Cell membrane structure

The basic structure of a bilayer is two phosholipids. In a normal lipid molecule there are 3 fatty acids and a glycerol. In a phospholipid there are only 2 fatty acids. The third fatty acid is replaced by a negatively charged phosphate group. The phosphate head of the molecule is polar; one end is slightly positive and the rest is slightly negative. This makes the phosphate head attract other polar molecules, like water, and it is therefore hydrophillic. The fatty acid tails are non-polar and therefore hydrophobic. So when added to water phospholipids arrange themselves to avoid contact between the hydrophilic tails and water. They may form a layer on the surface with their hydrophillic tails directed out of the water, arrange themselves into spherical clusters called micelles or form a bilayer. The bilayer is favoured by phosolipids because the 2 fatty acids are too bulky to fit into the interior of a micelle. The formation of bilayers by phospholipids is of critical biological importance. A lipid bilayer will tend to close on itself so that there are no edges with exposed hydrocarbon chains, thus forming compartments.

The cell surface membrane phospholipids tend to adopt their most stable arrangement themselves, which is a bilayer. This arrangement avoids the hydrophobic fatty acid tails having any contact with the water on either side of the membrane but ensures the hydrophilic heads are in contact with water.

The cell surface membrane is not simply a phospholipid bilayer. It also contains proteins, cholestrol, glycoproteins and glycolipids. The model shows phospholipids pointing head  up. This is because the head section of the phospholipds is hydrophilic (soluble in water) where else the tail is hydrophobic (insoluble in water)

• Thus, having to know that, the bilayer only allows non-polar molecules to diffuse through a concentration gradient. Molecules which are sufficiently small (such as water, oxygen and carbon dioxide diffuses through the membrane freely regardless of being polar or not. Polar molecules are only allowed to enter if they are incorporated into a carrier protein at where it is actively transported into the cell or undergo facilitated diffusion (diffusion using a protein molecule).
• Glycolipids are also present on the cell surface membrane since it allows cell recognition and also to promote an immune response. 
• Carbohydrates and cholesterol function is to reduce the fluidity of the membrane and maintain its stability to ensure that no leaking occurs.

An experiment was done involving fusing mouse cells with human cells. Before the cells were fused a specific membrane protein was labelled in each cell type. The mouse membrane protein was given a green fluorescent label, and the human membrane protein was given a red fluorescent label. A light microscope was used to follow where the green and red proteins moved. Immediately after fusing the cells, the coloured labels remained in their respective halves, but after 40 minutes at 37C there was complete intermixing of the proteins. The only way the proteins could have intermixed was to have diffused through the membranes, showing the components were indeed fluid. 

More unsaturated phospholipids - more fluid
The more phospholipids containing unsaturated fatty acids there are present in the membrane, the more fluid it is. The 'kinks' in the hydrocarbon tails of the unsaturated phospholipids prevent them from packing closely together, so more movement is possible. Cholesterol reduces the fluidity of the membrane by preventing the movement of the phospholipids.