Monosaccharides are single sugar monomers, disaccharides contain two monosaccharides joined together, and polysaccharides are polymers containing many monosaccharides. Their different sizes and structures determine their biological functions.
Structure and Examples
| Carbohydrate | Structure | Examples and functions |
|---|---|---|
| Monosaccharide | One sugar unit | Glucose is transported and oxidized in cell respiration; ribose is part of RNA. |
| Disaccharide | Two monosaccharides joined by a glycosidic bond | Sucrose transports carbohydrate in plants; lactose occurs in milk. |
| Polysaccharide | Many monosaccharides joined by glycosidic bonds | Starch and glycogen store energy; cellulose provides structural support in plant cell walls. |
Monosaccharides are joined by condensation reactions. An hydroxyl group from one molecule and hydrogen from another are removed, producing water and forming a glycosidic bond. For example:
Adding further monosaccharides repeatedly produces a polysaccharide. The reverse process is hydrolysis, in which water is used to break glycosidic bonds during digestion.
Structure is closely related to function. Monosaccharides are relatively small and soluble, making glucose suitable for transport and rapid use in respiration. Polysaccharides are much larger and relatively insoluble, so they do not diffuse out of cells or strongly affect osmotic conditions.
Starch and glycogen are compact because their glucose chains coil and branch. Glycogen is more highly branched, allowing glucose to be added or removed rapidly. Cellulose contains alternating beta-glucose monomers, producing straight chains that form strong bundles through hydrogen bonding.
A common misconception is that all carbohydrates are sugars. More precisely, monosaccharides and disaccharides are sugars, whereas polysaccharides are complex carbohydrates.
Exam Technique
For an IB distinguish question, compare both carbohydrates directly: state the number of sugar units, type of reaction involved, and one named biological function. Do not describe only one carbohydrate or confuse condensation, which builds carbohydrates, with hydrolysis, which breaks them down. This content applies at both SL and HL in B1.1.
