1.2—Monosaccharides, disaccharides and polysaccharides

Monosaccharides are single sugars such as glucose, fructose and galactose. A disaccharide contains two monosaccharides joined by a glycosidic bond; a polysaccharide is a long chain of sugar units.

Maltose is glucose–glucose, sucrose is glucose–fructose and lactose is glucose–galactose. Starch contains amylose and branched amylopectin; glycogen is even more highly branched, making glucose release faster in animal cells.

Small sugars are soluble and readily transported, whereas starch and glycogen are compact, insoluble stores with little osmotic effect. Branching creates more chain ends for enzymes to remove glucose.

A plant can transport sucrose in phloem but store glucose units as starch. A liver cell stores glycogen so it can release glucose rapidly between meals without filling the cytoplasm with many separate sugar molecules.

Do not treat every polysaccharide as a fuel store: cellulose is also a glucose polymer, but its structure makes it a strong fibre rather than a readily digested reserve.