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Experiment with a pond weeds light source and see how it affects photosynthesis!

Click here to experiment in a larger window, or click Fullscreen below to experiment in full screen.

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Photosynthesis is the means by which plants can convert carbon dioxide and water to oxygen and, most importantly, glucose. This happens when sunlight hits a pigment called chlorophyll, which is stored in the chloroplasts in plant cells.



Plants then use the glucose in a number of different ways:
- Respire the glucose to provide energy
- Use glucose as a building block for making cellulose, which is used for producing cell walls .
- Store the glucose as starch, so that the plant can use the glucose at another time.
- Combine with other molecules to make either fatty acids or amino acids.

There are a number of factors which affect the rate of photosynthesis, and one of them is light intensity. This interactive science experiment allows us to discover the relationship between these.

We can measure the rate of photosynthesis in a number of ways, and one of them is to use an underwater plant, such as Canadian pondweed (Elodea canadensis). We can then count the number of bubbles being released from the plant each minute.

1. Complete the experiment above. The method of how to measure the rate of photosynthesis is included with the experiment. Remember that the more measurements you take at each distance away from the light, the more accurate and reliable your data will be.

2. Plot your results on a graph, with a suitable length axes and a meaningful title. Remember to state what is being measured on each axis, and to include units of measurement. Then, draw a line of best fit. An example is below.

3. From the graph, work out the mean number of bubbles produced at each distance.

4. How does the distance from the light source relate to the light intensity?

5. What gas do you think the bubbles contain?

6. Using your data and your graph, describe the trend that it shows. Why do you think this trend occurs?
  i) Can you think of any other factors, apart from light, which could affect the rate of photosynthesis in Elodea canadensis?
 ii) How do these factors affect photosynthesis?

How could you improve this experiment to achieve:
  i) more accurate results
  ii) more precise results
  iii) more reliable results?

7. Extension activity: Have a go at rewriting the experiment for one of the two factors you mentioned in question 6.
  i) How would you adjust the experiment to reduce errors in your method?
  ii) How about reducing uncertainties in your measurements?