Iodine Staining Differentiates Starch from Cell Wall

Steps:

• Iodine solution tests for starch by forming a colored complex with it.
• Starch grains in plant cells turn blue-black due to the iodine-starch reaction.
• Cell walls, composed of cellulose, do not react with iodine.
• Thus, cell walls retain the orange-brown color of the iodine solution.

Why D is correct:

• Iodine reacts specifically with starch to produce a blue-black color, while cellulose cell walls show no reaction and remain orange-brown, per the standard iodine test for polysaccharides.

Why the others are wrong:

• A: Cell wall does not turn blue-black, as it lacks starch.
• B: Cell wall does not turn blue-black; starch does not remain orange-brown.
• C: Starch grains turn blue-black, not orange-brown.

Final answer: D