The cell membrane, a fundamental boundary separating a cell’s interior from its external environment, presented a significant challenge to early biologists. Its precise composition and arrangement remained uncertain, leading to various models. The Davson-Danielli model emerged as a prominent early attempt to describe this crucial cellular structure, influencing cell biology for decades.
The Davson-Danielli Model Explained
Proposed in 1935 by Hugh Davson and James Danielli, the model offered a specific architectural blueprint for the cell membrane, positing a “protein-lipid-protein sandwich” structure. At its core lay a phospholipid bilayer, a double layer of lipid molecules with their hydrophobic tails facing inward and hydrophilic heads facing outward. This central lipid bilayer was flanked on both its inner and outer surfaces by two continuous layers of globular proteins. While the concept of a phospholipid bilayer had been suggested earlier by Gorter and Grendel in 1925, the Davson-Danielli model’s novel contribution was the inclusion of these protein layers. The model suggested these proteins provided structural support and played a role in regulating substance movement across the membrane.
Evidence from Chemical Composition Studies
Early investigations into the chemical makeup of cell membranes provided support for the Davson-Danielli model. Researchers conducted chemical analyses, often using red blood cells, to determine the membrane’s constituents. These studies consistently revealed significant amounts of both phospholipids and proteins. Plasma membranes, for instance, consisted of approximately 50% lipid and 50% protein by weight.
A particularly influential finding came from Gorter and Grendel in 1925. They extracted lipids from red blood cell membranes and calculated that the total surface area these lipids would occupy in a single layer was roughly twice the surface area of the red blood cells themselves. This observation strongly suggested the cell membrane was composed of a double layer of lipids. The amphipathic nature of phospholipids, possessing both water-attracting (hydrophilic) heads and water-repelling (hydrophobic) tails, naturally lent itself to forming such a bilayer structure in an aqueous environment.
Evidence from Surface Behavior and Permeability
Two lines of experimental evidence, surface behavior and permeability, were central to the Davson-Danielli model’s initial acceptance. Experiments measuring the surface tension of cell membranes demonstrated values significantly lower than those of a pure lipid-water interface. This reduced surface tension led Davson and Danielli to propose that protein layers coated the lipid bilayer, effectively lowering the membrane’s interfacial tension.
The selective permeability of the cell membrane also aligned with the model’s predictions. Observations showed that membranes allowed small, non-polar molecules to pass through relatively easily, while restricting larger or charged molecules. Davson and Danielli further proposed that the membrane’s permeation properties could be explained by its hydrophilic and lipophilic regions, with pores potentially allowing for the exchange of certain materials.
Early Electron Microscopy Findings
The advent of electron microscopy (EM) provided what appeared to be direct visual confirmation for the Davson-Danielli model. Early electron micrographs of cell membranes, especially after being stained with osmium tetroxide, consistently displayed a characteristic “railroad track” or “tramline” appearance. This visual pattern consisted of two distinct dark lines separated by a lighter space.
Scientists interpreted these dark lines as the protein layers, which stained heavily with osmium, and the lighter, unstained space as the lipid bilayer situated between them. The observed total thickness of the membrane, typically around 7.5 to 8.0 nanometers, also closely matched the dimensions predicted by the Davson-Danielli model, which proposed a 6.0 nanometer lipid bilayer flanked by 1.0 nanometer protein layers. This visual evidence supported the proposed protein-lipid-protein sandwich structure.