Monosaccharides: form and function

Molecules (Form and function) · Carbohydrates and lipids · note 3 of 8

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Monosaccharides: form and functionSpec B1.1.4

In short

Monosaccharides are single sugar units, such as pentoses with five carbon atoms (ribose, deoxyribose) and hexoses with six (glucose). Glucose suits its role because it is soluble, so it is easily transported in blood plasma; chemically stable, so it does not react while stored or carried; and yields energy when oxidised in cell respiration.

Monosaccharides are the monomers of carbohydrates. In water they mostly exist as rings. You need to recognise two groups from ring diagrams.

Pentoses and hexoses
Group	Carbon atoms	Ring	Examples
Pentose	5	Five-membered ring: four carbons and one oxygen; C5 sits outside the ring	Ribose, deoxyribose
Hexose	6	Glucose and galactose: six-membered ring of five carbons and one oxygen, with C6 (CH₂OH) outside the ring. Fructose: five-membered ring, with C1 and C6 outside	Glucose, galactose, fructose

Exam tip:

To identify a sugar from a diagram, count all the carbon atoms, including any outside the ring. Five means pentose, six means hexose. Fructose is a hexose even though it forms a five-membered ring.

Glucose (C₆H₁₂O₆) exists as alpha-glucose and beta-glucose. They differ only at carbon 1: in alpha-glucose the –OH on C1 is below the ring; in beta-glucose it is above the ring.

Ring diagrams of ribose (a five-membered ring with carbons 1 to 5 numbered and the ring oxygen), alpha-glucose and beta-glucose (six-membered rings with carbons 1 to 6 numbered). The –OH on C1 is circled: below the ring in alpha-glucose and above the ring in beta-glucose. — Hexoses have six carbons, pentoses five. Alpha- and beta-glucose differ only in the position of the –OH on C1.

Glucose: properties linked to use

How glucose's properties suit its function
Property	Reason	Use
Soluble	Polar –OH groups form hydrogen bonds with water	Dissolves in cytoplasm and blood plasma
Transportable	Small and soluble	Carried in blood plasma to every cell
Chemically stable	The ring structure does not react readily	Can be carried and held in cells without unwanted reactions
Yields energy when oxidised	Oxidation releases energy, captured as ATP in cell respiration	The main respiratory substrate in most cells

Its solubility is also a drawback for storage: a high concentration of glucose would draw water into cells by osmosis. So glucose is stored as polysaccharides instead.

Exam tip:

Linking question: what are the roles of oxidation and reduction? Oxidising glucose to carbon dioxide and water in respiration releases energy; reducing carbon dioxide in photosynthesis stores it.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between alpha and beta glucose?

Alpha and beta glucose differ only at carbon 1: in alpha-glucose the –OH group is below the ring and in beta-glucose it is above. This small difference matters. Alpha-glucose forms coiled, branched starch and glycogen for energy storage, while beta-glucose forms straight cellulose chains that bundle into strong fibres.

Why are starch and glycogen good energy stores?

Starch and glycogen are good energy stores because coiling and branching make them compact, and their large size makes them relatively insoluble, so they have little osmotic effect. Glucose can easily be added by condensation or removed by hydrolysis, and glycogen's many branch ends let it release glucose quickly when needed.

Why are lipids better than carbohydrates for long-term energy storage?

Triglycerides release about twice as much energy per gram as carbohydrates when oxidised. They are insoluble, so they have no osmotic effect, and they are stored without water, whereas glycogen is stored bound to water. A fat store therefore adds much less mass for the same energy, which suits long-term storage.

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