Phospholipids:
- Lipids in plasma membranes are MAINLY PHOSPHOLIPIDS.
- Phospholipids are pretty similar to triglycerides except ONE of the fatty acid molecules is replaced by a phosphate group/molecule.
- Fatty acid molecules repel water (hydrophobic); phosphate molecules attract water (hydrophilic).
- A phospholipid is therefore made up of two parts:
- Hydrophilic head = Interacts with water (attracted to it) but not with fat (phosphate group).
- Hydrophobic tail = Orients itself away/repels from water, but mixes readily with fat.
- Molecules that have to ends (poles) that behave differently in this way are said to be polar.
- This means that when these polar phospholipid molecules are placed in water they position themselves so that the hydrophilic heads are as close to the water as possible and the hydrophobic tails are as far away from the water as possible.

- All membranes around and within cells (inc those around and within cell organelles) have the same basic structure and are known as PLASMA MEMBRANES.
- The cell-surface membrane is the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment.
- Therefore allows different conditions to be established inside and outside a cell.
- Controls movement of substances IN AND OUT of the cell.
Phospholipid Bilayer Structure:
- Phospholipids form a bilayer sheet.
- Phospholipids are important components of cell-surface membranes for the following reasons:
1. One layer of phospholipids has its hydrophilic heads pointing inwards, interactive with the water in the cell cytoplasm.
2. The outer layer of phospholipids has its hydrophilic heads pointing outwards (interacting with the water which surrounds the cells).
3. The hydrophobic tails of both phospholipid layers point into the centre of the membrane, protected from the water on both sides.