Starch testing in leaves
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of uses of glucose from photosynthesis and biochemical tests.
What storage molecule is glucose converted to in plants?
Starch.
What chemical is used to test for starch in leaves?
Iodine solution.
What colour does iodine solution turn if starch is present?
Blue-black.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains starch testing in leaves, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Why Test a leaf for Starch?
Starch is produced from glucose made during photosynthesis. Testing for starch shows whether photosynthesis has taken place in a leaf.
Method for Starch Test in Leaves
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Boil the leaf in water for 1–2 minutes.
This kills the leaf, stopping all chemical reactions, including photosynthesis. -
Boil the leaf in ethanol (in a boiling tube) in a hot water bath for 5–10 minutes.
This removes chlorophyll, making the leaf pale so colour change is easier to see. Ethanol is flammable – use a hot water bath – rather than a Bunsen burner. -
Rinse the leaf in warm water.
Softens the brittle leaf after ethanol treatment. -
Place the leaf flat on a white tile and cover with iodine solution.
Iodine turns blue-black if starch is present. -
If iodine stays yellow / brown this indicates no starch, and therefore no photosynthesis had been taking place (as there was no glucose made to convert to starch).
Example 1: Variegated Leaf (Green and White Areas)
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Only the green areas contain chlorophyll.
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After the starch test, only the green areas turn blue-black, indicating the presence of starch.
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This shows photosynthesis only happens in parts of the leaf with chlorophyll.
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As chlorophyll is needed to absorb light energy for photosynthesis.
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Example 2: Leaf Covered with Opaque Card or Foil
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A strip of black card or foil is fixed across the leaf during light exposure.
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After testing with iodine solution, the exposed parts turn blue-black as they absorb light for photosynthesis and make glucose which they convert into starch.
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The part of the leaf under the card does not turn blue-black. The iodine solution will remain yellow / brown as there is no starch present.
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This proves light energy is needed for photosynthesis.
Key Terms
- Starch - Insoluble carbohydrate made from glucose and stored in plants.
- Iodine solution - Chemical that tests for starch, turns blue-black if present.
- Variegated leaf - A leaf with both green and white areas (only green areas photosynthesise as white areas have no chlorophyll).
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Exam Tip
Always explain that photosynthesis makes glucose, but that glucose is then converted or stored as starch. This is why we can test a leaf for starch to see if photosynthesis has been occurring.
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Practice Question
A student carried out a starch test on a variegated leaf. Only the green parts turned blue-black. Explain why this happened. [4 marks]
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Only the green parts contain chlorophyll (1).
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Which is needed to absorb light for photosynthesis (1).
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Only those parts can make glucose (1).
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Which is then stored as starch (1).
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!