Lipids (AQA A Level Biology): Revision Note

Lipids

• Lipids are macromolecules that contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates, lipids contain a lower proportion of oxygen

• Lipids are non-polar and hydrophobic

• There are two main groups of lipids:

  • Triglycerides (the main component of fats and oils)

  • Phospholipids (a type of lipid molecule that is a major component of cell membranes)

Triglycerides: structure & function

• Triglycerides, as with all lipids, are non-polar, hydrophobic molecules

• The monomers are glycerol and fatty acids

  • Glycerol is an alcohol

  • Fatty acids contain a methyl group at one end of a hydrocarbon chain known as the R group (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group

    • The shorthand chemical formula for a fatty acid is RCOOH

Triglyceride structure

• Fatty acids can vary in two ways:

  • Length of the hydrocarbon chain (R group)

  • The fatty acid chain (R group) may be saturated or unsaturated

• Unsaturated fatty acids can be mono- or  poly-unsaturated:

  • If hydrogen (H) atoms are on the same side of the double bond, they are cis-fatty acids and are metabolised by enzymes

  • If H atoms are on opposite sides of the double bond, they are trans-fatty acids and cannot form enzyme-substrate complexes; therefore, they are not metabolised. They are linked with coronary heart disease

Bonding in triglycerides

• Triglycerides are formed by esterification

• An ester bond forms when a hydroxyl (-OH) group on glycerol bonds with the carboxyl (-COOH) group of the fatty acid:

  • An H from glycerol combines with a hydroxyl group (OH) from the fatty acid to make water, therefore, this is a condensation reaction

• Three fatty acids join to one glycerol molecule to form a triglyceride

  • Therefore, for one triglyceride to form, three water molecules are released

    

Properties of triglycerides

• Properties of triglycerides in organisms include:

  • energy storage – Triglycerides are an efficient form of long-term energy storage due to their high calorific value

  • thermal insulation – They reduce heat loss by acting as an insulating layer beneath the skin

  • buoyancy – Their low density helps aquatic organisms stay afloat

  • protection – Triglycerides form a cushioning layer around vital organs, protecting them from physical damage

Energy storage

• Triglycerides have long hydrocarbon chains with many C–H bonds and little oxygen, making them highly reduced

  • Oxidation of these bonds during respiration releases large amounts of energy for ATP production

  • They provide more energy per gram (≈37 kJ g⁻¹) than carbohydrates or proteins (≈17 kJ g⁻¹)

• Being hydrophobic, they do not cause osmotic water uptake, allowing efficient storage in cells

• Plants store triglycerides as oils in seeds and fruits. These oils are often liquid at room temperature due to unsaturated fatty acids with kinks in their chains

• Mammals store triglycerides in adipose tissue (e.g. for energy during hibernation).

• Triglyceride oxidation also releases metabolic water, important for:

  • desert animals with limited water access

  • bird and reptile embryos within eggs

• Inside cells, triglycerides form insoluble droplets, suitable for energy storage, with the hydrophobic fatty acids on the inside and the glycerol molecules on the outside

Insulation

• Triglycerides are part of the myelin sheath, insulating nerves and speeding up nerve impulse transmission

• Stored in adipose tissue under the skin, triglycerides reduce heat loss (e.g. whale blubber)

Buoyancy

• Fat has a low density, aiding buoyancy in aquatic animals

Protection

• Adipose tissue cushions and protects internal organs from damage