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| What
do scientists know about lipid organization in membranes? |
Scientists believe that
lipids
form a double layer, because the alternative
is for the charged groups to face each other. Because the electrical
charges would repel each other, and because the charge groups are
attracted to water, this is the logical way for lipids to self-organize
(see below).
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| Membranes
self-organize |
Ever notice that when your mother cooks soup, when it cools the fat rises
to the top and congeals? When hot, the fats do not aggregate well,
but rather mix in with the water part of the soup. What happens if you pour a little
oil or other kind of fat into a bowl of water? It will self-organize
as a film of oil at the top. It floats because fat is lighter than
water. It forms a film because the electrical forces of water grab
the charged head groups and make them line up.
- The amount of lipid in red
blood cells, which have only protein on the inside, is double
what it would take to cover the outside diameter of the cell. Thus, it must occur in
two layers.
(See " How We Find Out" for more details.)
- In water,
lipids group together spontaneously to form membranes. Think about
what this says about the importance of water.
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So
how are proteins and lipids related to each other in the membrane?
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- Scientists know that proteins are closely associated with
membranes, because dissolving
membranes will pull out both protein and lipid.
- Drs. Danielli and Davson (see "Story
Time") thought that membranes were built
like a sandwich, with the protein "bread" on the outside
and a double layer of lipid "meat" on the inside.
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This is how Danielli and Davson thought about the structure of
membranes. The blobs at the top and bottom of the diagram
represented assumed layers of protein. |
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