A-Level Biology (OCR) – Lipids: introduction

2.8 - Lipids: Introduction

1The roles of lipids in living organisms
2The difference between saturated and unsaturated fatty acids
3How to test for lipids

8 min

What are lipids?

Lipids are biological molecules that contain the elements carbon (C), hydrogen (H), and oxygen (O). However, lipids contain a much lower proportion of oxygen than carbohydrates.

Lipids are not made up of long chains of monomers, meaning they are not considered as polymers.

Roles of lipids

The main functions of lipids:

Energy supply - Lipids can be oxidised to provide energy to cells.
Structural components - Phospholipids are used in cell membranes.
Waterproofing - Insoluble lipids are used to form water-resistant barriers.
Insulation - Lipids can help retain heat or act as electrical insulators.
Protection - Delicate organs are surrounded by a layer of fat.

Fatty acids

Most lipids are made up of fatty acids combined with an alcohol (usually glycerol).

Diagram showing the structure of a fatty acid with a carboxyl group and a hydrocarbon chain.

Fatty acids consist of a carboxyl group (-COOH) attached to a hydrocarbon chain (R group).

Saturated fatty acids and unsaturated fatty acids

There are two types of fatty acid: saturated fatty acids and unsaturated fatty acids.

Saturated fatty acids:

These have hydrocarbon chains that are 'saturated' with hydrogen, meaning all carbon atoms are bonded to the maximum number of hydrogen atoms.
The hydrocarbon chain has no carbon-carbon double bonds.
Lipids that contain saturated fatty acids have higher melting points and so are usually solid at room temperature (fats).

Diagram showing the structure of a saturated fatty acid with a hydrocarbon chain and no carbon-carbon double bonds.

Unsaturated fatty acids:

These have hydrocarbon chains that do not contain the maximum number of hydrogen atoms bonded to the carbon atoms.
The hydrocarbon chain has at least one carbon-carbon double bond, which causes the chain to kink.
Lipids that contain unsaturated fatty acids have lower melting points and so are usually liquid at room temperature (oils).

Diagram showing the structure of an unsaturated fatty acid with a carbon-carbon double bond.

Unsaturated fatty acids may either be monounsaturated or polyunsaturated:

Monounsaturated - One double bond.
Polyunsaturated - Two or more double bonds.

Testing for lipids

To find out whether a sample contains lipids, you must carry out the emulsion test.

Illustration showing the steps of the lipid emulsion test including adding ethanol and water to a food sample and observing a milky white emulsion for positive results.

Steps to find out whether a sample contains lipids:

Place your food sample in a test tube.
Add 2 cm³ of ethanol.
Shake.
Add 2 cm³ of distilled water.
If lipids are present, a milky white emulsion will appear.