IGCSE Biology / Plants and food
Practical: light, carbon dioxide and chlorophyll revision notes
Edexcel International GCSE Biology (4BI1), chapter 10: Plants and food, specification point 2.23. Free lesson.
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Photosynthesis investigations remove one factor at a time and test leaves for starch, or measure oxygen released by pondweed. These methods show which conditions are needed and how the rate changes with light intensity.
How to test whether light, carbon dioxide and chlorophyll are needed
Each test starts by destarching the plant, which means using up its stored starch. Leave it in the dark for two days. Then remove one factor, give the plant bright light for several hours and test the leaf for starch using the ethanol method. To remove light, cover part of a leaf with black paper. To remove carbon dioxide, enclose a leaf in a flask with soda lime, which absorbs carbon dioxide from the air inside. Use a control flask containing sodium hydrogencarbonate solution instead of soda lime. This checks that any difference is caused by the lack of carbon dioxide, not by…
What the starch test shows
After the plant has been in bright light for several hours, test the leaves for starch. Iodine turns blue-black where starch is present and stays orange-brown where it is absent. The results show that starch is made only in the parts that had access to all three factors. The area covered by black paper stays orange-brown, so light is needed for starch to form. The leaf enclosed in the flask with soda lime stays orange-brown, so carbon dioxide is needed. The white parts of the variegated leaf stay orange-brown, while green parts turn blue-black, showing that chlorophyll is needed.
How to measure photosynthesis with pondweed
The oxygen released by Canadian pondweed can be used to estimate the rate of photosynthesis. Sodium hydrogencarbonate solution supplies carbon dioxide. Keep other conditions controlled so that changing the lamp distance tests the effect of light.
How the rate changes with light intensity
As light intensity increases, the pondweed releases oxygen faster at first. The graph then levels off because carbon dioxide or temperature becomes limiting. On the rising part of the graph, light intensity is limiting photosynthesis. On the flat part, another factor, such as carbon dioxide or temperature, is limiting the rate. Bubble counts estimate the rate, but measuring the volume of oxygen collected is more accurate because bubbles can be different sizes.