The steps below are involved in the testing of a leaf for starch. Study them carefully and answer the…

Analyze the question

You are testing a leaf for starch. Starch is a product of photosynthesis, so the question is really asking: when does a leaf contain starch that we can detect?

Concept

Photosynthesis needs light:

CO₂ + H₂O → (light, chlorophyll) → C₆H₁₂O₆ → stored as starch

• In sunlight, the leaf makes glucose and stores the excess as starch.
• In darkness, photosynthesis stops, and the plant uses up or moves out its stored starch.

The standard test is:

1. Place the leaf in boiling water (kills the leaf and stops reactions, and softens the tissues). (Step III)
2. Decolorize it in hot ethanol/alcohol to remove chlorophyll. (Step I)
3. Dip it in warm water to soften it again. (Step II)
4. Add iodine solution. A blue-black color shows starch is present. (Step IV)

The correct order is III → I → II → IV.

Solution: when to pluck the leaf

The aim is to show that a leaf has starch. Starch is only reliably present after the plant has photosynthesized, which means after exposure to sunlight.

• B, C, D (after sunset or before sunrise): The plant has had no light for hours. Starch would have been used up or translocated, so the test would be negative or weak. These are the times you pluck a leaf to get a destarched leaf for a control, not to show starch.
• A (after being exposed to sunlight): The leaf has photosynthesized and stored starch, so the iodine test gives a blue-black color.

Answer: A

Tip: Remember "No light, no starch." If the question asks for a leaf that has starch, choose after sunlight. If it asks for a leaf that is starch-free, choose after darkness (destarching), such as before sunrise.