Carbohydrate Structure

Carbohydrates are one of the four main groups of biological molecules. They contain the elements carbon, hydrogen, and oxygen, with hydrogen and oxygen typically present in a 2:1 ratio (as in water). The general formula for a carbohydrate can be written as Cₙ(H₂O)ₙ, although this is a simplification and does not apply to all carbohydrates.

Carbohydrates serve two principal functions: energy provision (monosaccharides and disaccharides) and structural support (cellulose in plant cell walls).

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Monosaccharides

Key Definition: A monosaccharide is the simplest carbohydrate unit — a single sugar molecule that cannot be hydrolysed into smaller carbohydrate units.

Monosaccharides are classified by the number of carbon atoms they contain:

Type	Number of Carbons	Examples
Triose	3	Glyceraldehyde (G3P — an intermediate in glycolysis and the Calvin cycle)
Pentose	5	Ribose (in RNA and ATP), deoxyribose (in DNA)
Hexose	6	Glucose, galactose, fructose

Alpha and Beta Glucose

Glucose exists as two structural isomers: α-glucose and β-glucose. Both have the molecular formula C₆H₁₂O₆ and a six-membered ring structure. The difference lies in the orientation of the hydroxyl (–OH) group on carbon-1:

• In α-glucose, the –OH on C1 is positioned below the plane of the ring (same side as the –OH on C4).
• In β-glucose, the –OH on C1 is positioned above the plane of the ring (opposite side to the –OH on C4).

This seemingly minor difference has profound consequences for the polysaccharides they form. Polymers of α-glucose (starch, glycogen) are coiled and compact — ideal for energy storage. Polymers of β-glucose (cellulose) are straight and rigid — ideal for structural support.

Properties of Monosaccharides

• Soluble in water (polar –OH groups form hydrogen bonds with water).
• Sweet-tasting.
• Reducing sugars — they can donate electrons to other molecules, which is the basis of Benedict's test.
• Provide energy when oxidised during cellular respiration.

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Condensation and Hydrolysis Reactions

Key Definition: A condensation reaction is a chemical reaction in which two molecules are joined together with the removal of a water molecule (H₂O).

Key Definition: A hydrolysis reaction is the reverse — a covalent bond is broken by the addition of a water molecule.

When two monosaccharides undergo a condensation reaction, a glycosidic bond forms between them, and one molecule of water is released. The reverse process (hydrolysis) requires water and is catalysed by specific enzymes.

These reactions are fundamental to biology:

• Condensation builds polymers from monomers (anabolic).
• Hydrolysis breaks polymers into monomers (catabolic), for example during digestion.

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Disaccharides

Key Definition: A disaccharide is a sugar composed of two monosaccharides joined by a glycosidic bond formed through a condensation reaction.

Disaccharide	Component Monosaccharides	Glycosidic Bond	Where Found
Maltose	α-glucose + α-glucose	α-1,4	Germinating seeds; intermediate in starch digestion
Sucrose	α-glucose + fructose	α-1,2	Transported in plant phloem sap
Lactose	β-galactose + α-glucose	β-1,4	Mammalian milk

The numbers in the bond name (e.g., 1,4) refer to the carbon atoms on each monosaccharide that are involved in the bond. For example, an α-1,4 glycosidic bond links carbon-1 of one glucose to carbon-4 of the next.

Reducing and Non-Reducing Sugars

• All monosaccharides and most disaccharides are reducing sugars — they have a free aldehyde or ketone group that can reduce Cu²⁺ ions in Benedict's reagent.
• Sucrose is a non-reducing sugar because both anomeric carbons are involved in the glycosidic bond, leaving no free reducing group.
• To test for a non-reducing sugar, the sample must first be hydrolysed (by boiling with dilute hydrochloric acid), neutralised, and then tested with Benedict's reagent.

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Polysaccharides

Key Definition: A polysaccharide is a polymer consisting of many monosaccharide units joined by glycosidic bonds.