Experiment 2 - Light exclusion

A similar method described on the previous page can be used to prove that photosynthesis doesn't occur in parts of leaves that have been kept in the dark.


To determine that photosynthesis only occurs in areas of a leaf exposed to the light.


  1. Wrap silver foil around half a leaf and leave the plant in its normal position for 48 hours.
  2. Remove the leaf from the plant and place in boiling water to kill and preserve it.
  3. The leaf is left for 10 minutes in hot ethanol in a boiling tube to remove chlorophyll.
  4. The leaf is dipped in boiling water to soften it.
  5. The leaf is spread out in a Petri dish and covered with iodine solution.


The areas that were exposed to the light stain blue-black.

The areas that were covered by the silver foil remain pale.


  • Independent variable – the presence or absence of light.
  • Dependent variable – whether starch is present or not.
  • Control variables – methods and equipment used.


Care must be taken when using boiling water.

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

Iodine solution is an irritant.

Eye protection must be worn.


Starch is produced from glucose made during photosynthesis. Areas of the leaf exposed to the light should turn blue–black when iodine is added proving starch is present. This in turn, proves that photosynthesis occurred. The areas of the leaf that were covered by the silver foil could not photosynthesise. This means they could not produce glucose and turn this to starch. So, when tested with iodine, these areas should remain pale.