The big idea: Monosaccharides are single sugar units — the monomers (building blocks) of all carbohydrates. Join two together and you get a disaccharide, a 'double sugar'.
Key facts
- Glucose is the most important monosaccharide.
- Its formula is C₆H₁₂O₆, and it is usually drawn as a six-sided ring.
- Disaccharides include maltose, sucrose and 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.
| Disaccharide | Made from (two monosaccharides) | Where you meet it |
|---|---|---|
| Maltose | glucose + glucose | Germinating grain; breakdown of starch |
| Sucrose | glucose + fructose | Table sugar; transported sap in plants |
| Lactose | glucose + galactose | Milk sugar |
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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).
Alpha- and beta-D-glucose are isomers: the only difference is whether the -OH on carbon 1 points DOWN (alpha) or UP (beta).
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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.
Condensation: two glucose monosaccharides join, a glycosidic bond forms between them and one molecule of water (H₂O) is released.
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An easy way to remember: Condensation constructs (and condenses out water). Hydrolysis uses hydro (water) to split a bond.
So: joining sugars loses water; breaking them uses water.
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The big idea: To name a monosaccharide from its ring drawing, count its carbon atoms: five makes a pentose, six a hexose.
The points to remember
- A monosaccharide is a single sugar unit, usually drawn as a ring.
- A pentose has five carbon atoms: ribose and deoxyribose.
- A hexose has six carbon atoms: glucose, galactose and fructose.
- Count every carbon: the ring corners and the carbon in each CH₂OH group.
- Ring size can mislead: fructose has a five-cornered ring but six carbons.
Remember it as: Pent = five, hex = six: count the carbons, not the corners.
Real example: ribose in every cell: Ribose, a pentose, is part of every RNA molecule and of ATP, the energy carrier in all your cells.
The big idea: Glucose is the sugar living things use most, because its properties suit transport, storage and respiration.
The points to remember
- Soluble in water, so glucose dissolves in blood plasma and is transported round the body.
- Small and soluble, so it moves easily into cells where it is needed.
- Chemically stable, so it can be joined into starch, glycogen or cellulose and kept.
- Oxidised in respiration, releasing energy that is used to make ATP.
- Its polymers are easily broken down again, so stored glucose can be released when needed.
Remember it as: Soluble to move, stable to store, oxidised for energy.
Soluble
- Dissolves in plasma and sap.
- So it is easy to transport.
Stable
- Does not break down on its own.
- So it can be stored as starch or glycogen.
Energy-rich
- Oxidised in respiration.
- Releases energy to make ATP.
Real example: glucose in your blood: An adult's blood carries about 4 to 5 grams of glucose in total, dissolved in the plasma and delivered to every cell for respiration.
One trap when you write about energy:
Glucose does not produce energy: Say glucose releases energy when it is oxidised in respiration, and that the energy is used to make ATP. "Glucose produces energy" on its own is not enough.
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How this is tested: Paper 1A (multiple choice) and Paper 2 test this micro in different ways.
What you may be asked
- Paper 1A: state what distinguishes alpha- from beta-D-glucose (the direction of the carbon-1 -OH).
- Paper 1A: identify the products of a condensation reaction (a disaccharide plus water), sometimes from a labelled reaction diagram.
- Paper 2, Outline: how glucose's properties (soluble, stable, transportable, easily respired) make it useful in living things.
Exam-style question — joining two monosaccharides
Two glucose molecules react together to form maltose. Identify the type of reaction and state the products formed. [2]
How to score both marks
- Name the reaction. Two monosaccharides joining together is a condensation reaction — molecules are joined and a small molecule is removed.
- State the products. The products are maltose (the disaccharide) and one molecule of water (H₂O). (Mark 1: condensation. Mark 2: the disaccharide AND water — both products are needed.)
Final answer
A condensation reaction; the products are the disaccharide maltose and water (H₂O).
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.
The reaction you are describing: two glucose units join, a glycosidic bond forms, and water is released.
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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).
A two-mark Paper 2 version pairs one property with its function:
Monosaccharides are essential molecules in the body. Outline one property of monosaccharides and its related function.
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