Jim became famous for his work on biological membranes. Inhe wrote a famous book with his old friend, Hugh Davson, in which they developed a mental model of how membranes were built. Their model grew out of their research and that of others on the transport of materials across capillaries and red blood cell membranes. This model dominated thinking among world scientists for over a decade.
The following points highlight the top two types of membrane models. Micellar or Sub-Unit Models.
I Lipid Bilayer Hypothesis: Overton suggested that the cell membranes contain lipids. This conclusion was based on the fact that fat solvents dissolved the membrane easily and fat soluble substances passed easily through the cell membranes. Some workers like Hober and Fricke found low electrical conductivity in the cell membrane which indicated the presence of lipid layer.
On the basis of studies of cell membranes of erythrocytes they showed that the lipid extracted from the red cell ghost spread in area twice that of a simple molecular film.
So it was thought that the membrane consisted of double layers of lipid molecules, the polar hydrophilic groups of the molecules being situated on the outside and hydrophobic ends standing at right angles to the surface are oriented inwardly Fig.
Harvey and Coley and Danielli and Harvey studied surface tension of cell membrane and on the basis of their observation they pointed out the existence of protein molecules adsorbed on the surface of lipid droplets which reduce the surface tension of droplets. This conclusion led James Danielli and Hugh Davson in to suggest bimolecular leaflet model of cell membrane.
Danielli and Davson model was the first attempt to describe membrane structure in terms of molecules and to relate the structure to biological and chemical properties. According to bimolecular model of Danielli and Davson, plasma membrane consists of two layers of phospholipid molecules a bimolecular leaflet in which phospholipid molecules are arranged in such a way that hydrophilic heads of the phospholipid molecules face outside and hydrophobic non-polar lipid chains are associated in the inner region of leaflet.
The hypothesis also suggested that the polar ends of lipid molecules are associated with monomolecular layer of globular proteins.
The plasma membrane would thus consist of a double layer of phospholipid molecules sandwiched between two essentially continuous layers of protein Fig. This basic model has been modified from time to time. Danielli suggested the presence of two types of proteins; tangentially arranged in contact with the lipid and globular proteins on the outer surface.
Again Davson and Danielli and Danielli considered proteins to be in the form of a folded P-chain. Perhaps, these units form micelles of membranes indicated in recent electron micrographs. In still other variations the proteins are thought to be in coiled or globular form on both sides of lipid layers [Fig.
Models with globular proteins on both the sides or with folded P-proteins on both the surfaces and helical proteins extending into the pores are also suggested. David Robertson studied the cell membranes from electron micrographs of sectioned material. The preparations involved usual fixing in solutions of osmium tetraoxide and potassium permanganate KMnO4and dehydrating in solvents such as acetone before sectioning.
In late s Robertson summarised a large number of ultra-structural data obtained by him and some other workers and concluded that the plasma membrane and the membranes of all cell organalles were similar in structure.
Although the similarity is not resolved by light microscopy, it is clearly seen in electron micrographs. This conclusion led Robertson in to propose unit membrane hypothesis according to which all biological membranes show generalised unit membrane construction. The unit membrane model visualises cell membrane as a trilaminar and indicates structure consisting of two dark osmiophilic layers separated by a light osmiophilic layer.
The physical appearance of this trilaminar model has led to the term unit membrane. The unit membrane concept implies a trilaminar appearance with a bimolecular lipid layer between two protein layers Fig.
The clear Osmiophilic zone 35 A in thickness is a bimolecular layer of lipids without the polar groups. The plasma membrane surrounding the cell is thicker at the free surfaces of the cell than where it is in contact with other cells.- Analysis of evidence from electron microscopy that led to the proposal of the Davson- Danielli model.
- Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model. Phospholipid Bilayers: Some substances are attracted to water - they are hydrophilic. This conclusion led James Danielli and Hugh Davson in to suggest bimolecular leaflet model of cell membrane.
Danielli and Davson model was the first attempt to describe membrane structure in terms of molecules and to relate the structure to biological and chemical properties. c) Since it could not be reconciled with new evidence, the Davson & Danielli’s model was ultimately abandoned.
3 Figure 3: Membrane Research: Freeze-Fracture (’s). S2 Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model. S3 Analysis of the falsification of the Davson-Danielli .
membrane transport Davson-Danielli Sandwich Model () Sandwich Model () •. • Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model. • Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.
• Drawings of the fluid mosaic model of membrane structure can be two dimensional rather than three dimensional.