All cell membranes have a phospholipid bilayer, made of two layers of lipid molecules, forming a barrier around all cells. Each phospholipid consists of a hydrophobic interior and a hydrophilic exterior.
Phospholipids are a type of lipid that form cell membranes. They are composed of:
- An alcohol molecule: sphingosine or glycerol
- Two fatty acids: 1,2-diacylglycerol
- A phosphate group: linked by a phosphodiester bond to other molecules, such as choline, serine or ethanolamine.
Phospholipids are divided into two groups:
- Phosphoglycerides: where the alcohol is glycerol, a short-chain alcohol.
- Sphingolipids: the alcohol is sphingosine, a long-chain alcohol.
Each phospholipid is amphipathic, which means they have two hydrophobic tails and one hydrophilic head. The hydrophobic tails (also called apolar) point inward toward each other because it repels water, and the hydrophilic heads (also called polar) point outward because it attracts water. So they associate through hydrophobic interactions between the long-chain fatty acid portions of adjacent molecules. And this is why the polar heads are exposed outward (where there is water) where they can interact with other molecules. And the apolar tails are located towards the inside of the lipid bilayer, i.e. towards the inside of the cell.
So, a cell membrane has two layers of amphipathic phospholipids with their tails pointing inward toward each other, and the head pointing outward, in contact with the external environment.