Polysaccharides

Polysaccharides are long chains of sugar units. Those made of one kind of unit are homopolysaccharides; those with more than one kind are heteropolysaccharides. They work either as fuel stores or as building material.

Starch, the plants' food store, is a mix of two molecules:

• Amylose (15-20%): long unbranched chains joined by alpha(1→4) bonds. These chains coil, and the coils trap iodine, giving the blue colour of the starch-iodine test.
• Amylopectin (80-85%): also alpha(1→4) chains, but branched by alpha(1→6) bonds every 24 to 30 units.

Amylase enzymes in saliva and pancreatic juice digest starch.

Glycogen, the animals' food store, is like amylopectin but even more branched. Muscle stores 1-2% and the liver up to 10% of its dry weight as glycogen.

Some polysaccharides build structures:

• Cellulose, the most abundant biomolecule on Earth: unbranched chains of glucose joined by beta(1→4) bonds, up to 15,000 units long. Humans lack the enzyme cellulase, so we cannot digest it; cattle and termites can, with help from microbes in their gut.
• Chitin: beta(1→4) chains of N-acetylglucosamine, building the hard outer skeletons of insects and crustaceans and the walls of fungi.
• Peptidoglycan: chains of two sugars (NAM and NAG) cross-linked by short peptides, forming bacterial cell walls. The enzyme lysozyme, found in tears, breaks it down and so kills bacteria.
• Agar, from red algae (its main part is agarose): forms gels used to grow microbes in the lab and to separate DNA pieces by electrophoresis.

A simple pattern: alpha bonds mostly store fuel, beta bonds mostly build walls.