A consistent observation made for plasma membranes that was not explained by the bimolecular lipid leaflet model was the very low surface tension of the cell membrane.

In 1935, J. F. Danielli and E. N. Harvey proposed that oil droplets and other lipid inclusions in cells were bounded at their surfaces by an organized layer of the lipid and a layer of protein.

It was postu­lated that the protein, which consisted of a monomo­lecular layer of hydrated molecules, faced the aqueous cytoplasm and simultaneously interacted with the po­lar portions of the lipid layer.

The nonpolar portions of the lipid layer faced the hydrophobic oil phase of the droplet interior (Fig. 15-4).

In this structure, the nat­ural surface activity of the protein would account for the low interfacial tension of the droplet membrane. Shortly thereafter, Danielli and H. Davson suggested that the plasma membrane itself might be composed of two such lipid-protein bilayers—one facing the inte­rior of the cell and the other facing the external milieu (Fig. 15-5).

Danielli and Harveys 1935 Model of the Protein-Lipid Bilayer formed at the Interface between an Oil Droplet in a Cell and the Cells Aqueous Cytoplasm

 

Daielli-Davson Membrane Model

In this arrangement, the association be­tween the surface proteins and bimolecular lipid leaf­let would be maintained primarily by electrostatic in­teractions between the polar ends of each lipid molecule and charged amino acid side chains of the polypeptide layers.

Either electrostatic or van der Waals bonds could bind other groups to the outer pro­tein surface. Danielli and Davson proposed that such a membrane would exhibit selective permeability, being capable of distinguishing between molecules of differ­ent size and solubility properties and also between ions of different charge.

By the early 1950s, several modifications were made in the Danielli-Davson membrane model. For example, it was suggested that glycoproteins might be adsorbed to the outer membrane surface, thereby accounting for the antigenic properties of cell mem­branes. Pores through which certain materials are ex­changed between a cell and its environment were pre­sumed to exist, the channels formed by periodic continuities (bridges) between the outer and inner protein layers.

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