Triglyceride Function (AQA A Level Biology): Revision Note
Exam code: 7402
Triglycerides: structure & function
Lipids are macromolecules that contain carbon, hydrogen and oxygen atoms
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, 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


Examiner Tips and Tricks
You need to recognise saturated and unsaturated fatty acids from diagrams. Make sure you know the difference and how to spot it in a diagram.
Formation of 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

Function of triglycerides
Triglycerides have some key functions in organisms, including:
energy storage
insulation
bouyance
protection
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
Examiner Tips and Tricks
It is common to be asked why triglycerides are energy reserves (they store more energy per gram due to their hydrocarbon chains).
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