As previously mentioned, phospholipids form a "double layer", called a bilayer.
- This is due to their structure, they are composed of a "head" and a "tail".
- The head is hydrophilic, meaning it can interact with water, whereas the tail is hydrophobic and it repels water.
This forms a bilayer as the hydrophilic heads face outwards towards the water, whilst the hydrophobic tails "hide" on the inside of the bilayer, away from the water.
- This means the inside of the bilayer is also hydrophobic and water-soluble substances e.g. ions and polar molecules cannot diffuse directly through the membrane.
- However small, non-polar substances can diffuse through the membrane.