Plants make food using photosynthesis. This needs light, carbon dioxide and water. It produces glucose, and oxygen as a by-product. Leaves are adapted to carry out photosynthesis.
This is an example of a common
experiment used to investigate light intensity and the rate of
photosynthesis. This should help you understand how to work
scientifically.
Aim of the experiment:To use pondweed to see how light intensity affects the rate of photosynthesis.
Method
- Set up the apparatus as in the diagram.
- Leave for five minutes for the pondweed to acclimatise to the new light intensity.
- Count the number of bubbles given off in one minute.
- Move the light 10 cm further back.
- Leave for five minutes for the pondweed to acclimatise again.
- Count the number of bubbles given off in one minute.
- Repeat by moving the lamp away by 10 cm intervals until 50 cm is reached.
Variables
- Independent variable - the light intensity (how close the light is).
- Dependant variable - the number of oxygen bubbles given off (the rate of photosynthesis).
- Controlled variables - the size of the pondweed, the volume of water used and its temperature.
Risks:Care must be taken with glassware.
Expected results
| Distance from lamp to beaker (cm) | Number of bubbles given off per minute |
|---|---|
| 10 | 15 |
| 20 | 7 |
| 30 | 3 |
| 40 | 1 |
| 50 | 0 |
What the results mean
More oxygen bubbles are
produced when the light is closer to the beaker. So a greater light
intensity gives a greater rate of photosynthesis.
Evaluation
- Your measurements are accurate if they are close to their true value.
- Your measurements are precise if they are similar when completed again.
- Your experiment is repeatable if you get precise measurements when it is repeated.
- Your experiment is reproducible if others get precise measurements when they repeat it.
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