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