The Davson Danielli model (or the paucimolecular model) was an illustration that depicts the membrane in cells. It was developed in the year 1935 by Hugh Davson and James Danielli The model is a bilayer of phospholipids which is located within two layers of protein globular. the model is trilaminar as well as lipoproteinous.
The first theory to define the position of proteins in the bilayer of lipids found in membranes. Danielli and Davson suggested a model in which two layers of proteins flanked by a central bilayer of phospholipids.
The model was called a ‘lipo-protein sandwich’ since the layer of lipids could be sandwiched by two layers of protein. The Davson-Danielli model was dominant before Singer and Nicolson improved their fluid mosaic theory in the year 1972. It was a fluid mosaic that grew on the Davson and Daniels model by including transmembrane protein and eliminating the previously proposed flanking protein layers which were not substantiated by evidence from experiments.
Key Features of the Davson–Danielli Model
- Danielli and Davson suggested the sandwich model, to describe membrane structure, in which the bilayer of lipids is coated both sides with dried protein (globular protein).
- Therefore the plasma membrane could comprise two bilayers of lipids and proteins, one that faces the interior of the cell, and the other one facing the outside environment.
- In this configuration the link between surfaces proteins as well as the bimolecular lipid leaflet could be maintained through Electrostatic interaction between the poles end of each lipid molecule as well as charge-charged amino acids side chains in the polypeptide layers.
- Electrostatic or van der Waals bonds may bind different groups to the protein’s outer surface.
- Danielli and Davson thought that membranes could exhibit the ability of separating molecules of different sizes and solubility characteristics as well as between ions with different charges.
- Based on the speed at the rate at which different molecules pass through to the cell membranes, researchers estimated the bilayer of lipids to be approximately 6.0 millimeters thick and each of the proteins layers to be around 1.0 millimeter thickness, which gives an overall thickness of around 8.0 millimeters.
- A protein-lipid sandwich
- Lipid bilayer made up of the phospholipids (hydrophobic heads inside and hydrophilic head outside)
- Proteins coat the surface of the outside
- Proteins are not able to penetrate the bilayer of lipids.
Support of Davson–Danielli model
- The Danielli Davson model was supported by electron microscopy.
- In electron micrographs of high magnification the membranes appear as dark parallel lines , and an area of lighter color between.
- Proteins appear dark on electron micrographs while the phospholipids appear white, showing layers of proteins or central phospholipid.
- The size of membranes also was estimated to be around 7.5 millimeters.
Problems in the Davson–Danielli Model
- It was assumed that all membranes are uniform in thickness and had an equal ratio of lipids and proteins.
- It was assumed that all membranes have symmetrical external and internal surfaces (i.e. not bifacial)
- It didn’t take into account the permeability properties of some substances (did not realize the necessity of hydrophilic pores)
- It was observed that the temperatures in which membranes began to solidify were not in line with the temperatures predicted by the model.
Falsification Evidence for the Davson–Danielli Model
- Membrane proteins were found to be insoluble when in the water (indicating the existence of hydrophobic surface) and differed in dimensions. These proteins are not capable of forming a uniform and continuous layer on the exterior of membranes.
- Tagging membranes with fluorescent antibodies proteins revealed that they are mobile and were not fixed in the same place. Membrane proteins from two distinct cells were labeled by green and red fluorescent markers, respectively. When the two cells combined, the markers became interspersed across the membrane of the fused cell. This showed that the membrane proteins were able to move, and not create the static layer (as as per Davson-Danielli).
- Freeze fracture was used to open up the membrane. It revealed rough and irregular surfaces within the membrane.
- These rough surfaces were thought of as transmembrane proteins which suggests that proteins aren’t only restricted to the exterior of the membrane.
Based on those limitations, a brand new model was suggested in the work of Seymour Singer and Garth Nicolson in 1972. According to this model proteins were contained within the bilayer of lipids instead of having distinct layers. This model, referred to as the fluid-mosaic model is the one that is used by researchers to this day (with modifications).
- Lodish, H. F., Berk, A., Kaiser, C., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. L., Matsudaira, P. T. (2008). Molecular cell biology. New York: W.H. Freeman.
- Smith, C. M., Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’ basic medical biochemistry: A clinical approach. Philadelphia: Lippincott Williams & Wilkins.