The structure and function of lipids

In a nutshell

Lipids are important biological molecules made up of carbon, hydrogen and oxygen. Triglycerides and phospholipids are two important types of lipids. They have special features that allow them to perform their function. To test for lipids in sample, you should perform the emulsion test.

Triglycerides

Triglycerides are a form of lipid made up of one molecule of glycerol and three fatty acids.

Biology; Biological molecules; KS5 Year 12; The structure and function of lipids — Triglyceride structure: 1. Hydrocarbon fatty acid tail.

Fatty acids

Fatty acids are long chains with hydrocarbon tails. These tails are hydrophobic meaning they repel water which makes lipids insoluble in water. They all have the same basic structure with varying hydrocarbon tails.

Biology; Biological molecules; KS5 Year 12; The structure and function of lipids — The basic structure of a fatty acid: 1. Carbon atom linking fatty acid to glycerol, 2. Variable hydrocarbon chain (R).

Condensation reactions

Triglycerides are formed via condensation reactions. This means when a fatty acid and glycerol join they form an ester bond which releases a molecule of water when it is created.

Biology; Biological molecules; KS5 Year 12; The structure and function of lipids

1.	Fatty acid
2.	Two more fatty acids are attached in the same way here
3.	Condensation reaction
4.	Ester bond
A.	Glycerol
B.	Triglyceride

Saturated and unsaturated fatty acids

Saturated fatty acids do not have any double bonds between carbon atoms, therefore, the fatty acid is 'saturated' with hydrogen.

Biology; Biological molecules; KS5 Year 12; The structure and function of lipids — 1. A saturated hydrocarbon tail

On the other hand, unsaturated fatty acids do have at least one double bond between carbon atoms and this causes the chain to kink.

Biology; Biological molecules; KS5 Year 12; The structure and function of lipids — 2. An unsaturated hydrocarbon tail

Function of triglycerides

Triglycerides are energy storage molecules. They have certain features that help them carry out this function.

Feature	Function
Long hydrocarbon tails	These tails contain lots of chemical energy that will be released when it is broken down. This is why lipids contain approximately twice as much energy per gram than carbohydrates.
Insoluble	They do not affect the water potential of the cell, so water does not enter by osmosis and cause them to swell. Triglycerides are able to form insoluble droplets in cells. This is because their fatty acid tails are hydrophobic so they face inwards to protect themselves from water, whilst the glycerol head face outwards. An insoluble lipid droplet

Phospholipids

Phospholipids are another type of lipid. They are mainly found in cell membranes and consist of one molecule of glycerol, two fatty acids and a phosphate group. The phosphate group is hydrophilic and the fatty acids are hydrophobic.

Biology; Biological molecules; KS5 Year 12; The structure and function of lipids — 1. A phosphate group, 2. Hydrocarbon tail

Function of phospholipids

Phospholipids make up the bilayer of cell membranes. Cell membranes are important because they control what enters and leaves the cell.

Feature	Function
Hydrophilic head and hydrophobic tail	They form a double layer with their heads facing outwards towards the water on either side. A phospholipid bilayer
Hydrophobic centre of the bilayer	This means water-soluble substances cannot pass through easily.

Emulsion test

To detect if there is any lipid in a sample, you can perform the emulsion test.

Procedure

1.	Add ethanol to your sample
2.	Shake for a minute.
3.	Pour the solution into water.
4.	The presence of lipid (a positive result) will produce a milky emulsion. The more lipid there is, the greater the milky colour. If there is no change in the solution (negative result), there is no lipid in the sample.

Biology; Biological molecules; KS5 Year 12; The structure and function of lipids

Role of lipids

The structure of lipids relates to their function. This is explained below.

Role	Explanation
Energy storage	Triglycerides can be oxidised in respiration to produce ATP. Lipids release twice as much energy as carbohydrates in respiration. They are therefore useful as a fat store for organisms to use during the winter period.
Waterproofing	Lipids are hydrophobic which means they orientate themselves away from water. Some mammals secrete oil from their sebaceous glands. This prevents their fur becoming waterlogged.
Insulation	Lipids are poor heat and ion conductors. Myelin is a lipid that surrounds nerves and prevents the transport of sodium and potassium ions. This speeds up the rate of the neural impulse.