The Davson–Danielli model (or paucimolecular model) was a model of the
plasma membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
of a cell, proposed in 1935 by
Hugh Davson and
James Danielli. The model describes a
phospholipid bilayer that lies between two layers of
globular proteins, which is both trilaminar and lipoprotinious. The phospholipid bilayer had already been proposed by Gorter and Grendel in 1925; however, the flanking proteinaceous layers in the Davson–Danielli model were novel and intended to explain Danielli's observations on the surface tension of lipid bilayers (It is now known that the phospholipid head groups are sufficient to explain the measured surface tension).
Evidence for the model included
electron microscopy
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
, in which high-resolution
micrographs
A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnify, magnified image of an object. This is opposed to a macrograph or photomacrograph, an image which is also taken ...
showed three distinct layers within a cell membrane, with an inner white core and two flanking dark layers. Since proteins usually appear dark and phospholipids white, the micrographs were interpreted as a phospholipid bilayer sandwiched between two protein layers. The model proposed an explanation for why certain substances were not able to pass through cell membranes, while also accounting for the thinness of membranes. Despite the Davson–Danielli model being scientifically accepted, the model made assumptions, and couldn't account for observed phenomena. For example, the model assumed that all membranes had the same structure, which couldn't explain how different types of membranes could have different functions. Another shortcoming of the Davson–Danielli model is that proteins, while
amphipathic, are mostly hydrophobic, so the existence of proteins on the outside of the cell membranes in direct contact with water presented an issue.
The Davson–Danielli model was scientifically accepted until
Seymour Jonathan Singer
Seymour Jonathan Singer (May 23, 1924 – February 2, 2017) was an American cell biologist and professor of biology, emeritus, at the University of California, San Diego.
Biography
Singer was born in New York City and attended Columbia Univers ...
and
Garth L. Nicolson advanced the
fluid mosaic model in 1972.
The fluid mosaic model expanded on the Davson–Danielli model by including transmembrane proteins, and eliminated the previously-proposed flanking protein layers that were not well-supported by experimental evidence. The experimental evidence that falsified the Davson–Danielli model included membrane freeze-fracturing, which revealed irregular rough surfaces in the membrane and
fluorescent antibody tagging of membrane proteins, which demonstrated their fluidity within the membrane.
See also
*
Fluid mosaic model
References
{{DEFAULTSORT:DavsonDanielli model
Cell anatomy