IGCSE Biology / Plants and food
Practical: the starch test revision notes
Edexcel International GCSE Biology (4BI1), chapter 10: Plants and food, specification point 2.23. Free lesson.
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The starch test shows whether a leaf contains starch, which is evidence that photosynthesis has made glucose. Destarching the plant first makes the result reliable.
Why destarch the plant first
A plant may already contain starch from earlier photosynthesis. Leave it in the dark for two days before testing, so it uses up or moves away this stored starch. Then any starch found must have been made during the experiment. Without destarching, a positive result proves nothing because the starch may have been present before the experiment.
How to prepare the leaf
Follow the steps in order. Boiling water kills the leaf's cells, stops reactions and breaks down cell walls so chemicals can enter. Ethanol removes the green chlorophyll, making the iodine colour easier to see.
Test for starch with iodine
Spread the softened leaf on a white tile and add iodine solution. The colour change shows whether starch is present. Blue-black means starch is present. Orange-brown means starch is absent.
Use the test to show that light is needed
If part of a leaf is covered with black paper while the plant is left in light for six hours, only the uncovered part receives light. It can photosynthesise, make glucose and form starch. The covered part cannot do this. The uncovered part turns blue-black with iodine. The covered part stays orange-brown.