Why does cellulose fail to give a blue color with iodine solution, whereas starch produces an intense blue complex?
Answer and explanation
Correct answer: B. Cellulose lacks helical secondary structures and cannot trap iodine molecules
- Step 1: Starch contains amylose, which adopts a helical secondary structure with internal hydrophobic cavities.
- Step 2: Iodine () molecules slip into these helical coils, forming a coordination complex that absorbs red-yellow light and appears intensely blue.
- Step 3: In contrast, cellulose consists of unbranched -glucan chains held together by hydrogen bonds in linear rigid ribbons; it has no helices and cannot hold iodine.
Answer
Cellulose lacks helical secondary structures and cannot trap iodine molecules.
Why not the others: Cellulose is a neutral homopolymer, not an acidic mucopolysaccharide; Cellulose contains linkages, not ; Cellulose is strictly an unbranched linear homopolymer of glucose.
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