Reasonable Assumptions?

Scientists often engage in "ball-park" speculations similar to this one, but just how reasonable is the "numerology"? Well, the arithmetic seems straightforward, but what about the "average" molecular weights used in the calculations.

The two major types of lipids found in the plasma membranes of most eukaryotic cells are cholesterol and phospholipid. Cholesterol has a molecular weight of 387, but as you will see in Chapter 2, phospholipids are actually a complex clan of molecules. The molecular weight of a single family of phospholipid, phosphatidyl choline, varies with the fatty acid side-chains from less than 700 to over 800. The "average" lipid molecular weight of a membrane consisting of equal amounts of cholesterol and the heavier form of phosphatidyl choline is just about 600. Not a bad "ball-park figure"!

Membrane proteins on the other hand are far more variable than lipids in their molecular weights. The relative masses of mammalian erythrocyte plasma membrane proteins, for example, ranges from around 18,000 to over 220,000. So any "average" value is bound to be misleading, certainly more so than any average lipid value. However, proteins are also associated with biological membranes in different ways, and if we consider only those integral to the membrane - those intimately associated with lipid in the membrane core - then an "average" molecular weight of 60,000 (or 60 kiloDaltons; 60 kDa) is not a bad approximation for a single membrane peptide. Since many integral membrane proteins consist of two or four or even more peptides, the actual lipid/protein ratio may be very much higher than our simple example suggests!

Remember, however: approximations are just that - good guesses! Often, a good basis for speculation, but dangerous when substituted for the reality of supporting data!