Membranes may be studied in any number of different ways: for example, by looking at them with high-powered microscopes or by observing their various transport or signalling properties with electrical instruments and enzymatic assays. More basically, we can begin our study of membrane structure and function by examining how membranes are constructed. What is their composition? Specifically, what is the composition of the outermost cellular envelope or plasma membrane?
Perhaps surprisingly, plasma membranes from quite different cells have similar compositions, as illustrated in the Table on the facing page. These outer envelopes consist mainly of protein and lipid, with very little carbohydrate. Moreover, what little carbohydrate is present is covalently linked to either protein or lipid, making these latter molecules, respectively, glycoprotein or glycolipid. Basically, then, the plasma membranes of most eukaryotic cells contain equal weights of lipid and protein (with and without attached carbohydrate), but there are many more lipid than protein molecules Ón these membranes. How so?
A weight fraction of lipid contains many more molecules than an equal weight fraction of protein simply because lipids are much, much smaller than proteins. To illustrate this point, let's make some assumptions and perform some simple arithmetic. Let's assume membrane lipid has an "average" molecular mass of 600 and membrane protein, an "average" of 60,000. Thus, a membrane containing equal weights of lipid and protein contains about 100 lipid molecule for every protein (essentially the ratio of their molecular weights)! That's a lot of lipid, relatively speaking. And for myelin, the derivative of the glial cell plasma membrane that "insulates" a myelinated nerve, the lipid:protein ratio may rise as high as 400:1. At the other extreme, inner mitochondrial membranes (and the plasma membranes of many bacteria) are relatively rich in protein, containing 3 times more protein than lipid on a weight basis. Even so, bacterial and mitochondrial membranes still have about 30 lipid molecules for every protein!
Clearly, lipids are a major constitutent of biological membranes and it makes sense to
begin our study of membrane structure and function by first examining the structure of
membrane lipids in the next chapter.
If the "numerology" of the preceding paragraph, and the resulting conclusion,
strike you as suspicious you may wish to examine the underlying assumptions before