2. (10) The proposal that membranes are bilayers of lipid molecules was a major

Question

2. (10) The proposal that membranes are bilayers of lipid molecules was a major step forward in understanding membranes. This concept is generally attributed to Gorter and Grendel Below are the original data from Gorter and Grendel s breakthrough 1925 paper entitled "ON BIMOLECULAR LAYERS OF LIPOIDS ON THE CHROMOCYTES OF THE BLOOD" (chromocytes are red blood cells,RBC). This paper purported to show that red blood cells were covered with a 2 molecule thick bilayer. They measured blood cells under a microscope to estimate surface area (RBC's are distinct in different animals, column 1) and numbers of cells (units in column 3 are mm3), then they extracted phospholipids with acetone from a large packed volume of cells column 2 units are mLs or cm3) and spread the lipids on a film balance to measure total surface area of the lipids as a monolayer (see below in final Column are m2). Based on the data were they justified? Choose two data sets and show why or why not with calculations. Watch units! BTW, their lipid extraction procedure with acetone was later shown to remove only about 75% of RBC lipid. How does this change the results? Discuss possible sources of error in these measurements and analysis

Explanation / Answer

Gorter and Grendel first proposed the existence of a bilayer based on the data given in the table.

According to the data:

The membrane lipids from 4.74 x 109 erythrocytes form a monolayer with an area of 0.89 m2 the surface area of an erythrocyte is 100 m2.)

4.74 x 109 erythrocytes have a total surface area of (4.74 x 109)(100) = 4.74 x 1011 m2 or 0.474 m2.

Thus the ratio of lipid surface area to cell surface area is 1.89, which is close to 2.0, the value expected for a bilayer.

(They predicted that if a plasma membrane were really a bilayer then its surface area should be half of that occupied by all its amphipathic lipids spread out in a monolayer.)

First, their estimation of average erythrocyte surface areas from dried blood smears were too low.

For example, 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 Gorter and Grendel estimate of 99 sq microns.

In addition to this, the extraction technique employed could not isolate the totality of erythrocyte lipids from the samples. Acetone removes only about 75% of erythrocyte lipids (compared to a mixture of chloroform and methanol which removes 100% of the lipid). Acetone does not quantitatively extract all the lipids-they under-estimated the lipid content of the RBC membrane.

They spreaded lipid extract on to water surface in Langmuir trough where acetone evaporated (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 Grendel adjusted the movable barrier to provide the same degree of surface tension for all of their samples of extracted lipid. It's not clear how they chose this "standard" tension, but it was quite low because phospholipid monolayers can be compacted to a much greater extent.

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 that the film buckles and begins to spill over the sides of the trough. It is clear that the ratio can vary with the degree of monolayer packing.

Inspite of all these difficulties, they were absolutely correct in their conclusions. They calculated that there was sufficient lipid to encircle the erythrocyte twice, suggesting a lipid bilayer structure. These errors fortuitously cancelled one another, providing the correct answer after all.

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