Proving photosynthesis only occurs in the light

Aim

To experimentally prove that photosynthesis only occurs in the light.

Method

  1. Half cover a geranium plant leaf (or similar) with silver foil.
  2. Place the plant on a sunny windowsill for 48 hours.
  3. Drop the leaf into boiling water to kill and preserve it.
  4. Then place it in hot ethanol in a boiling tube for ten minutes to remove the chlorophyll.
  5. Dip the leaf in boiling water to soften it.
  6. Spread the leaf out in a Petri dish and covered with iodine solution.
  7. Record which areas remain the brown colour of the iodine stain and which are changed to blue/black as a result of the presence of starch.
An image showing the stages of leaf when dropped in boiling water.

Risks

Care must be taken when using boiling ethanol. Make sure that no Bunsen burners are turned on as the ethanol is highly flammable.

Results

The half of the leaf under the silver foil remained the brown colour of the iodine stain. This is because no photosynthesis occurred, so no glucose was produced and none was stored as starch. The half of the leaf not covered by the silver foil turned blue/black in the presence of iodine. This proves that glucose was produced during photosynthesis and was stored by the plant as starch.

curriculum-key-fact
Iodine tests for starch which is made by the plant from glucose produced during photosynthesis.