AQA GCSE FOUNDATION Biology: Photosynthesis and Light Intensity

Investigating the Effect of Light Intensity on Photosynthesis

This practical investigates the relationship between light intensity and the rate of photosynthesis in aquatic plants.

You will need:

• Pondweed (e.g., Elodea)
• Beakers
• Measuring cylinder
• Light source (e.g., lamp)
• Ruler
• Stopwatch
• Water

Method:

1. Set up your experiment:
   • Fill a beaker with water and place a piece of pondweed inside.
   • Place the beaker under a light source.
   • Ensure the distance between the light source and the beaker can be adjusted.
2. Measure the initial light intensity:
   • Use a light meter to measure the light intensity at the beaker's surface.
   • Record this value.
3. Observe oxygen production:
   • Allow the pondweed to photosynthesize for a set time (e.g., 1 minute).
   • Count the number of oxygen bubbles produced during this time.
4. Repeat with different light intensities:
   • Adjust the distance between the light source and the beaker, changing the light intensity.
   • Repeat steps 2 and 3 at each new light intensity.
5. Calculate the rate of photosynthesis:
   • The rate of photosynthesis is the number of oxygen bubbles produced per unit time.
   • To calculate this, use the formula: Rate = 1 / Time
   • For example, if 20 bubbles are produced in 1 minute, the rate of photosynthesis is 20 bubbles/minute.

Data analysis:

1. Plot your results:
   • Create a graph with light intensity on the x-axis and the rate of photosynthesis on the y-axis.
2. Interpret your results:
   • Look at the relationship between light intensity and the rate of photosynthesis.
   • You should observe that as light intensity increases, the rate of photosynthesis increases until a certain point.
   • After this point, further increases in light intensity will have little effect on the rate of photosynthesis.

Safety precautions:

• Do not look directly at the light source.
• Handle glassware with care.

Variations:

• You can repeat the experiment with different types of pondweed to investigate their photosynthetic responses.
• You can use different light sources (e.g., LED, fluorescent) to compare their effect on photosynthesis.

Understanding the results:

• The relationship between light intensity and photosynthesis is directly proportional up to a certain point. This is because light is needed for the light-dependent reactions of photosynthesis, where light energy is converted into chemical energy.
• However, at very high light intensities, other factors, such as the availability of carbon dioxide, can limit the rate of photosynthesis.