Carbohydrates

Carbohydrates are organic compounds containing only carbon, hydrogen and oxygen. Hydrogen and oxygen are usually present in the same 2:1 ratio as in water, which gives the general formulaC_x(H₂O)_yand the name, which means 'hydrated carbon'. For simple sugars x = y, so the formula is (CH₂O)_n. For example glucose, with n = 6, is C₆H₁₂O₆.Carbohydrates are classified by size into three groups:• monosaccharides: single sugar units, such as glucose, fructose and galactose
• disaccharides: two monosaccharides joined together, such as maltose, sucrose and lactose
• polysaccharides: many monosaccharides joined together, such as starch, glycogen and celluloseMonosaccharides and disaccharides are called sugars. They are sweet, small and soluble in water. Polysaccharides are not sweet and are insoluble or only slightly soluble.Roles of carbohydrates:
• energy source: glucose is the main respiratory substrate in most cells
• energy storage: starch in plants, glycogen in animals and fungi
• structure: cellulose in plant cell walls
• parts of other molecules: ribose and deoxyribose in nucleotides, and short carbohydrate chains on glycoproteins and glycolipids of cell surface membranesWorked example: a monosaccharide with 5 carbon atoms has the formula (CH₂O)₅ = C₅H₁₀O₅, which is ribose.