# Carbohydrates
Carbohydrates are organic molecules containing C, H, and O. They serve as energy sources, energy stores, and structural components. Understanding their structure and function is essential for A-Level Biology.
1. Monosaccharides
Simple sugars — the monomers of carbohydrates.
| Monosaccharide | Formula | Found In |
|---|---|---|
| Glucose (α and β) | C₆H₁₂O₆ | Blood, fruit |
| Fructose | C₆H₁₂O₆ | Fruit, honey |
| Galactose | C₆H₁₂O₆ | Milk |
| Ribose | C₅H₁₀O₅ | RNA |
| Deoxyribose | C₅H₁₀O₄ | DNA |
α-glucose vs β-glucose: differ in the position of —OH on carbon 1. This structural difference leads to different polysaccharides (starch vs cellulose).
2. Disaccharides
Two monosaccharides joined by a glycosidic bond via condensation reaction (releases H₂O).
| Disaccharide | Components | Bond |
|---|---|---|
| Maltose | Glucose + Glucose | α-1,4 glycosidic |
| Sucrose | Glucose + Fructose | α-1,2 glycosidic |
| Lactose | Glucose + Galactose | β-1,4 glycosidic |
Hydrolysis: breaking glycosidic bond by adding water → splits back into monosaccharides.
3. Polysaccharides
| Polysaccharide | Monomer | Structure | Function |
|---|---|---|---|
| Starch (amylose + amylopectin) | α-glucose | Amylose: unbranched helix; Amylopectin: branched | Energy storage in plants |
| Glycogen | α-glucose | Highly branched | Energy storage in animals (liver, muscle) |
| Cellulose | β-glucose | Straight chains with H-bonds between chains → microfibrils | Structural in plant cell walls |
Why Starch Is a Good Storage Molecule
- Compact (helix shape)
- Insoluble (doesn't affect osmosis)
- Easily hydrolysed when energy needed
- Amylopectin: many branches → many ends for rapid hydrolysis
Why Cellulose Is a Good Structural Molecule
- β-glucose → alternate molecules flipped 180°
- Straight chains → hydrogen bonds between chains
- Microfibrils → high tensile strength
4. Testing for Carbohydrates
| Test | Reagent | Positive Result |
|---|---|---|
| Reducing sugars | Benedict's reagent (heat) | Blue → green → yellow → orange → brick red |
| Non-reducing sugars | Boil with HCl, neutralise with NaHCO₃, then Benedict's | Colour change after hydrolysis |
| Starch | Iodine solution | Brown → blue-black |
5. Practice Questions
- Compare the structure and function of amylose and cellulose.
- Describe how a condensation reaction forms maltose.
- Why is glycogen more highly branched than amylose?
- Explain why starch is insoluble and why this is important.
- Describe the test for non-reducing sugars.
Want to check your answers and get step-by-step solutions?
Summary
- Monosaccharides: glucose (α/β), fructose, galactose
- Disaccharides: condensation → glycosidic bond; hydrolysis breaks them
- Polysaccharides: starch/glycogen (energy storage); cellulose (structural)
- α-glucose → starch/glycogen; β-glucose → cellulose
- Tests: Benedict's (reducing sugars), iodine (starch)