Lesson 7 of 14 · 6 min
Polysaccharides
NCERT §9.5
Farah drops iodine on a slice of raw potato and on a strip of cotton wool. The potato turns blue-black; the cotton does not change. Both are made of glucose.
The lesson in notes
In short
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.