Showing experimentally that light is necessary for photosynthesis

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Aim

To show experimentally that light is necessary for photosynthesis.

Materials Required

A potted plant, black paper strips or Gangong’s light screen, clips or cellotape, alcohol, iodine solution, water bath, beaker, burner, water, petri dish, dropper.

Theory

Light is one of the requirements for photosynthesis. When a part of the leaf is covered from both upper and lower sides with a black paper strip, light will not reach to covered part. Hence, photosynthesis will not take place in covered part of the leaf. On the other hand, as light will be available to uncovered part of the leaf, cells of this region will carry out photosynthesis actively and hence, glucose will be sythesised in the uncovered part of the leaf. In the leaf a part of this glucose is utilised to carry out various metabolic activities and rest of the amount converts into starch. Presence of starch in the leaf, which can be detected by performing iodine test, indicates occurrence of photosynthesis.

Procedure

• Select a healthy potted plant and destarch the plant by placing it in complete darkness for at least 48 hours.
• After 48 hours, using clips or cellotape cover a part of the leaf of the plant from both upper and lower sides with black paper strips.
• Now, keep the potted plant in sunlight for atleast 3-4 hours.
• After 3-4 hours, pluck the leaf and remove black paper strips.
• Boil the leaf in water for about 2 minutes.
• Now, to decolourise the leaf, transfer the leaf from boiling water to a beaker containing ethanol and boil the leaf in a water bath.
• Remove the leaf from beaker and wash it in water at room temperature.
• Place this leaf in petri dish and add few drops of iodine solution over the decolourised leaf.
• The area of the leaf that was uncovered performed photosynthesis and will have starch, therefore turned blue-black with iodine solution.
• The area of the led that was uncovered performed photosynthesis and will have starch, therefore turned blue-black with iodine solution.
• The covered portion of the leaf remains yellow as it does not perform photosynthesis.

Observations

• The uncovered part of the leaf turns blue-black. This confirms the presence of starch.
• The covered portion of the leaf remains yellow showing that no starch synthesis occured in this region.

Result

This proves that light is necessary for the photosynthesis.

Precautions

• Use healthy potted plant.
• Plant should be destarched by placing in the dark.
• The black paper strip should be attached tightly on both sides of leaf so that no sunlight enters in covered portion of leaf.
• Always use a water bath for boiling leaf in alcohol as alcohol is inflammable.
• Wash the leaf before performing iodine test.

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