Scientists often speculate using "ball-park calculations" similar to this one involving the relative number of membrane lipids and proteins, but just how reasonable is such "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 very complex clan of molecules (consisting of several related families). The molecular weight of a single family of phospholipid - phosphatidyl choline, the most common plasma membrane phospholipid - 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. First, many membrane proteins are functional aggregates of two or more peptide subunits, while other proteins are simply single peptides. Moreover, the relative mass of mammalian erythrocyte plasma membrane peptides, for example, ranges from around 18,000 to over 220,000 (or 18 to 220 kilo Daltons or kDa). 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 embedded in it and intimately associated with lipid in the membrane core - then an "average" molecular weight of 60 kDa is not a bad approximation for a single membrane peptide. Since many integral membrane proteins are aggregates of two or four or even more peptides, the actual lipid/protein ratio may actually be very much higher than our simple example suggests!
Scientists engage in such imaginary ball games for much the same reasons that athletes perform calisthetics: they're limbering and, ultimately, refreshing. Remember, however: approximations are just that! They're a necessary prelude to useful research, but a poor substitute for real data!