How light affects photosynthesis

The other week we did a nice and fairly straightforward experiment to look at the factors affecting photosynthesis. For this we used some pondweed, water, baking powder and a light. Sounds rather straight forward doesn’t it? It mostly is apart from it not being entirely reliable. I found this quite a nice experiment to run with my GCSE group as it allowed them to get a hands on feeling for this. We only spent 30 minutes on this and this should have been longer. Around 2 out of 8 groups got reliable data. We followed this up with homework to repeat the experiment using an online tool.

Here is what you need and and do..

Large beaker, funnel, graduate test tube, light source (lamp), metre ruler, stopwatch, paperclip, sodium hydrogencarbonate solution (5g/l), sharp scissors, Elodea sample.

Elodea experiment


  1. Place your pondweed (Elodea) in a large beaker of water containing a pinch of sodium hydrogen carbonate (ensures a good supply of carbon dioxide in the water).

  2. Attach a paperclip to the uncut end of the pondweed. Makes the stem float upwards but keeps the plant at the bottom.

  3. Cut the end of the pondweed while it is under water

  4. Place the funnel over the weed to cover it completely. The end of the funnel should be under the water ā€“ see diagram.

  5. Using the metre ruler ā€“ position the light 10cm away from the plant and leave for 5 minutes to adjust to surroundings.

  6. Whilst waiting: Write a prediction. How will the rate of bubbles change as you move the light away?

  7. Whilst waiting: Create a table of results ā€“ Distance away (cm), Number of bubbles

  8. Count the number of bubbles given off by the stem in one minute and record your answer.

  9. Now move to 20cm, 30cm, 40cm. Record your result in a table. Make sure you give the plant 5 minutes at each new light intensity before you start to measure.


Questions (answer in book):

What do you notice?

Does this accurately represent the rate of photosynthesis?

What factor may also play a part as the lamp is moved closer to the plant? How would this affect your results? How could you overcome this?


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