A-Level Biology (OCR) – Lipids: Triglycerides, phospholipids & cholesterol

2.9 - Lipids: Triglycerides, Phospholipids & Cholesterol

1The structure and function of triglycerides and phospholipids
2The synthesis and breakdown of triglycerides
3The similarities and differences between triglycerides and phospholipids
4The structure and function of cholesterol

11 min

Triglycerides

A triglyceride is a type of lipid used as a store of energy in animals, plants, and some bacteria.

Diagram showing the structure of a triglyceride with a glycerol backbone and three fatty acid tails.

The left diagram is a simplified version of a triglyceride. The diagram on the right shows the atoms a triglyceride is made up of.

A triglyceride consists of a glycerol backbone attached to three fatty acid tails. Each fatty acid tail contains a hydrocarbon chain (R) which can vary in length and may be saturated or unsaturated.

Features that allow triglycerides to store energy efficiently:

Long hydrocarbon tails - Their many carbon-hydrogen bonds can be broken to release energy.
Low mass to energy ratio - Lots of energy can be stored in a small volume.
Insoluble - They do not affect the water potential of cells as they are large and non-polar.
High ratio of hydrogen to oxygen atoms - Triglycerides will release water when oxidised.

Triglyceride formation and breakdown

Triglycerides are synthesised via condensation reactions and broken down via hydrolysis reactions. These reactions involve the formation or the breakdown of covalent bonds known as ester bonds.

Condensation reaction

Condensation:

The hydroxyl groups (OH) on the glycerol and on the three fatty acids react together to release three water molecules (H₂O).
This results in three ester bonds between the glycerol and the fatty acids.

Hydrolysis reaction

Hydrolysis:

The addition of three water molecules (H₂O) breaks the ester bonds.
This separates the glycerol and the fatty acids.

Phospholipids

A phospholipid is a type of lipid used as a structural component of the cell membrane.

Diagram showing the structure of a phospholipid with a phosphate group, glycerol, and two fatty acid tails.

They are similar to triglycerides except one of the fatty acid tails is replaced by a phosphate group.

Phospholipids are polar

A phospholipid is made up of two parts:

A hydrophilic 'head' - This contains glycerol and phosphate.
A hydrophobic 'tail' - This contains fatty acids.

Diagram showing the structure of a phospholipid with a hydrophilic head containing glycerol and phosphate and a hydrophobic tail containing fatty acid.

The phosphate group is polar and so attracts water (hydrophilic) whereas the fatty acid tails repel water (hydrophobic).

Phospholipid bilayer

When phospholipids are placed in water, they arrange themselves into a double layer (bilayer) so that the hydrophilic heads are facing out (towards the water) and the hydrophobic tails are facing in (away from the water).

Diagram showing the arrangement of phospholipids in a bilayer with hydrophilic heads facing outwards and hydrophobic tails facing inwards.

This arrangement creates a hydrophobic centre in the bilayer so that water-soluble substances cannot pass through.

Cholesterol

Cholesterol is a type of lipid known as a sterol and is used by animal cells to increase the stability of the cell membrane.

Cholesterol is also used to make vitamin D, steroid hormones, and bile.

Diagram showing the structure of a cholesterol molecule with hydrophilic and hydrophobic regions.

Like phospholipids, cholesterol is a polar molecule. The hydroxyl group (OH) is hydrophilic whereas the rest of the molecule is hydrophobic.

Cholesterol in the cell membrane

Illustration showing cholesterol molecules within a cell membrane, binding to phospholipid tails to reduce membrane fluidity.

Cholesterol binds to phospholipid fatty acid tails, causing the phospholipids to pack more closely together. This reduces the fluidity of the cell membrane.

Comparing triglycerides and phospholipids

Comparison table showing the differences between triglycerides and phospholipids, including function, number of fatty acid tails, presence of phosphate, and polarity.