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The effect of light intensity on the rate of photosynthesis


(* private, @ shared)
* Boiling tube * plasticine or other weight
* sprig of Canadian pondweed * squared paper
@ Dilute solution of sodium hydrogencarbonate (sodium bicarbonate)
@ Bench lamp @ Ruler @ stop clock


1) Anchor the pondweed to the bottom of the boiling tube, with the cut end projecting upwards. Three-quarters fill the tube with sodium hydrogencarbonate solution.

2) Place the boiling tube 30 cm from a bench lamp. [Optional- measure the light intensity].

3) Wait two minutes.

4) Count the number of bubbles rising from the cut end during the next five minutes.

5) Record your results in an orderly fashion (table).

6) Repeat steps 2-5 but with the tube at 60 cm., then 90 cm., and finally 120 cm. from the light.

7) Plot your results on the squared paper provided.

8) If you have time, you may repeat the experiment, but with different distances.

Interpretation of results

1) Why must you wait two minutes before counting the bubbles?

>To allow the pondweed to adjust to the new conditions and make sure oxygen is bubbling out, not just filling a gap in the stem.

2) Does the distance between the lamp and the plant affect the rate of bubble production? Is there a straightforward relationship?

>It is likely to follow the inverse square law, so doubling the distance between light and plant should give a quarter of the number of bubbles!

3) What other factor might have affected the rate of photosynthesis?

>Amount of dissolved CO2 (=hydrogencarbonate ions), temperature, (minerals?)

4) How could the experiment be improved?

>Catch the bubbles in a capillary, and measure their volume?

5) Why was sodium hydrogencarbonate used?

>As a source of CO2

6) What were the bubbles made of?

How would you prove it?
>Glowing splint test (if you can get enough!)

7) How can extra experimental readings be made which are most significant? Would these require any special procedure?

>Replication - Repeats - best done x2 or x3 in gradual steps from low intensity to high

8) Do you think that these results apply to ordinary land plants? Give reasons.

>Should do but no-one sees the gas they give out.

9) What is the advantage of Canadian pondweed in a fishtank? Does it need any special treatment?

>It is an oxygenator. Must have the light on regularly, and be cut back if it grows too much

Web links

Photolab an excellent animation from kscience.co.uk

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