Levels of Organization Image Map
How do modern scientists picture the position of proteins in the cell membrane?

Cell Structure Image
Scientists discovered that portions
of membrane proteins "hang outside" the membrane, both on the inner and outer surfaces. The portion of the protein embedded inside the membrane is coiled (red part of diagram), but outside the membrane, the chemical forces of water partially uncoil the proteins. 

Why do parts of the protein remain outside the membrane?

These portions of membrane proteins MUST reside outside the membrane, because the terminals of proteins have electrically charged regions that are attracted by the water on the inner and outer surfaces. These parts of the protein are not attracted to lipid, because the lipid core has no electrical charge.

With this new understanding of how proteins and membranes interact, an overall model of the structure of cell membranes was developed.

 In 1972, S. J. Singer and G. L. Nelson proposed the modern model of membranes below: 

Membrane and Protein Diagram Image taken from ZY 560 Mammalian Physiology Homepage at http://www.auburn.edu/academic/classes/zy/560/membrane/sld003.htm 

Diagram of the modern view of membrane proteins (the orange, pink, and purple structures in the membrane). The globs are 3-dimensional representations of protein, which in reality consist of multiple coils bunched together in ways that a pore could be created. Blue structures are lipid. 

Note how old ideas in science sometimes get re-instated.  In the process of disproving the Danielli-Davson model, modern scientists have actually proved that the original ideas were partially right. There IS protein on the outside and inside of membranes. 


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