Required Practical: Light Intensity and the Rate of Photosynthesis

The rate of photosynthesis in a plant can be measured with the following simple experiment.

A graphical representation of the oxygen production by photosynthesis. A beaker of water contains pond weed, which is illuminated from both sides by lamps. The lamps emit light, signifying light energy, which is absorbed by the pond weed. A thermometer is immersed in the beaker to monitor the temperature. As photosynthesis occurs, oxygen bubbles rise from the pond weed. Below the setup, there's an equation showing the process of photosynthesis: Carbon dioxide and water, with light energy, produce glucose and oxygen, represented by the chemical formulas CO2 + 6H2O → C6H12O6 + 6O2.

Method:

1. Set up the apparatus above, placing the funnel upside down in the beaker. However, do not insert the measuring tube into the beaker until the pondweed has adjusted

2. Cut a piece of pondweed around 7 cm long and use forceps to place it in the beaker.

3. Using a ruler, place the lamp 15 cm away from the beaker containing the pondweed. The lamp does not have to be placed like it is in the diagram, as long as it is directed at the pondweed in the beaker

4. Leave the apparatus for around 10 minutes to allow the pondweed to adjust.

5. Position the boiling tube upside down in the beaker, over the upside-down funnel. We will use the boiling tube to count the number of bubbles released, which is oxygen gas

6. Count the number of bubbles in the boiling tube after one minute

7. Repeat the count five times, recording your results in a table, such as the one below:

DistanceNumber of bubbles (1)Number of bubbles (2)Number of bubbles (3)
15 cm
20 cm
25 cm
30 cm
35 cm

8. Repeat the experiment at different distances from the lamp

9. Plot the results on a graph to see a pattern or trend

Alternatively, you can use a gas syringe to measure the volume of oxygen produced by the pondweed, instead of counting bubbles. This will give you more accurate results. If you decide to take this approach, you can plot a graph:

  • X-axis = Distance from the lamp
  • Y-axis = Change in gas volume

Variables

Independent variable – The distance from the lamp (light intensity)

Dependent variable – The number of bubbles (oxygen) produced per minute

Control variables – Maintain a consistent temperature and use the same piece of pondweed throughout.

Safety Precautions

Ensure your hands are dry before handling the lamp to prevent electric shocks.

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