Carbohydrates: Mono-, Di- and Polysaccharides · NCERT Biology

Why does cellulose not give a blue color with iodine test?

  1. It is composed of only fructose units.
  2. It contains nitrogen in its backbone.
  3. Its linear structure lacks iodine-trapping helices.
  4. Its molecular weight is too low to react.
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Correct answer: Its linear structure lacks iodine-trapping helices.

Starch has a helical secondary structure (in amylose) that can hold iodine molecules, producing a blue-black color. Cellulose has a linear, non-helical structure and therefore cannot trap iodine to produce a color change.

Difficulty: Medium Question 12 of 20

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