Phospholipids

Cell membranes have two main components, phospholipids and protein.

Chemically, phospholipids consist of glycerol bonded to two fatty acids and a phosphate group. The fatty acids are hydrophobic (water hating); they repel water and polar molecules. The phosphate group is hydrophilic (water friendly) and attracts water and polar molecules. Phospholipids have a hydrophilic ‘head’ and hydrophobic ‘tails’.

The chemical structure of a phospholipid is shown in the diagram.

The black circle highlights the charged phosphate group. The green rectangle highlights the two fatty acid components.

You do not need to be able to draw this!

A simpler representation of the structure is shown below.

This can be further simplified to:

If isolated phospholipids are placed on water, they form a monolayer with the hydrophilic heads in the water and the tails which repel water sticking out into the air.

In cells, as cytoplasm is watery and cells are bathed in watery solutions, phospholipids arrange themselves in a bilayer – with the hydrophilic heads next to the water on both sides and the hydrophilic tails in the centre.