Demonstrate that carbon dioxide is necessary for photosynthesis by moll’s half leaf experiment

Requirements:

Apparatus: A wide-mouthed bottle, split cork, Petri dish, beaker, test tube, sprit lamp.

Material:  Destarch plant with healthy leaf.

Chemicals: water, alcohol, iodine solution, Vaseline, KOH.

Theory:

Photosynthesis (G.K photo-light, synthesis production) is the anabolic process in which green plants produce their food in the presence of sunlight and chlorophyll and produce glucose and oxygen. The volume of oxygen released during photosynthesis is co-equal to the volume of carbon dioxide absorbed. The released oxygen is exclusively formed from water and not from carbon dioxide. The process of photosynthesis can be represented by the following chemical equation.

The overall reaction is not a single and simple reaction between carbon dioxide and Water but a highly complex process in which a number of reactions occur. In it, the radiant energy absorbed by chlorophyll is transferred into chemical energy which is available for all vital activities of living cells. Photosynthesis consists of oxidation and reduction. The overall process of photosynthesis is completed into two main phases.

1. Light reaction or Hill reaction
2. Dark reaction or Blackman reaction

Procedure:  A wide-mouthed bottle with a split cork in two equal halves is taken. The bottle Is partially filled up with KOH solution. A leaf detached from a plant previously kept half in darkness for at least 48 hours is pressed in between the two halves of the cork of the bottle so that half of the leaf remains within the bottle and the other half outside the bottle. The petiole of the leaf outside is kept in a water-filled beaker. So that the leaf may not become dry soon. The apparatus is made airtight by applying Vaseline so that the atmospheric air may not enter the bottle. Thereafter the apparatus is kept in the sunlight for photosynthesis.

Test for starch:  Boil the experimental leaf in a test tube containing alcohol for five minutes to decolorized the leaf by making a water bath. Wash the leaf with water to remove the alcohol. Place the leaf on a Petri dish containing iodine solution.

Observation: After the starch test, the portion of the leaf outside the bottle turns blue-black in color and color remains unchanged in a portion of the leaf that kept inside the bottle.

Result of conclusion: The carbon dioxide within the bottle is absorbed by KOH solution and in the absence of CO_2, the photosynthesis does not take place and the starch is not formed. The portion of the leaf which remained outside the bottle could receive all the necessary factors for photosynthesis and the photosynthesis did take place in the portion forming starch. The portion of the leaf gives a positive starch test and becomes blue-black in contact with iodine solution. This way this experiment proves that CO₂ is necessary for the process of photosynthesis.

Precautions:

1. The leaf should be de-starched.
2. The mouth of the bottle should be sealed completely.
3. The portion of the leaf inside the bottle should not touch the KOH solution.
4. Setup should be placed in the light.
5. The decolorized leaf should be washed with water before the iodine test.