The Different Components of Cell Membranes have Different Roles

PHOSPHOLIPIDS FORM A BARRIER TO DISSOLVED SUBSTANCES

Phospholipid molecules have a 'head' and a 'tail'.
The head is hydrophilic - it attracts water - and the tail is hydrophobic - it repels water.
The molecules automatically arrange themselves into a bilayer - the heads face outwards towards the water on either side of the membrane.
The centre of the bilayer is hydrophobic so the membrane doesn't allow water soluble substances (like ions) through it - it acts as a barrier to these dissolved substances.

CHOLESTEROL GIVES THE MEMBRANE STABILITY

Cholesterol is a type of lipid.
It's present in all cell membranes (except bacterial cell membranes).
Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
Cholesterol helps to maintain the shape of animal cells (which don't have cell walls). This is particularly important for cells that aren't supported by other cells, e.g. red blood cells, which float free in the blood

	Created by Lucy Nove over 9 years ago

A2 Biology (Cell Membranes) Note on Cell Membrane Structure, created by Lucy Nove on 22/03/2017.