Triglyceride Function (AQA A Level Biology): Revision Note

Exam code: 7402

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Cara Head

Updated on

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

Triglycerides_Basics - types of fatty acids (1), downloadable IGCSE & GCSE Biology revision notes
Diagram of a polyunsaturated fatty acid chain, featuring a methyl group (CH3) and carboxyl group (COOH), illustrating double bonds.
Examples of different types of fatty acids with the functional groups and presence of double bonds highlighted

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

      Triglycerides Basics - Formation of a triglyceride (1)_2, downloadable AS Level & A Level Biology revision notes
Diagram illustrating triglyceride synthesis via condensation of glycerol and three fatty acids, forming ester bonds and releasing three water molecules.
Formation of a triglyceride from a glycerol molecule and three fatty acid molecules by the process of esterification.

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

Diagram illustrating an insoluble droplet with hydrophilic glycerol heads and hydrophobic fatty acid tails, showing water-soluble molecules blocked.
Triglycerides are suitable as energy storage molecules as they form insoluble droplets inside cells

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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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Cara Head

Reviewer: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding

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