Investigating the effect of light intensity on photosynthesis rate

Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen. Light intensity can affect the rate of photosynthesis, and this can be investigated using an aquatic plant, such as Cabomba or Canadian pondweed. During photosynthesis, the plant releases oxygen, and by measuring the oxygen production, we can determine the rate of photosynthesis.

Once the experiment is complete, you will have collected data on the number of oxygen bubbles produced (or the volume of oxygen if using a gas syringe) at different light intensities (distances from the light source). This data can be analysed to understand how light intensity affects the rate of photosynthesis.

Organising the Data

• Create a table to record your results, showing the distance from the light source and the corresponding average number of bubbles (or volume of oxygen produced). Example of a table format:

Distance from Light Source (cm)	Number of Bubbles per Minute (Trial 1)	Number of Bubbles per Minute (Trial 2)	Number of Bubbles per Minute (Trial 3)	Average Number of Bubbles per Minute
10 cm	15	17	16	16.0
20 cm	10	12	11	11.0
30 cm	7	8	7	7.3
40 cm	4	5	4	4.3

Calculating Averages

For each distance, calculate the average number of bubbles per minute by adding the bubble counts from each trial and dividing by the number of trials. This helps reduce the effect of any anomalies or outliers in your data.

Graphing the Data

• Plot a graph with the distance from the light source on the x-axis and the average number of bubbles per minute (or volume of oxygen produced) on the y-axis.
• The graph should show how the rate of photosynthesis changes as light intensity decreases (as the distance from the light increases).

Interpreting the Graph

• Trend: You are likely to observe that as the distance from the light source increases, the rate of photosynthesis decreases. This is because light intensity decreases as the distance from the light source increases, reducing the energy available for photosynthesis.
• Shape of the Graph: You might see a linear relationship at first, where the rate of photosynthesis decreases steadily as distance increases. However, beyond a certain distance, the rate of photosynthesis may level off because the light intensity becomes too low to sustain significant photosynthesis.
• Conclusion: The data should support the conclusion that higher light intensity (closer distance to the light source) increases the rate of photosynthesis, while lower light intensity (greater distance) decreases it.

Further Analysis

• Limiting Factors: Consider that light intensity is just one factor affecting photosynthesis. If light intensity is too low, it may become a limiting factor, slowing the rate of photosynthesis. However, at high light intensities, other factors (e.g., CO₂ concentration, temperature) may become limiting, preventing further increases in the rate of photosynthesis.
• Sources of Error: Identify any potential errors in your experiment, such as:
  • Human error in counting bubbles.
  • Variations in temperature or CO₂ concentration.
  • Inconsistencies in the light source (e.g., flickering).

Variables

Independent Variable:

• The distance from the light source (or light intensity). By changing the distance, you alter the intensity of light reaching the plant. Dependent Variable:

• The number of bubbles produced per minute, which corresponds to the rate of oxygen production and hence the rate of photosynthesis. Control Variables:

• Concentration of sodium hydrogen carbonate solution: Ensure it remains constant 1% to provide a consistent CO₂ supply.

• Temperature: Keep the temperature stable, as changes in temperature could affect the rate of photosynthesis.

• Same piece of Cabomba pondweed: Use the same pondweed throughout the experiment to maintain consistency.