Carbohydrates
Carbohydrates are polyhydroxy aldehydes or ketones and their derivatives. This domain tests classification, reducing properties, glycosidic linkages, and the structural difference between storage and structural polysaccharides.
Core rules
- The empirical formula is often (CH2O)n; glucose is C6H12O6, a hexose.
- Monosaccharides are aldoses or ketoses; glucose is an aldohexose and fructose is a ketohexose. Ribose is a pentose.
- Disaccharides form by condensation: maltose has an α-1,4 bond, lactose a β-1,4 bond, and sucrose an α-1,2 glycosidic bond.
- Reducing sugars have a free anomeric carbon and reduce Benedict's or Fehling's solution; sucrose is non-reducing.
- Starch contains amylose and amylopectin; glycogen is animal storage; cellulose is β-1,4-linked and indigestible by humans; chitin is a nitrogen-containing structural polysaccharide.
Worked example
How many glycosidic bonds and water molecules are involved when 12 glucose units form a linear amylose chain? Each bond joins two residues and releases one water. Twelve residues in one chain require 12 − 1 = 11 glycosidic bonds and release 11 water molecules. If the chain were branched, the count would rise because each branch point adds another bond.
Traps that cost marks
Sucrose is non-reducing despite containing glucose and fructose. Cellulose is not a source of glucose for humans because we lack cellulase. Starch itself does not reduce Benedict's solution until it is hydrolysed to maltose or glucose. Do not confuse a glycosidic bond with a peptide bond.