Biology · Biomolecules · NEET
The monomer is the same (glucose), but the shape is different. In starch the glucose units link in a way that lets the chain coil into a helix (spring). This helix has a hollow centre that traps iodine (I2) molecules, and the trapped iodine gives the blue-black colour. In cellulose the glucose units link so the chains lie straight and flat, packed side by side. There is no coil and no pocket, so iodine cannot be trapped and no blue colour forms. For NEET, remember: same building block, different secondary structure.
The blue colour comes from iodine (I2) molecules sitting inside the helical part of the starch molecule. NCERT states directly: 'Starch forms helical secondary structures. In fact, starch can hold I2 molecules in the helical portion. The starch-I2 is blue in colour.' The iodine itself is brown; only when it is trapped inside the starch helix does the blue-black colour appear.
No, cellulose does not give the blue colour with iodine. NCERT says: 'Cellulose does not contain complex helices and hence cannot hold iodine.' Cellulose forms straight fibres (like cotton thread), not a coil, so there is no space to hold iodine molecules. This is why cotton or paper (which are cellulose) do not turn blue in the iodine test.
No. This is a common trap in NEET. Cellulose is NOT helical. It is a long straight chain (a homopolymer of glucose) that forms fibres. Only starch forms the helical secondary structure. So if an option says 'cellulose is a helical molecule', it is wrong - the correct reason cellulose gives no blue colour is that it does NOT contain helices.
No new covalent bond forms. The iodine is simply held (trapped) inside the helix by weak physical forces, not by a chemical reaction that breaks the starch. So options saying 'cellulose breaks down when iodine reacts' or 'starch reacts chemically with iodine' are wrong. It is a physical trapping inside the helix, which is why heating removes the colour and cooling brings it back.
Cellulose does not form blue colour with Iodine because
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is based on the secondary structure. The helix is a secondary structure (the way the glucose chain folds). The primary structure (just the glucose sequence) is not enough - it is the helical folding that traps iodine and makes the blue colour.
Cotton and paper are made of cellulose. Cellulose has no helix, so it cannot trap iodine. This is why the iodine test is used to detect starch in food (like potato or bread) but never gives colour with cotton or paper.
Glycogen (the animal storage polysaccharide) gives a reddish-brown colour, not the deep blue of starch, because its branching pattern gives shorter helical portions. For NEET, focus on the NCERT fact: starch gives blue, cellulose gives none.
Starch forms helices that hold I2 giving blue colour; cellulose has no helices, so it cannot hold iodine and gives no colour. Same glucose, different secondary structure.
