How Would You Test the Presence of Starch in Leaves?

Plants make their food (glucose) through photosynthesis, but they store this extra food inside their leaves in a solid chemical form called Starch. To scientifically prove that a leaf has been performing photosynthesis in the sunlight, biologists use a highly famous chemical test using Iodine.

Question (Click to Flip)

Can I do this test on a potato?

Answer

Yes! Potatoes are massive underground storage tanks for starch. If you cut a raw potato and put a drop of iodine on it, it will instantly turn dark blue-black.

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Key Facts

If you take a potted plant and lock it inside a completely dark cupboard for 3 days before doing this test, the leaf will NOT turn blue-black. This proves that without sunlight, the plant cannot make starch.

Iodine solution is naturally a dark yellow/brown liquid. It only turns blue-black when it physically touches a starch molecule.

Step 1: Boiling in Water (Killing the Leaf)

First, pluck a green leaf from a plant that has been sitting in bright sunlight for a few hours.
Drop the leaf into a beaker of boiling water for 2 minutes.
Why?: Boiling physically kills the living cells, stops all chemical reactions inside the leaf, and softens the massive, tough cell walls.

Step 2: Boiling in Alcohol (Removing the Green Color)

Take the boiled leaf out and put it inside a small test tube filled with Ethanol (Alcohol).
Place this test tube into a hot water bath and let it boil. (Never heat alcohol directly over a flame, as it is highly flammable and will explode!)
Why?: The hot alcohol completely dissolves and extracts the green 'Chlorophyll' pigment. The leaf will turn completely pale, white, and brittle. This is necessary because if the leaf remains dark green, you will not be able to see the color change in the next step.

Step 3: The Iodine Test

Carefully remove the pale leaf, dip it in warm water to soften it, and place it flat on a white tile.
Drop 2-3 drops of Iodine solution directly onto the leaf.
The Result: If starch is present, the iodine will chemically react with it, instantly turning the pale leaf into a massive, deep Blue-Black color. If the leaf remains brown/yellow, it means there is absolutely zero starch.

Questions and Answers

Can I do this test on a potato?+

Yes! Potatoes are massive underground storage tanks for starch. If you cut a raw potato and put a drop of iodine on it, it will instantly turn dark blue-black.

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