5.1.7 Investigating the Rate of Photosynthesis

Practical Methods to Measure Photosynthesis

1. Measuring Oxygen Production (Aquatic Plants):

• Use an aquatic plant like Elodea or Cabomba.
• Place the plant in a beaker of water with sodium hydrogencarbonate ($NaHCO₃$) to ensure $CO₂$ is not a limiting factor.
• Shine light at the plant and count the number of oxygen bubbles produced over a set time or collect oxygen in a gas syringe to measure its volume.

2. Measuring Carbon Dioxide Uptake:

• Use an indicator like hydrogencarbonate solution.
• Hydrogencarbonate changes colour depending on $CO₂$ levels (yellow in high CO₂, red in atmospheric levels, purple in low CO₂).
• Place plant leaves in sealed containers with hydrogencarbonate solution and monitor the colour change.

3. Measuring Biomass Increase:

• Measure the dry mass of a plant before and after a set period of growth.
• Dry the plant at 80°C until no further change in mass is recorded.
• Biomass increase indicates the rate of photosynthesis over time.

Controlling Variables

To ensure valid results:

• Light Intensity: Use a lamp at fixed distances to alter light intensity while using a lux meter to measure it accurately.
• $CO₂$ Concentration: Add a fixed amount of sodium hydrogencarbonate to maintain a constant $CO₂$level.
• Temperature: Conduct the experiment in a temperature-controlled environment or water bath to prevent fluctuations.

Investigating Limiting Factors

4. Light Intensity:

• Place the light source at varying distances from the plant.
• Measure the oxygen produced at each distance to assess how light intensity affects the rate of photosynthesis.

5. Carbon Dioxide Concentration:

• Use different concentrations of sodium hydrogencarbonate in the water.
• Measure oxygen production to determine the impact of $CO₂$ availability.

6. Temperature:

• Place the plant in water baths of different temperatures.
• Monitor oxygen production to identify the optimum temperature.

Data Analysis

• Plot graphs to analyse the relationship between limiting factors and the rate of photosynthesis.
• Identify trends, such as the plateauing of the rate when another factor becomes limiting.
• Understand the interaction between multiple limiting factors.

Common Errors & Precautions

• Ensure the plant is healthy and has been pre-illuminated to activate photosynthetic enzymes.
• Remove air bubbles from apparatus like gas syringes to improve accuracy.
• Repeat experiments and calculate mean values to improve reliability.