Investigating photosynthesis – starch and chlorophyll

Photosynthesis can be investigated to show the production of starch and the importance of chlorophyll.

Starch testing

Iodine solution is used to test leaves for the presence of starch. You need to:

  1. heat a plant leaf in boiling water for 30 seconds (this stops its chemical reactions)
  2. heat it in boiling ethanol for a few minutes (this removes most of its colour)
  3. wash with water and spread onto a white tile
  4. add iodine solution from a dropping pipette

After a few minutes, the parts of the leaf that contain starch turn blue-black.

Note that ethanol is heated using a hot water bath. Ethanol boils at 78°C, so a tube of it boils when placed in a beaker of hot water. This is safer than using a Bunsen burner because ethanol is flammable.

Variegated leaves have green parts (where the cells contain chlorophyll) and white parts (where there is no chlorophyll). Only the parts that were green become blue-black with iodine solution, showing the importance of chlorophyll in photosynthesis.

A plant can be ‘de-starched’ by leaving it in the dark for a few hours. Parts of its leaves are covered with dark paper, and the plant is left in the light for a few hours. Only the uncovered parts become blue-black with iodine solution, showing the importance of light in photosynthesis.

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