lipids

Elemental composition of lipids

Biological molecules composed of carbon (C), hydrogen (H), and oxygen (O), containing a much lower proportion of oxygen than carbohydrates.

Polymer status of lipids

Lipids are not considered polymers because they are not formed from long repeating chains of monomers.

Five main biological functions of lipids

Energy supply, structural components, waterproofing, insulation, and physical protection of delicate organs.

Structure of a fatty acid

Consists of a carboxyl group ($-COOH$) attached to a hydrocarbon chain (R group).

Saturated fatty acid

A hydrocarbon chain with no carbon-carbon double bonds, where all carbon atoms are bonded to the maximum number of hydrogen atoms; usually solid at room temperature.

Unsaturated fatty acid

A hydrocarbon chain with one or more carbon-carbon double bonds, causing kinks that lower the melting point and make it liquid at room temperature.

Monounsaturated vs. polyunsaturated fatty acids

Monounsaturated fatty acids contain exactly one double bond in their hydrocarbon chain, whereas polyunsaturated fatty acids contain two or more double bonds.

Emulsion test

A test where a sample is mixed with $2\,cm^3$ of ethanol, shaken, and added to $2\,cm^3$ of distilled water; a milky white emulsion indicates the presence of lipids.

Triglyceride structure

A lipid formed from one glycerol backbone attached to three fatty acid tails via ester bonds.

Triglyceride adaptations for energy storage

High number of C-H bonds in hydrocarbon tails, low mass-to-energy ratio, insolubility in water (no osmotic effect), and high hydrogen-to-oxygen ratio that releases water upon oxidation.

Formation of triglycerides

A condensation reaction between the hydroxyl groups ($-OH$) of glycerol and three fatty acids, releasing three $H_2O$ molecules and forming three ester bonds.

Breakdown of triglycerides

A hydrolysis reaction requiring the addition of three $H_2O$ molecules to break three ester bonds, yielding glycerol and three fatty acids.

Phospholipid structure

A lipid made up of a glycerol molecule attached to two fatty acid tails and one phosphate group.

Polarity of phospholipids

Contains a hydrophilic head (glycerol and phosphate) that attracts water and hydrophobic tails (fatty acids) that repel water.

Phospholipid bilayer function

Arranges in water with hydrophilic heads facing outward and hydrophobic tails facing inward, creating a hydrophobic centre that blocks water-soluble substances.

Role of cholesterol in cell membranes

A polar sterol lipid that binds to phospholipid fatty acid tails, packing them closer together to reduce membrane fluidity and increase stability.