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3 The External Surfaces of Cells in Culture


For an all too long period, it has been obvious that we should try to discover the molecular composition and architecture of the outer surfaces of cells. Forearmed with this knowledge we might begin to understand why a given cell can have only certain cells as neighbours and what goes wrong when a cell becomes cancerous and loses its normal affinities. But until recently most articles on cell surfaces went little beyond the seminal formulation of Goiter and Grendel (1925), namely, that the essence of a membrane was a bimolecular layer of lipids. Over the past several years however, membranes have become much more accessible to meaningful experimentation, and there seems to be no good reason why the essential molecular features of the surfaces of several cell types should not be elucidated within this decade.

An essential function of cell surface membranes is to maintain the large differences between the concentrations of molecules inside and outside cells. To perform this function membranes must be semi-permeable, if not effectively impermeable, to many compounds and this explains why membranes contain so much lipid. For lipids, which are not soluble in water, provide a barrier against the movement of water-soluble compounds. But at the same time cell surface membranes must allow the rapid entry into the cell of particular water-soluble molecules, for example, foods such as amino acids and simple sugars. To meet this demand enzymes have evolved which, situated in surface membranes, pump water-soluble molecules into the cell...

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