A monosaccharide is the simplest form of carbohydrate, consisting of a single sugar unit that cannot be broken down into smaller sugars by hydrolysis. They serve as basic building blocks for more complex carbohydrates such as disaccharides, oligosaccharides, and polysaccharides.
Key Characteristics:
| Property | Description |
|---|
| Chemical formula | General formula: (CH₂O)ₙ, where n = 3–7 |
| Structure | Carbon chain with hydroxyl groups (–OH) and a carbonyl group (C=O) |
| Solubility | Highly soluble in water due to hydrogen bonding |
| Taste | Often sweet-tasting (e.g., glucose, fructose) |
| Examples | Glucose, fructose, galactose, ribose |
Classification of Monosaccharides:
- By number of carbon atoms:
| Carbon Atoms | Name | Example |
|---|
| 3 | Triose | Glyceraldehyde |
| 4 | Tetrose | Erythrose |
| 5 | Pentose | Ribose |
| 6 | Hexose | Glucose, Fructose |
| 7 | Heptose | Sedoheptulose |
- By type of carbonyl group:
| Type | Group Present | Examples |
|---|
| Aldose | Aldehyde (–CHO) | Glucose, Galactose |
| Ketose | Ketone (C=O) | Fructose |
Biological Importance:
| Role | Description |
|---|
| Energy source | Monosaccharides like glucose are primary fuels for cells |
| Structural components | Ribose and deoxyribose are found in RNA and DNA |
| Precursor molecules | Used to synthesize amino acids, nucleotides, and lipids |
| Building blocks | Form disaccharides (e.g., sucrose) and polysaccharides (e.g., starch, cellulose, glycogen) |
Common Monosaccharides:
| Monosaccharide | Type | Function |
|---|
| Glucose | Aldohexose | Main energy source for cells |
| Fructose | Ketohexose | Found in fruits; sweeter than glucose |
| Galactose | Aldohexose | Component of lactose (milk sugar) |
| Ribose | Aldopentose | Sugar in RNA |
| Deoxyribose | Aldopentose | Sugar in DNA (missing one oxygen compared to ribose) |
Chemical Structure:
Monosaccharides can exist in:
- Linear form: open-chain (less common in solution)
- Cyclic form: ring structure (more common and stable)
Example:
- Glucose cyclizes to form a 6-membered ring called pyranose.
Summary Table:
| Feature | Description |
|---|
| Type of Molecule | Simple carbohydrate |
| Number of Sugar Units | 1 |
| Basic Formula | (CH₂O)ₙ |
| Main Functions | Energy, structure, biosynthesis |
| Common Forms | Glucose, fructose, galactose, ribose |
| Can Form | Disaccharides, polysaccharides |
