Approximations, Expectations and the Reality of Membranes as Lipid Bilayers

Gorter and Grendal obtained a range of values for the ratio of erythrocyte surface area to the surface area of erythrocyte membrane lipids spread in a monolayer, which clustered around 1:2. Why weren't the experimental values always a single value of 0.5?

The obvious explanation is statistical variation, both in terms of the measurements being made and and in terms of the cells themselves. Such an explanation is accurate, but more careful examination of their data can provide us with greater insight into both the nature of biological investigation as well as the structure of plasma membranes.

For example, Gorter and Grendal based their monolayer measurements on lipids extracted with acetone. We now know this solvent removes only about 75% of erythrocyte lipids compared with a more stringent method using a mixture of chloroform and methanol which removes 100% of the lipid. Moreover, their estimates of an average human erythrocyte surface area of 99 sq microns were based on microscopical observations of dried blood films. More recent measurements made of living cells with a differential interference microscope of living cells indicate human erythrocytes are much larger than the Gorter and Grendal estimates and have an average surface area of about 138 sq microns. How do these newer data <change our view> [hyperlink to MembStruct4aside2aside1] of the bilayer structure of the plasma membrane?

To explore a second example of why the Gorter/Grendal data may have varied and be flawed, we need to learn more about how they measured the surface areas of their monolayers. These measurements were made by adding lipid extracts to a saline solution contained in a Langmuir trough, which is shallow container with a movable barrier and a device for measuring surface tension, as depicted on the facing page. In the trough, the lipids formed a monolayer on the surface of the saline as the acetone evaporated, but they spread out to cover the available surface. To standardize their measurements Gorter and Grendal adjusted the movable barrier to provide the same degree of compaction (or surface tension). It's not at all clear how they chose the tension at which to measure the monolayer surface areas, but it is certain the tension was quite low: phospholipid monolayers can be compacted to a much greater extent. More recent experiments indicate the ratio of monolayer surface area to RBC surface area can approach 1:1 at pressures where the lipid are so compacted the film buckles and begins to spill over the sides of the trough. <So what is the appropriate degree of lipid packing to use?> [hyperlink to MembStruct4aside2aside2.htm]