Levels of Organization Image Map

How did they prove that membranes exist as two layers of lipid?

When lipid molecules are in water, the nature of their chemistry is such that they separate in a way that is a little more complicated than just a single film of lipid. A key discovery came in 1925, when two scientists, Groter and Grendel, discovered that the total surface area of a lipid that could be separated from a red blood cell was directly related to the surface area of the red blood cells. Indeed, the amount of lipid that could be extracted was exactly double that needed to completely surround the cell. 

Maybe the lipid wrapped around the cell twice!  We could guess at this just by knowing the chemistry. A lipid molecule has two chains of carbon atoms. Think of it like a kite with two tails. These carbon chains are not attracted to water, but they are attracted to the carbon chains of other lipids. 

 Lipid Bilayer
Lipid Image
The surface area of lipid extracted from a red blood cell is exactly double the surface area of the cell. This observation led to the idea that cell membranes must contain two layers of  lipid.
If we know the chemical properties of lipids, we can figure out how they must self organize in cells, which contain mostly water.
  • One end has chains of carbon, which are repelled by water
  • The other end dissolves in water (that is, is attracted to water)
  • Thus, at the surface of water, one end of the lipid is attracted to water and the carbon tails point out away from water.
  • A second layer can occur because the carbon tails of two lipids attract each other.
How do you think membrane lipids interact with proteins?


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