Lipids are another group of organic compounds. They make up the cell membrane and are used as energy stores amongst other things in the cell. They are insoluble in water but not alcohol. Fats and oils are examples of triglycerol lipids which are joined together by ester bonds.

Fatty Acids

Specification Reference

"Understand how a triglyceride is synthesised, including the formation of ester bonds during condensation reactions between glycerol and three fatty acids."
"Know the differences between saturated and unsaturated lipids".
Fatty acids all have a long hydrocarbon chain with a carboxyl group attached. They can either be saturated (have single carbon-carbon bonds) or unsaturated (double carbon-carbon bonds).

Synthesising Triglycerides
A fat or oil results when one, two or three fatty acids combine with glycerol to produce either a monoglyceride, a diglyceride or a trigylceride. These molecules are joined together by an ester bond which is formed during a condensation reaction. A molecule of water is removed.

The Nature of Lipids

Specification Reference

"Understand how the structure of lipids relates to their role in energy storage, waterproofing and insulation"
  • Lipids are very compact meaning that they release twice as much energy as carbohydrates do gram for gram.
  • The fatty acid tail is hydrophobic meaning that lipids repel water. This makes them useful as waterproofers.
  • Lipids have a low density meaning that thick layer can be used to insulate animals well whilst still allowing them to move easily and float.
  • Finally, they are insoluble in water so do not interfere with reactions in the cell.


Specification Reference

"Understand how the structure and properties of phospholipids relate to their function in cell membranes."
Inorganic phosphate ions (PO43-) are present in the cytoplasm. Sometimes, the hydroxyl groups of glycerol undergoes an esterfication with a phosphate group forming a phospholipid. The phosphate part is polar therefore meaning it is hydrophilic and dissolves in water. In contrast, the lipid tails are hydrophobic so do not dissolve in water. This means that phospholipids organise themselves into different structures in order to achieve stability in water. There are two possible arrangements.
  • Monolayers form between the water and the air.
  • Micelles are clusters of phospholipids which organise themselves into spherical shapes with the hydrophilic heads pointing outwards and the hydrophobic tails pointing inwards. This behaviour forms the basis of all cell membranes.