Chapter 4: Planar Lipid Bilayers and Plasma Membranes.

Phospholipids will spontaneously associate into an extended monolayer at the interface between oil and a saline solution. They will behave similarly at the air surface of any aqueous solution, where the hydrocarbon tails form a very thin oil film and the polar heads project into the solution. Given this behavior it's not too hard to imagine how a stable phospholipid bilayer might be formed within an aqueous solution from two monolayers, laid "back-to-back". The hydrocarbon tails from each monolayer would touch, and together these tails would constitute the hydrophobic core of the bilayer. The polar heads of each monolayer would interact with the adjacent solution and their interactions would exclude the tails and help stabilize the core. Such a bilayer is depicted on the facing page.

Real bilayers can be constructed from high aqueous concentrations of many amphipathic lipids. If a saline solution containing very high concentrations of phospholipids is rapidly aggitated, microscopic spheres called liposomes are formed. These liposomes contain a continuous lipid bilayer at their surface, which effectively separates the saline solution into an interior compartment and an exterior one. In fact, liposomes look like very simple, minature cells. <But are plasma membranes around real cells really bilayers?> [link to aside MembStruct4aside1]

A good scientific way of answering this question would be to make a specific, testable prediction based on current information and then to test the accuracy of the prediction. So what do you know so far about plasma membranes so far? Yes, they contain alot of amphipathic lipids (Chaps. 1 and 2)! And what do you know about amphipathic lipids? They'll assemble in aqueous environments into stable, organized structures such as monolayers and liposomes (Chaps 3 and 4). Now, make a testable prediction!

Gorter and Grendal, the "founding fathers" of our current model of plasma membrane structure, made one such prediction. They reasoned quite simply that if a plasma membrane were really a bilayer then its surface area should be half that occupied by all its amphipathic lipids spread out in a monolayer. They then made independent measurements of the surface areas of different mammalian erythrocytes. These scientists also extracted the lipids from erythrocyte membranes, spread the lipids out at an air/saline interface and measured the surface areas of the resulting monolayers. Comparison of these two measurements indicated a cell:monolayer surface area ratio of <about 1:2> [hyperlink to MembStruct4aside2], confirming for Gorter and Grendal the bilayer nature of the plasma membrane.

But where's all the membrane protein? Continue on to the <next chapter> [hyperlink forward] to examine this question.