Edexcel IGCSE Biology · Spec 2.23
Photosynthesis Practical
Required practical: measuring the rate of photosynthesis in pondweed and how light intensity affects it.
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Photosynthesis Practical
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Explained
Counting bubbles, and testing a leaf for starch
Photosynthesis produces oxygen, so if you can count the oxygen you can measure the rate. Water plants make this easy, because the gas comes off as visible bubbles.
The pondweed method
Put a piece of pondweed such as Elodea in water containing sodium hydrogencarbonate, which supplies dissolved carbon dioxide so that the carbon dioxide never runs short. Shine a lamp at it and count the bubbles given off in a minute.
Move the lamp to different distances and repeat. Light intensity falls as distance increases, so the bubble count falls with it.
Place a beaker of water, or a glass sheet, between the lamp and the pondweed. This is a heat shield, and it absorbs the infrared so the lamp does not warm the water. Without it you would be changing the temperature as well as the light intensity, and you could not say which caused the change in rate.
Leave the pondweed a minute or two to settle after each move before counting, and repeat each distance to take a mean.
Limitations of counting bubbles
Bubbles are not all the same size, so counting them measures volume only roughly. Collecting the gas in a syringe or a capillary tube and measuring its volume is more precise, and mentioning that is the standard improvement.
The bubbles are also not pure oxygen, though they are mostly oxygen, and the plant is respiring at the same time, so what you are really measuring is the net rate.
Testing a leaf for starch
The leaf is first destarched by leaving the plant in the dark for a day or two, so any starch found afterwards must have been made during the experiment.
Then boil the leaf in water to kill it and break down the cell membranes, heat it in ethanol to remove the chlorophyll so the colour change is visible, rinse it in water to soften it, and add iodine solution. Blue black means starch is present; orange brown means it is not.
The ethanol is heated in a water bath rather than over a flame, because ethanol is flammable. That is a standard safety mark.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks why a plant is destarched before the experiment, for two marks. It credits showing that no starch is present or that the starch has been used up, that this happens by respiration, and that it makes the test fair or valid because any starch produced afterwards must be from photosynthesis.
That last clause is the reason, and it is the one usually missing. Destarching is not preparation; it is the control.
For the negative result, the same mark scheme wants the chain rather than the observation: no photosynthesis because there was no carbon dioxide, so no carbohydrate or starch was made, so the iodine solution stayed unchanged.
On plan questions of this kind, the mark scheme awards separate marks for how the variable is changed, for the organisms being the same species and size, for repeating and taking a mean, for what is measured, for a stated time period, and for the variables kept the same. On one such question it notes that candidates must state how the variable is changed, that counting alone is not enough without a time, and that certain plausible controls are simply ignored because they are not relevant to that experiment.
Turning the results into a graph
Plot the number of bubbles per minute against light intensity, not against distance, because the two are not proportional. Light intensity is proportional to one over the distance squared, so doubling the distance quarters the intensity.
The curve rises steeply at first, then levels off. Light is the limiting factor on the rising part, and something else, usually carbon dioxide or temperature, is limiting once it flattens. Naming which factor is limiting where is what the graph question is testing.
Spec 2.23
What you need to know
- Measure the rate of photosynthesis
- Describe the pondweed method
- Explain how light intensity affects the rate
Active recall
Quick check
Answer each question before opening the answer.
How can you show a leaf needs light to make starch?
Destarch the plant, cover part of a leaf, leave in light, then test with iodine — the covered part stays orange (no starch).
Why is the leaf boiled in ethanol during the test?
To remove the chlorophyll so the iodine colour change can be seen clearly.
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