Fluid mosaic model
It refers to the fluid nature of the membrane. Some of the proteins are fixed but others can move around within the bilayer. Mosaic refers to the mosaic like structure of all the parts together. It refers to some peripheral proteins that are loosely attached on the surface of the membrane whilst integral membrane proteins are fully embedded within the phospholipids, some even spanning both layers.
Protein Sandwich model
The most widely accepted model until the early 1970s was a three-layer protein-lipid sandwich, based on the evidence of the electron micrographs in which the dark outer layers were thought to be proteins and the lighter region within was the lipid. The protein-lipid sandwich does not allow the hydrophilic phosphate heads to be in contact with water, nor does it allow the non-polar hydrophobic amino acids on the inside of the membrane proteins to be kept away from water. Interpretation of the electron micrograph changed to support the new model of membrane structure. The phosphate heads are more electron dense and show up as the darker edges to the membrane with the tails forming the lighter inner parts of the sandwich.
Several integral proteins were investigated further and shown to have regions at their ends that had polar hydrophilic amino acids, with the middle portion being mainly composed of non-polar hydrophobic amino acids.
Frozen membrane sections were fractured along the weak point between the lipid layers, and the inner fractured surface coated in a heavy metal. Scanning electron microscopy revealed a smooth mosiac-like surface interspersed by much larger particles.
Several experiments were carried out using only labelled molecules that only attach to other specific molecules. In one experiment plant proteins called lectins that bind to polysaccarides were labelled with ferritin which is visible under the electron microscope. When mixed with membrane samples, the lectins only bond to the outer surface of the mambrane, and never to the inner. This showed that membranes are asymmettric, i.e. the outside surface of the membrane is different to the inside.
Mouse and Human cell experiment
Involved fusing mouse cells with human cells. Before the cells were fixed a specific membrane protein was labelled in each cell type. The mouse membrane protein was given a green flourescent label, and the human membrane protein was given a red flourescent 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 halfs, but another 40 minutes at 37°C there was complete intermixing of the proteins. The only way the proteins could have intermixed was to have diffused through the membranes, showing that the componants were indeed fluid.
What affects the fluidity of the membrane?
The more phosolipids containing unsaturated fatty acids there are present in the membrane, the more fluid it is. The 'kinks' in the hydrocarbon tails of the unsaturated phosolipids prevent them from packing closely together, so more movement is possible. Cholesterol reduces the fluidity of the membrane by preventing movement of the phosolipids.
What is the role of the cell membrane?
Many different types of protein are found in within the membrane, each tyoe having a specific function. Some function as enzymes. Others function as carrier and channel proteins involved in the transport of substances in and out of cells. Glycoproteins and glycolipids have important roles in cell to cell recognition and as receptors.