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Many cells have a cell membrane composed of two layers of lipids, why is it two layers and not just one?

What purpose do the membranes serve?

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By "double membranes" do you mean two bilayer membranes (i.e. as is the case for mitochondria)? Because if you do, then you are wrong to say that almost all cells have double membranes. –  Alan Boyd Jul 13 '13 at 17:12
The title edit has made the question worse. Most cells do not have double membranes. A bilayer is not a double membrane. –  Alan Boyd Jul 13 '13 at 18:33
@alanboyd sorry I didn't see your comment when I made the edit but you are right –  rg255 Jul 14 '13 at 7:53

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The lipids that make up the cell membrane have a hydrophilic (water loving) head and a hydrophobic (water hating) tail - the wiki article explains this in more detail and I will add more to this answer soon but here is a key quote from the wiki page -

"The cell membrane consists primarily of a thin layer of amphipathic phospholipids which spontaneously arrange so that the hydrophobic "tail" regions are isolated from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the intracellular (cytosolic) and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer."

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The lipid bilayer creates an environment that allows the passage of some gasses and ions in a passive manner. Alternatively, important integral membrane proteins such as proton pumps are embedded in the bilayer and create a voltage differential in the cell. The pumps are powered by ATP - consider studying symporters and antiporters. Important proteins get anchored in the bilayer in order to perform their functions.

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Actually there are monolayers in some extremophilic archeon, wich have both layers of the regular membranes attached by covalent bonds.

However, the main reason is that membranes are formed by virtue of hydrophobic interactions. Phospholipids are amphipathic mollecules wich have a polar "head" (given by the negative charge of the phosphate group) and an hydrophobic "tail" (usually two alliphatic chains).

The mollecule tries to avoid the state of minimal entropy, wich in this case would be to "swim" freely in the water. In this hypothetical state, water mollecules would need to orientate themselves in order to minimize the contact with the hydrophobic tail (wich are very big mollecular gropus who doesn't atract the polar water mollecules) and to maximize the contact with the polar head. If this mollecule finds more mollecules of their kind they will stick together by their hydrophobic tails, because that will reduce the urge to organize water mollecules, and by so, raising the entropy of the system. There are some posible conformation wich allow to do this, and one or other will produce depending of the composition of the lipid mix.

Lastly, membranes serve mainly as selective barriers wich separate the internal cell, whose conditions are regulated, and the outside. Membranes can have embeded enzymatical systems, such as photosystems, respiratory chains or some key metabolic paths. Also, they usually contain sensitive proteins who respond to the external condictions and regulate the behaviour of the cell. Other functions can be found, but those are the most important.

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