distance makes the light intensity four times weaker, causing oxygen production to drop by four times too. This demonstrates the inverse square law perfectly.
Pro Tip: Always calculate mean averages from multiple readings - this makes your data much more reliable and helps you spot any anomalous results.
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Is Knowunity really free of charge?
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