What is Monosaccharide? — Biology

Definition 10.1 University Biology — Year 1 · Chapter 10 — Carbohydrates

Carbohydrates are molecules of composition $(\mathrm{CH_2O})_n$ and their derivatives. A monosaccharide (simple sugar) is a chain of three to seven carbons, one of which carries a carbonyl group and the others hydroxyls: an aldose when the carbonyl is terminal (an aldehyde: glucose, galactose, ribose), a ketose when it is internal (a ketone: fructose). By length: trioses ( $n = 3$ : glyceraldehyde), pentoses ( $n = 5$ : ribose, deoxyribose), hexoses ( $n = 6$ : glucose, fructose, galactose). Every carbon bearing four different groups is a chiral centre, so each formula stands for several stereoisomers: glucose has four chiral carbons and sixteen isomers, of which living things use one, D-glucose. Epimers differ at one carbon only (glucose and galactose at C4).

The two anomers of D-glucose in Haworth projection: the ring seen edge-on with its thick edge toward the reader, the hydroxyl of the anomeric carbon 1 below () or above () the plane. They interconvert through the open-chain aldehyde. — The two anomers of D-glucose in Haworth projection: the ring seen edge-on with its thick edge toward the reader, the hydroxyl of the anomeric carbon 1 below ( $\alpha$ ) or above ( $\beta$ ) the plane. They interconvert through the open-chain aldehyde.

Examples

Example 10.4 (Sugars that are not $(\mathrm{CH_2O})_n$ )

Deoxyribose lacks the hydroxyl at C2 (Chapter 11); glucosamine and N-acetylglucosamine carry an amine instead of the C2 hydroxyl (chitin, peptidoglycan); glucuronic acid has a carboxyl at C6 (the matrix polysaccharides); sugar phosphates (glucose-6-phosphate, ribose-5-phosphate) are the forms in which sugars enter metabolism; sugar alcohols (glycerol, sorbitol) have no carbonyl. The core is the same: a small polyhydroxylated carbon chain that water loves.

Example 10.15 (A day’s sugar)

A human eats some $300\,\mathrm{g}$ of carbohydrate a day, nearly all as starch and sucrose; the gut hydrolyses it to glucose, fructose and galactose, the liver turns the last two into glucose, and glucose circulates at $5\,\mathrm{g}$ in the whole blood — twenty minutes’ supply for the brain, which takes $120\,\mathrm{g}$ a day, refilled continuously from the liver’s glycogen. The lens of the eye, the red cells and the kidney medulla live on glucose alone; nothing else in the diet can replace it, and when it runs out the liver makes it from protein.

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