Monosaccharides and disaccharides (Biology HL)

The big idea: Monosaccharides are single sugar units — the monomers (building blocks) of all carbohydrates.

Glucose is the most important monosaccharide. Its formula is C₆H₁₂O₆, and it is usually drawn as a six-sided ring.

Join two monosaccharides together and you get a disaccharide — a 'double sugar' such as maltose, sucrose or lactose.

Carbohydrate

A molecule made of carbon, hydrogen and oxygen, with hydrogen and oxygen usually in a 2:1 ratio (as in water). Sugars, starch and cellulose are all carbohydrates.

Monosaccharide

A single sugar unit — the monomer of a carbohydrate. Examples: glucose, fructose, galactose.

Disaccharide

A sugar made of TWO monosaccharides joined together. Examples: maltose, sucrose, lactose.

Glucose

The most common monosaccharide, formula C₆H₁₂O₆, drawn as a six-carbon ring; the main respiratory substrate (the molecule cells break down to release energy).

Count the sugars in the name: 'Mono-' means one and 'di-' means two.

Monosaccharide = one sugar unit. Disaccharide = two sugar units joined.

Many smaller units joined together make a polysaccharide (many sugars) — that is the next micro.

Glucose comes in two slightly different forms called alpha-D-glucose and beta-D-glucose.

They are isomers: they have exactly the same formula (C₆H₁₂O₆) and the same atoms, but those atoms are arranged differently at one point on the ring.

Isomers

Molecules with the SAME chemical formula but a DIFFERENT arrangement of their atoms.

Alpha-D-glucose (α-glucose)

The form of glucose in which the hydroxyl (-OH) group on carbon 1 points DOWNWARDS (below the plane of the ring).

Beta-D-glucose (β-glucose)

The form of glucose in which the hydroxyl (-OH) group on carbon 1 points UPWARDS (above the plane of the ring).

Hydroxyl group (-OH)

An oxygen joined to a hydrogen; the group on carbon 1 whose direction tells alpha- and beta-glucose apart.

The ONE structural difference to know: Alpha- and beta-D-glucose differ in just one thing: the direction of the -OH group on carbon 1.

Alpha (α): the -OH on carbon 1 points DOWN.

Beta (β): the -OH on carbon 1 points UP.

That tiny difference has huge consequences later — alpha-glucose builds starch and glycogen (energy stores), while beta-glucose builds cellulose (a tough structural fibre).

To make a disaccharide, two monosaccharides are joined by a condensation reaction.

A new bond — the glycosidic bond — forms between them, and one molecule of water (H₂O) is removed. Because water is taken out, condensation is sometimes called a 'water-losing' reaction.

Condensation reaction

A reaction that joins two molecules together and releases a molecule of water (H₂O).

Glycosidic bond

The covalent bond that links two sugar units together in a disaccharide or polysaccharide.

Hydrolysis

The reverse of condensation: a molecule of water is ADDED to break the glycosidic bond and split a disaccharide back into two monosaccharides.

Condensation builds, hydrolysis breaks: Condensation: glucose + glucose → maltose + water. A glycosidic bond forms; one water molecule is released.

Hydrolysis runs it backwards: maltose + water → glucose + glucose. A water molecule is added to break the bond.

Same idea builds all biological polymers — proteins and nucleic acids are also assembled by condensation and broken by hydrolysis.

A memory hook: Condensation constructs (and condenses out water). Hydrolysis uses hydro (water) to split a bond.

So: joining sugars loses water; breaking them uses water.

How this is tested: On Paper 1A (multiple choice) the favourite is to state what distinguishes alpha- from beta-D-glucose (the direction of the carbon-1 -OH), or to identify the products of a condensation reaction (a disaccharide plus water) — sometimes from a labelled reaction diagram.

On Paper 2 an Outline question can ask how glucose's properties (soluble, stable, transportable, easily respired) make it useful in living things.

The marks live in the water: Students often write only 'maltose' and lose the second mark.

In a condensation answer you must also name the water that is released — it is the give-away that the reaction is condensation rather than hydrolysis.

Watch for the reverse question: If the diagram shows a disaccharide being split into two sugars with water being added, the reaction is hydrolysis, not condensation — and the products are the two monosaccharides.

Read the arrow direction and look for whether water is released (condensation) or added (hydrolysis).