Water carbohydrates and lipids

Water

Water is a polar molecule, which means its oxygen carries a slight negative charge and its hydrogens carry slight positive charges.

This dipole lets water surround and dissolve charged and polar substances, so solutes can be transported in solution and react inside cells.

Carbohydrates

Carbohydrates are classified by size into monosaccharides, disaccharides and polysaccharides.

Small soluble monosaccharides such as glucose are respired and transported, while large insoluble polysaccharides such as glycogen and starch act as compact energy stores.

Glycogen and amylopectin are branched for rapid hydrolysis, and amylose coils tightly for compact storage.

Benedict's reagent tests for reducing sugars and iodine solution tests for starch, and both become semi-quantitative when the colours are matched against known standards.

Lipids

Lipids such as triglycerides are non-polar and therefore insoluble in water; a triglyceride consists of one glycerol molecule joined to three fatty acids, and its many carbon–hydrogen bonds make it an energy-rich, long-term store.

The fatty acid chains may be saturated, with no carbon–carbon double bonds, so the straight chains pack closely and the lipid is solid at room temperature (a fat), or unsaturated, with one or more double bonds that kink the chains and prevent close packing, so the lipid is liquid at room temperature (an oil).

Condensation and hydrolysis

These molecules are built and broken using the same two opposite reactions.

Condensation joins monosaccharides through glycosidic bonds and joins glycerol to fatty acids through ester bonds, releasing water each time.

Hydrolysis reverses this by adding water to split the bonds during digestion.

Exam theme The link between structure and function is the central exam theme: insolubility suits storage, branching suits rapid energy release, and straight or kinked chains explain why lipids are solid or liquid.

Exam tip Always name the precise bond and connect each structural feature to its specific role.