Chlorophyll is Necessary for Photosynthesis – Variegated Leaf Experiment

Aim: To show that chlorophyll is necessary for photosynthesis.

Materials: A variegated plant (e.g., Coleus or Croton), iodine solution, ethanol, beaker, water bath.

Procedure:

1. Keep the variegated plant in the dark for 48–72 hours to remove all existing starch (destarching).
2. Expose the plant to sunlight for several hours.
3. Draw an outline map of the leaf, noting which parts are green and which are white/yellow.
4. Pluck the leaf and boil it in water for a few minutes to kill and soften the leaf cells.
5. Place the leaf in boiling ethanol (alcohol) to remove the green chlorophyll pigment (leaf becomes pale/whitish).
6. Remove the decolourised leaf and dip it in water to rehydrate.
7. Add a few drops of iodine solution to the leaf.

Observation:

• Green parts (had chlorophyll): Turn blue-black → presence of starch → photosynthesis occurred
• White/pale parts (no chlorophyll): Remain brown/orange → no starch → photosynthesis did NOT occur

Conclusion: Chlorophyll is essential for photosynthesis. Only regions containing chlorophyll can produce starch.

Chlorophyll is the green pigment found in the chloroplasts of plant cells. It is essential for photosynthesis because:

1. Light absorption: Chlorophyll absorbs sunlight, mainly in the red (680 nm) and blue-violet (430 nm) wavelengths. This light energy is used to drive the light reactions of photosynthesis.

2. Energy conversion: The absorbed light energy is used to split water molecules (photolysis) and generate ATP and NADPH, the energy carriers used in the Calvin Cycle to synthesise glucose.

3. Electron transport: Chlorophyll a in the reaction centres (P680 and P700) loses electrons that initiate the electron transport chain.

Without chlorophyll, plants cannot capture solar energy and therefore cannot produce food (glucose) by photosynthesis.

The iodine test is used to detect the presence of starch:

• Iodine solution (iodine dissolved in potassium iodide): Brown/orange colour
• When iodine reacts with starch: Turns blue-black
• When there is no starch: Remains brown/orange

Starch is the storage product of photosynthesis in plants. If starch is present, photosynthesis has occurred. The iodine test is the standard method to verify whether photosynthesis has taken place in a plant tissue.

While chlorophyll is necessary, photosynthesis also requires:

1. Sunlight: Energy source for the light reactions
2. Carbon dioxide (CO₂): Raw material for glucose synthesis in the Calvin Cycle
3. Water (H₂O): Split during photolysis; source of oxygen released

The overall equation for photosynthesis: 6CO₂ + 6H₂O + Light energy → C₆H₁₂O₆ (glucose) + 6O₂