Polysaccharides and nucleic acids

Polysaccharides are long chains of sugar units, like a necklace of many beads (a short chain of 2 to 9 sugars is an oligosaccharide).

• Cellulose: a homopolymer of glucose (only one kind of unit). It builds plant cell walls; paper and cotton are cellulose.
• Starch: the food store of plants.
• Glycogen: the food store of animals; its chain is branched.
• Inulin: a polymer of fructose.

Each chain has two different ends: the reducing end (written on the right) and the non-reducing end (on the left).

A simple test tells starch from cellulose. Starch chains coil into helices that can trap iodine (I₂), giving a blue colour. Cellulose has no such helices, so it gives no colour with iodine.

Some complex polysaccharides contain sugars carrying an amino group (amino-sugars, such as glucosamine and N-acetyl galactosamine). Chitin, which forms the hard exoskeleton of arthropods (insects, crabs), is one of them.

Nucleic acids (DNA and RNA) are polynucleotides: long chains of nucleotides. Each nucleotide has three parts: a nitrogen base, a five-carbon (pentose) sugar and a phosphate. The bases are of two kinds: purines (adenine and guanine) and pyrimidines (cytosine, uracil and thymine). The sugar is ribose in RNA and 2'-deoxyribose in DNA.