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This lesson covers the structure, properties and biological importance of monosaccharides and disaccharides as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand the general formula of carbohydrates, the structures of key sugars, condensation and hydrolysis reactions, and the tests used to identify reducing and non-reducing sugars.
Carbohydrates are organic molecules containing the elements carbon (C), hydrogen (H) and oxygen (O). They have the general formula:
(CH₂O)ₙ
where n is the number of carbon atoms. For example, glucose has the formula C₆H₁₂O₆ (n = 6).
Carbohydrates are classified by size:
| Category | Number of Sugar Units | Examples |
|---|---|---|
| Monosaccharide | 1 | Glucose, fructose, galactose, ribose, deoxyribose |
| Disaccharide | 2 | Maltose, sucrose, lactose |
| Polysaccharide | Many (hundreds to thousands) | Starch, glycogen, cellulose |
Monosaccharides are the simplest carbohydrates — they are single sugar units that cannot be hydrolysed into smaller carbohydrates. They are the monomers from which larger carbohydrates are built.
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