Carbohydrates, lipids, proteins and nucleic acids

Carbohydrates fall into three groups. Monosaccharides (glucose, fructose, ribose, deoxyribose) are simple sugars that cannot be hydrolysed; glucose, C₆H₁₂O₆, is kept in a healthy person’s fasting blood at about 3.9–5.5 mmol/l and is the main energy source. Disaccharides form from two monosaccharides with the release of water: sucrose – glucose + fructose, maltose – glucose + glucose, lactose – glucose + galactose. Polysaccharides (starch, glycogen, cellulose, chitin) are built of many glucose or similar monomers and are insoluble in water: starch is the reserve in plants, glycogen the reserve in animals and fungi, cellulose is the basis of the plant cell wall, and chitin is found in fungal cell walls and the covering of arthropods. Lipids are water-insoluble (hydrophobic) substances divided into neutral fats (glycerol + three fatty acids), waxes, phospholipids (the basis of membranes) and steroids (for example cholesterol). They serve as an energy source (1 g of fat gives about 38.9 kJ), as a reserve, as heat insulation, for protection, and as hormones.