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According to my textbook, Davson-Danielli's model of a phospholipid bilayer sandwiched between two layers of globular protein was incorrect. The nonpolar protein portions would separate the polar portions of the phospholipids from water, causing the bilayer to dissolve. Meaning, the Davson-Danielli model is not only incorrect, but it is also impossible.

I understand why the model is incorrect, but not why it is impossible. In my view, the nonpolar proteins would remain together in a micelle shape due to hydrophobic exclusion. Because of this, the phospholipid bilayer sandwiched on the inside would remain isolated from the water.

Why would a bilayer dissolve if the proteins were in contact with water and the phospholipids were isolated from the water?

I think that my confusion stems from many misunderstandings about the chemistry and structure of the cell membrane. In depth illustration of the error that my book mentioned would be most helpful.

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2 Answers 2

up vote 6 down vote accepted

The original figure that Danielli and Davson proposed looks like this (from the original publication):

enter image description here

It shows the phospholipid bilayer of the membrane (which is correct) embedded between two layers of globular proteins. The hydrophobic tails of the lipids are orientated towards each other, while the hydrophilic heads are oriented to the outside.

Although the membrane composition is correct (this was already published 1925 by Gorter and Grendel), there are some problems with the proposed model:

  • Membranes are not identical. The differ in thickness and the ratio of proteins:lipids.
  • Membranes have distinct inside and outside layers (defined by the membrane proteins which are present on the surface of the membrane)
  • Other than predicted by the model, the membrane proteins do not have a very good solubility in water - in fact they are amphiphatic, meaning they have hydrophilic and hydrophobic regions. The hydrophobic side is anchored inside the membrane.
  • When the membrane proteins would cover the lipid bilayer, their hydrophobic regions would be in contact with water, which destabilizes this construct. Even if they would be oriented towards the membrane, they would face towards the hydrophilic heads of the phospholipids causing the same effect. Additionally the proteins would also seperate the hydrophilic phospholipid heads from the water. So there is no real stable solution in embedding the membrane with proteins.
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Just remember that some proteins can stick to the membrane kind of that diagram propose, although they are not true membrane proteins. For example, Bee Venom Phospholipse A2 has a membrane binding surface where it binds to the membrane and hydrolyzes phospholipids. Of course it doesn't stick on the membrane forever, it binds to the membrane, breaks down a few lipids, then floats away to repeat the cycle. –  user137 Aug 6 at 15:02
    
But these proteins doesn't cover the whole membrane as the model suggested. Most proteins simply "float" in the membrane. –  Chris Aug 6 at 15:08

Problems with the davson danielli model :

  1. freeze-etched micrographs:

This is a technique of rapidly freezing the cell and then fracturing them. The fracture occurs along lines of weakness and in these micrographs there were two prominent dark lines which indicate the phospholipid bilayer and but there were transmembrane proteins even in between the phospholipid bilayer unlike what the davson danielli model suggested (that the phospholipid bilayers contained two layers of proteins on top and on bottom.)

enter image description here enter image description here

2)structure of membrane proteins :

improvements in biochemical tech allowed people to extract proteins from the membranes. They were different sizes and globular in structure.They were unlike the type that would form continuous layers on the periphery of the membrane. Proteins were hydrophilic in nature and hence should have been attracted to the tails and not the heads.

3)Fluorescent anti-body tagging :

red and green markers were attached to the antibodies that bind to membrane proteins. these two cells were fused together and 40 minutes later it was seen that the red and green markers were mixed throughout the membrane of the fused cells. therefore the proteins were free to move within the membrane rather than being fixed in a peripheral layer.

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