1.3 - B - Membrane Models
The Davson-Danielli model was constructed in the 1930's by when Davson and Danielli proposed that layers of proteins surrounded the phospholipid bilayer. They believed this because the membrane was such an effect barrier despite being so thin that it must be sandwiched by proteins on either side.
In the 1960's, Singer and Nicolson proposed a new model whereby the proteins were embedded in and across the membrane. These would be the periperhal and integral proteins that we know today. They likened the proteins to tiles in a mosaic which gives the model its name, the fluid mosaic model.
Due to the movement of the phospholipids laterally and between the layers, proteins are able to move as if they are 'floating' in a liquid. As mentioned previously, the cholesterol molecules inside of the membrane prevent it from being too fluid or solid.
Falsification of Davson-Danielli Model
Early micrographs of cell membranes showed two dark lines with a lighter middle. Proteins appear dark in micrographs and phospholipids appear light, so it fit the model at the time. This was accepted for ~30 years until evidence was presented in the 1950-60's that did not fit the model. The primary experiments are discussed here:
Freeze-etching is a process that involves the rapid freezing and fracturing of cells. Then, the cells are fractured, which splits them along the center of the membrane which can be visualized under an electron microscope.
When they observed the micrographs, they noticed that the inner membrane was dotted with tiny structures. They determined that these were globular proteins that were embedded in the membrane, which contradicted the protein-sandwich structure proposed by the Davson-Danielli model.
Membrane Protein Structure
Biochemical techniques allowed scientists to proteins from the cell membrane. This provided several ideas that contradicted the sandwich model:
Antibodies are molecules that are able to attach to specific proteins. Scientists tagged two types of cells with either red or green fluorescent antibodies. Then, they fused the cells together and allowed them to sit. As shown in the diagram below, after some time the antibodies mixed on the surface of the membrane instead of staying in separate halves. This indicated the membrane proteins were not fixed, but rather were able to float freely on the membrane surface.