Question

Design an experiment to demonstrate that carbon dioxide is essential for photosynthesis. Write the observation and conclusion of the experiment.

Answer 1

Step 1: Purpose of the experiment:
To demonstrate that carbon dioxide (CO₂) is essential for the process of photosynthesis in plants.

Step 2: Preparation – Destarching the plant:
- Place a healthy potted plant in a dark room for 48 hours.
- This ensures that all pre-existing starch is used up, so that any new starch formed is due to photosynthesis during the experiment.

Step 3: Experimental setup:
- Cover one of the leaves of the plant using a conical flask containing potassium hydroxide (KOH).
- KOH absorbs carbon dioxide from the air inside the flask, creating a CO₂-free environment around that leaf.
- Leave another leaf exposed to the air (control).

Step 4: Exposure to sunlight:
- Place the plant in sunlight for a few hours to allow photosynthesis to occur.

Step 5: Testing for starch:
- Pluck both the KOH-covered leaf and the normal leaf.
- Boil the leaves in alcohol to remove chlorophyll (decolorize).
- Rinse in warm water to soften them.
- Add iodine solution to test for starch.

Step 6: Observation:
- The leaf that was inside the flask with KOH does not turn blue-black – showing no starch formation.
- The leaf that was exposed to air turns blue-black – indicating the presence of starch.

Step 7: Conclusion:
The experiment proves that carbon dioxide is essential for photosynthesis. Without CO₂, the plant cannot make starch, as seen in the leaf that was deprived of carbon dioxide.