Required Practical: Photosynthesis Rate (AQA GCSE Biology Combined Science): Revision Notes

Aim: This practical experiment is designed to investigate how different factors affect the rate of photosynthesis in aquatic plants, such as pondweed. By measuring the amount of oxygen produced, you can determine how factors like light intensity, carbon dioxide concentration, and temperature influence the rate of photosynthesis.

Method:

1. Setup:

• Place the pondweed in a test tube filled with water, ensuring the cut end is at the top. Add a small amount of sodium hydrogen carbonate to the water to provide a consistent source of carbon dioxide.
• Seal the test tube with a bung, and connect a capillary tube filled with water to the test tube. The other end of the capillary tube should be attached to a syringe.

2. Positioning the Light Source:

• Place a lamp at a specific distance from the test tube. This distance can be varied to investigate the effect of light intensity on the rate of photosynthesis.

3. Measuring Oxygen Production:

• As the pondweed photosynthesises, it produces oxygen, which forms a gas bubble in the capillary tube. Use the syringe to move the bubble to a measurable point.
• Start the stopwatch and measure the distance the oxygen bubble travels over a set period (e.g., 5 minutes). The distance moved by the bubble is proportional to the volume of oxygen produced, which indicates the rate of photosynthesis.

4. Varying Conditions:

• Repeat the experiment by changing the independent variable:
• Light Intensity: Move the lamp closer or further away from the test tube.
• Temperature: Use a water bath to control the temperature of the water around the pondweed.
• Carbon Dioxide Concentration: Adjust the amount of sodium hydrogen carbonate in the water.

5. Control Variables:

• Keep all other factors constant except the one being tested (e.g., if testing light intensity, keep temperature and CO₂ concentration constant).

6. Recording and Analysing Data:

• Record the distance the oxygen bubble travels at each condition.
• Plot a graph of the rate of photosynthesis (oxygen production) against the independent variable being tested (e.g., light intensity or temperature).

Data Analysis:

• Interpreting Results:
• Light Intensity: The rate of photosynthesis should increase as light intensity increases, up to a point where it levels off due to another limiting factor.
• Temperature: The rate increases with temperature up to an optimum point, after which it decreases as enzymes denature.
• Carbon Dioxide Concentration: The rate increases with more CO₂ until another factor becomes limiting.
• Identifying Limiting Factors:
• The experiment helps identify which factor is limiting the rate of photosynthesis under different conditions.

Conclusion: This practical provides insight into how various environmental factors affect the rate of photosynthesis. Understanding these factors is crucial for optimising plant growth, particularly in controlled environments like greenhouses.