Plasma Membranes of Mammalian Cells

The plasma membrane surrounding living cells "plays a role in intracellular metabolism, but what distinguishes this structure from other organelles is that the plasma membrane also mediaces interactions between the cell and its external environment. The simplest of these interactions maintain a desirable intracellular milieu by bringing in needed substances and getting rid of waste products. The plasma membrane acts as a passive diffusion barrier to charged and large molecules, and in addition carries out facilitated diffusion, active transport, endocytosls (pinocytosis and phagocytosis), and exocytosis. Further, this membrane is involved in locomotion and chemotaxis, processes which could remove a cell from noxious surroundings or to a more nutritive medium. On a more complex level, cells must communicate with other cells; this is especially important in the formation and maintenance of multicellular organisms. The plasma membrane functions in the secretion and reception of hormones, conduction of nerve impulses, and direct cellular interactions such as adhesion and contact inhibition. Finally, on yet another level the plasma membrane riaay be involved in such sophisticated processes as immuno-logical defense and information storage and retrieval. The "isolate-and-characterize" approach was first applied to plasma membranes around 1960. Unfortunately, our present state of knowledge in this field is not as great as might have been anticipated at that time. This slow progress is partially due to the considerable complexities inherent in working with membranes of any sort; but there are other factors as well. For example, many investigators have not been rigorous enough in their choice of plasma membrane markers and have not thoroughly analyzed their plasma membrane fractions for contamination by other subcellular particles; consequently, the true identity of many so-called plasma membrane preparations is uncertain. The vast majority of plasma membrane isolations have been preparative rather than analytical. Although the preparative approach is potentially a very valuable shortcut to an understanding of the entity being isolated, it can also lead to conclusions that are wrong unless really rigorous criteria are applied. Finally, most studies on plasma membranes are concerned simply with their chemical and enzymatic compositions; by isolating plasma membranes from the proper cell types, it should be possible to design experiments with more direct bearing on the functions of this organelle. Isolation of plasma membranes may conveniently be divided into five sequential steps-choice of the tissue to be used, selection of markers, disruption of the tissue, fractionation, and analysis. The following discussion combines some …