Gorter and Grendel obtained a range of values for the ratio of erythrocyte surface area to the surface area of erythrocyte membrane lipids spread in a monolayer that clustered around 1:2. Why weren't the experimental ratios always a single value of 0.5?
The obvious explanation is statistical variation, both in the measurements being made and in the cells themselves. Such an explanation is accurate and surely explains some of the variation in their data. With the benefits of hind-sight, closer scrutiny of their experiment reveals other sources of variation and provides greater insight into both the nature of science as well as the structure of plasma membranes.
For example, Gorter and Grendel 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 average erythrocyte surface areas from dried blood smears were too low. For example, more recent measurements made of living cells with a differential interference microscope indicate human erythrocytes have an average surface area of about 138 sq microns, which is much larger than the Gorder/Grendel estimate of 99 sq microns.. How do these newer data change our view of the bilayer structure of the plasma membrane?
To explore another example of why the Gorter/Grendel data may have varied from 1:2, we need to learn more about how they measured monolayer surface areas. Their 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. In the trough, the lipids formed a monolayer on the surface of the saline as the acetone evaporated and they spread out to cover all the available surface. To standardize their measurements Gorter and Grendal adjusted the movable barrier to provide the same degree of compaction (or surface tension) for all their various samples of extracted lipid. It's not at all clear how they chose this "standard" tension, but it was quite low because 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 is so compacted the film buckles and begins to spill over the sides of the trough. Clearly, the ratio can vary with the degree of monolayer packing, so what is the appropriate degree of lipid packing to use?