Lipids 3.1.3

Roles of lipids

Energy supply, structural component (cell membranes), waterproofing (insoluble), insulation, protection (of delicate organs).

Fatty acids

Consist of a carboxyl (COOH) group and a hydrocarbon chain (R group). They can be either saturated (no carbon-carbon double bond and have higher melting points) or unsaturated (contain at least 1 carbon-carbon double bond causing the chain to kink, and have lower melting points).

Test for lipids

Emulsion test:
Add ethanol, shake and add water. If lipids are present, a milky white emulsion will form.

Triglycerides

An energy store consisting of a glycerol attached via ester bonds to 3 fatty acid tails - which can be saturated or unsaturated.
Glycerol + 3 fatty acids ←→ Triglyceride + 3 water (condensation and hydrolysis)

Phospholipids

A structural component of cell membranes made up of a phosphate group, a glycerol and 2 fatty acids. The hydrophilic head (containing glycerol and phosphate) attracts water and the hydrophobic tail (fatty aids) repels water - this causes a phospholipid bilayer (hydrophilic heads facing out and hydrophobic tails facing in) to form when phospholipids are placed in water, meaning water-soluble substances cannot pass through.