Chapter 27 — Chapter 27 — Biochemistry

Biochemistry deals with chemical substances in living organisms and their reactions; biomolecules carry the same functional groups as simple organic compounds. Carbohydrates (sugars, starch, cellulose) are major energy sources in our diet; chemically they are polyhydroxy aldehydes or ketones or their derivatives. Monosaccharides are the simplest sugars and cannot be hydrolysed further; general formula Cn(H₂O)n with n = 3 to 7: triose, tetrose, pentose, hexose. By functional group they are aldoses (glucose) or ketoses (fructose). Pentoses and hexoses are more stable as rings: fructose forms a five-membered furanose ring, glucose a six-membered pyranose ring; open and cyclic forms interconvert. Many –OH groups give extensive hydrogen bonding, so pure monosaccharides are solids and are highly soluble in water (honey is thick and viscous for this reason). Glucose is blood sugar; fructose is abundant in fruits and honey. Disaccharides (C₁₂H₂₂O₁₁: sucrose, lactose, maltose) consist of two monosaccharides joined by a glycosidic linkage C–O–C formed by condensation with loss of water; sucrose = glucose + fructose.