Investigating the effect of light intensityon photosynthesis rate (Edexcel GCSE Biology): Revision Notes
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.
Aim To investigate the effect of light intensity on the rate of photosynthesis.
Method
- Set Up the Apparatus:
- Fill a boiling tube with 45 cm³ of sodium hydrogen carbonate solution (1%). This solution provides carbon dioxide, which is essential for photosynthesis.
- Let the boiling tube stand for a few minutes and gently shake to disperse any air bubbles that might form.
- Prepare the Pondweed:
- Cut a piece of Cabomba pondweed to a length of 8 cm.
- Using forceps, carefully place the pondweed in the boiling tube with the cut end facing upwards. Avoid damaging the plant or causing the solution to overflow.
- Position the Light Source:
- Place the boiling tube so that the pondweed is 10 cm away from a light source (such as a lamp).
- Let the apparatus stand for five minutes to allow the plant to acclimate and start photosynthesising.
- Measure the Rate of Photosynthesis:
- Count the number of oxygen bubbles emerging from the cut end of the pondweed over one minute. The bubbles represent the oxygen produced during photosynthesis.
- Repeat this measurement five times and record the results.
- Calculate the average number of bubbles produced per minute.
- Repeat at Different Distances:
- Move the light source to different distances (e.g., 20 cm, 30 cm, 40 cm) from the pondweed.
- For each distance, repeat the steps of counting the bubbles and calculating the average to determine how light intensity affects the photosynthesis rate.
Data Analysis
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).
Conclusion
- The analysis should confirm that light intensity directly influences the rate of photosynthesis. Closer light sources (higher intensity) result in higher rates of photosynthesis, shown by more bubbles or greater oxygen production.
- You may also discuss the possible plateau effect where further increases in light intensity no longer increase the photosynthesis rate due to limiting factors.
Variables
Independent Variable:
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The distance from the light source (or light intensity). By changing the distance, you alter the intensity of light reaching the plant. Dependent Variable:
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The number of bubbles produced per minute, which corresponds to the rate of oxygen production and hence the rate of photosynthesis. Control Variables:
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Concentration of sodium hydrogen carbonate solution: Ensure it remains constant 1% to provide a consistent CO₂ supply.
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Temperature: Keep the temperature stable, as changes in temperature could affect the rate of photosynthesis.
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Same piece of Cabomba pondweed: Use the same pondweed throughout the experiment to maintain consistency.