The structure and function of lipids
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Explainer Video
Summary
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.
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.
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.
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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.
On the other hand, unsaturated fatty acids do have at least one double bond between carbon atoms and this causes the chain to kink.
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.
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. |
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. |
FAQs - Frequently Asked Questions
What are saturated fatty acids?
Saturated fatty acids do not have any double bonds between carbon atoms, therefore, the fatty acid is 'saturated' with hydrogen.
What are 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.
What are triglycerides?
Triglycerides are a form of lipid made up of one molecule of glycerol and three fatty acids.