describe the formation of a glycosidic bond by condensation, with reference to disaccharides, including sucrose, and polysaccharides

Carbohydrates and Lipids – Formation of Glycosidic Bonds

1. What is a carbohydrate?

Carbohydrates are organic compounds composed of carbon, hydrogen and oxygen in a roughly 1:2:1 ratio. They are built from the basic unit – the monosaccharide – which can join together to form larger molecules.

2. Glycosidic bond formation (condensation reaction)

A glycosidic bond is formed when the hydroxyl group (‑OH) of one monosaccharide reacts with the anomeric carbon (C‑1) of another. The reaction is a condensation (dehydration) process, releasing a molecule of water.

General equation (shown in LaTeX):

\$\text{Monosaccharide}1\;+\;\text{Monosaccharide}2\;\xrightarrow{\text{condensation}}\;\text{Disaccharide}\;+\;H_2O\$

The type of glycosidic bond (α or β) depends on the orientation of the –OH groups involved.

3. Disaccharides – examples and bond types

Disaccharides consist of two monosaccharide units linked by a single glycosidic bond.

3.1. Sucrose – a detailed look

Sucrose is formed when the anomeric carbon of glucose (α‑C1) bonds to the anomeric carbon of fructose (β‑C2). Because both anomeric carbons are involved, sucrose has no free reducing end and is therefore a non‑reducing sugar.

Reaction schematic (LaTeX):

\$\text{Glucose}{\alpha\text{-C1}} + \text{Fructose}{\beta\text{-C2}} \;\xrightarrow{\text{condensation}}\; \text{Sucrose}{\alpha\text{-C1}\rightarrow\beta\text{-C2}} + H2O\$

4. Polysaccharides – long chains of monosaccharides

Polysaccharides are polymers formed by the repeated condensation of many monosaccharide units. The type of glycosidic linkage determines the properties of the polymer.

• Starch – a storage polysaccharide in plants, composed of α‑1,4 linked glucose units with occasional α‑1,6 branches (amylopectin).
• Glycogen – the animal analogue of starch, highly branched with α‑1,4 and α‑1,6 linkages, providing rapid glucose release.
• Cellulose – a structural polysaccharide in plant cell walls, consisting of β‑1,4 linked glucose units, forming straight, rigid fibres.

4.1. General polymerisation reaction

For a polymer of n glucose units (e.g., starch):

\$n\;\text{Glucose} \;\xrightarrow{\text{condensation}}\; (\text{Glucose})n \;+\; (n-1)H2O\$

5. Summary of key points

1. Glycosidic bonds are formed by condensation, releasing water.
2. The bond can be α or β, depending on the orientation of the reacting –OH groups.
3. Sucrose is a non‑reducing disaccharide formed by an α‑1,2 linkage between glucose and fructose.
4. Polysaccharides are long chains of monosaccharides; their physical properties depend on the type of glycosidic linkage (e.g., α‑linkages give flexible storage polysaccharides, β‑linkages give rigid structural polysaccharides).