Monosaccharides

Monosaccharides are the simplest carbohydrates — crystalline, sweet-tasting, water-soluble molecules that cannot be split (hydrolysed) into any smaller carbohydrate unit. They are the monomer 'building blocks' from which all more complex carbohydrates are built, and share the general formula (CH₂O)n, where n may be 3, 4, 5, 6 or 7 — giving rise to the names triose, tetrose, pentose, hexose and heptose according to how many carbon atoms the sugar contains.

Chemically, a monosaccharide carrying a free aldehyde group (–CHO) is called an aldose (e.g. glucose, xylose), while one carrying a free ketone group (>C=O) is called a ketose (e.g. fructose, ribulose). Because every monosaccharide has this free aldehyde or ketone group, all monosaccharides are reducing sugars: they can donate hydrogen/electrons to another compound — a chemical reduction — and in doing so reduce Benedict's reagent, turning its blue Cu²⁺ into the brick-red precipitate of Cu⁺. This colour change is the basis of the standard laboratory test for a reducing sugar.

Three biologically important hexose monosaccharides are:

• Glucose — the single most important fuel for living cells (present in human blood at roughly 90 mg per 100 ml); being small and water-soluble, it passes readily across the cell membrane, and yields energy when broken down by cellular respiration. …