Experiments to investigate photosynthesis

Experiment 1 - The effect of chlorophyll

The effect of the presence or absence of chlorophyll on photosynthesis can be investigated using a variegated plant. Variegated plants have regions of their leaves with, and without, chlorophyll.

Only those areas of the leaf with chlorophyll photosynthesise. They will test positive for starch, which is built up from the glucose produced.

Aim

To investigate photosynthesis in areas of a leaf with and without chlorophyll

An image showing the stages of leaf when dropped in boiling water.

Method

  1. The leaf of a variegated Pelargonium is dropped in boiling water to kill and preserve it.
  2. The leaf is left for 10 minutes in hot ethanol in a boiling tube to remove chlorophyll.
  3. The leaf is dipped in boiling water to soften it.
  4. The leaf is spread out in a Petri dish and covered with iodine solution.
  5. The areas that had the chlorophyll stain blue-black.
  6. The areas that had no chlorophyll remain pale.

Variables

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

Risks

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.

Results

Starch is produced from glucose made during photosynthesis. Areas of the leaf with chlorophyll should turn blue–black when iodine is added proving starch is present. This in turn, proves that photosynthesis occurred. The white areas of the leaf without chlorophyll 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.