The iodine test for starch.

Starch coils into a helix that traps I2 (blue); cellulose chains stay straight and cannot; glycogen branches, each with one reducing end.

Turn the sample blue with iodine, then pick the polymer that stays colourless and explain from its shape.

Cellulose also gives the blue iodine test.

• Polysaccharides (carbohydrates) are another class of macromolecule in the acid-insoluble pellet. They are long chains, or threads, of monosaccharide units.
• Cellulose is made of one type of monosaccharide only, glucose, so it is a homopolymer. Starch is a variant that stores energy in plant tissues; glycogen is the animal variant. Inulin is a polymer of fructose.
• In a polysaccharide chain such as glycogen, the right end is the reducing end and the left end the non-reducing end. Glycogen is branched.
• Starch forms helical secondary structures and can hold I2 molecules inside the helix; the starch-iodine complex is blue. Cellulose has no complex helices, so it cannot hold iodine.
• Plant cell walls are made of cellulose, and paper from plant pulp and cotton fibre is cellulosic.
• More complex polysaccharides are built from amino-sugars and chemically modified sugars such as glucosamine and N-acetyl galactosamine. Chitin, in the exoskeleton of arthropods, is one; these complex polysaccharides are mostly homopolymers.

Teaching this idea? Put the simulation on your own page or LMS with this code.