Membranes

Phospholipids make up the cell membrane

Each phospholipid consists of a phosphate head linked to 2 fatty acid chains

The head is hydrophilic and interacts with water and the tails are hydrophobic (amphipathic)

Phospholipids create two layers to make the cell's double membrane

--> naturally form continuous sheet-like bilayers in water

Phospholipids are synthesised by the smooth ER and transported as vesicles to the cell membrane

Large molecules and hydrophilic particles don’t pass easily between the hydrophobic hydrocarbon chains

This gives membranes low permeability

Low permeability allows for the maintenance of a concentration gradient

Holds useful substances inside the cell and prevents harmful substances form entering
The properties of the barriers formed by phospholipid bilayers enable them to carry out life's processes. These properties include:

• Low permeability

• Fluidity