Investigating photosynthesis (AQA GCSE Biology): Revision Notes
Investigating photosynthesis
What are we investigating?
Scientists need to understand how different factors affect the rate of photosynthesis. One important factor is light intensity - how bright the light is.
We can investigate this using aquatic plants like pondweed. These plants produce oxygen bubbles that we can count during photosynthesis.
Core practical experiment
Aim
The goal is to find out how light intensity affects the rate of photosynthesis in pondweed.
Equipment you need
Equipment List:
- Boiling tube and test-tube rack
- Bright lamp with ruler and stop watch
- Sodium hydrogen carbonate solution
- Fresh piece of pondweed
Method
Follow these steps carefully to ensure accurate and reliable results:
- Set up the apparatus - Put the boiling tube in the test-tube rack
- Add the solution - Fill with sodium hydrogen carbonate solution
- Add the plant - Lower the fresh pondweed into the boiling tube
- Position the lamp - Place it 10cm away from the lamp and wait 5 minutes
- Count bubbles - Count how many bubbles are produced in 1 minute
- Repeat - Do this twice more at the same distance
- Change distance - Repeat steps 4 and 5 at different distances (20cm, 30cm, 40cm)
Take your time when counting bubbles. Rushed counting leads to inaccurate results that won't show the true relationship between light intensity and photosynthesis rate.
Recording your results
Make a table with these columns:
- Distance (cm)
- Run 1, Run 2, Run 3 (bubbles per minute)
- Mean (average)
Calculate the mean by adding the three results and dividing by 3:
Analysis
- Calculate the mean rate of bubbling at each distance
- Draw a graph with:
- Distance on the horizontal axis
- Mean rate of bubbling on the vertical axis
- Describe the pattern - explain how the rate changes as distance increases
Worked Example: Describing Your Results
Step 1: Look at your data
At 10cm: mean = 25 bubbles/minute
At 20cm: mean = 15 bubbles/minute
At 30cm: mean = 8 bubbles/minute
Step 2: Describe the pattern "As the distance from the lamp increases, the mean rate of bubble production decreases. This shows an inverse relationship between distance and photosynthesis rate."
Important tips
About the plant: Pondweed (like Elodea) works well for this experiment. You can also use algae.
About the solution: Sodium hydrogen carbonate provides carbon dioxide. This ensures CO₂ concentration doesn't limit the reaction.
About the lamp: Use an LED lamp to avoid heating the pondweed. Hot temperatures can affect the results and give you misleading data about light intensity effects.
About reliability: Repeat the counting at each distance. If results are very different, do more repeats. Reliable results should be identical or very similar.
About the graph: Choose sensible scales so your plotted points fill at least 50% of the graph area. Draw a smooth curve of best fit.
What the results show
Pattern: As the lamp gets further away (distance increases), fewer bubbles are produced per minute.
Explanation: More distance = less light intensity = slower rate of photosynthesis = fewer oxygen bubbles.
This proves that light intensity affects the rate of photosynthesis.
Key Points to Remember:
- Light intensity affects photosynthesis rate - more light means faster photosynthesis
- We measure this by counting oxygen bubbles from aquatic plants
- Sodium hydrogen carbonate solution provides CO₂ so it's not a limiting factor
- Always repeat measurements and calculate means for reliability
- Use LED lamps to avoid heating effects that could change results