The plasma membrane

The plasma membrane has many special properties which derive from the polar nature of the phospholipid molecule. One end, the so called head is hydrophilic. This means it is attracted to water. The other end, the so called tails, are hydrophobic. This means they are

repelled by water. These attraction and repulsion forces mean the molecules have a property of self organisation or assembly when they are in aqueous(water based) solutions. They can form the classical plasma membrane bilayer but also can form liposomes or micelles.

This property is very important in vesicle formation in the transport of substances into cells (endocytosis), out of cells (exocytosis) and the transport of substances between organelles within cells. The membranes of organelles are also made of this bilayer.

These diagrams show how the hydrophobic and hydrophilic propeties of phospholipid maintains a flexible membrane structure

The hydrophobic heads are repelled and this rotates the phospholipids away from the fluid inside and outside the cell.

This results in a stable but fluid and flexible double layered membrane. It forms a water proof barrier between the inside and outside of the cell

The stability of the structure

is also controlled by the number of cholesterol molecules in the phospholipid bilayer. The phospholipid tails are attracted to one end of the cholesterol molecules which are also polar.

This structure allows some small molecules (for instance Carbon dioxide and oxygen) to diffuse through the bilayer. It also allows the membrane to change shape, form vesicles and allow different types of proteins to attach to its surface (peripheral proteins) or fit into its structure (integral proteins).