Surface phenomena in vitro

Surface phenomena.and drug action. Diuretics. Cardiac glycosides. 592 lonophores. 598 

The injury of membranes by biologically active agents. 603 The preservation of membranes by biologically active agents. 608 

The intense cytological studies of the last two decades have shown that cells, as well as most of the organelles within them, are covered with lipoprotein membranes (see Section 5.4). Life is possible only because of the presence of these biphasic membranes (lipophilic within, and hydrophilic on both outer surfaces), which provide for the separation of reactants. They also impose on the sequence of reactions an order that would otherwise be almost impossible to arrange. 

Often in, but sometimes apart from, these membranes, enzymes offer surfaces of the very greatest biological selectivity. Macromolecules, including enzymes and other large protein molecules, whether particulate or dissolved in the bulk phase, present interfaces for reactions. Human blood serum, which has 100 m of protein surface in each cubic centimetre, exemplifies the large interfaces available in protein solutions . 

The short-range (van der Waals) forces, described in Section 8.0, exert an attraction between all molecules which are in contact with one another. The presence of these forces in liquids becomes particularly evident at surfaces. Molecules in the bulk of the liquid are subject to these forces equally and in all directions. However, the molecules located at an air-water interface experience 

A liquid-liquid interface (i.e. the surface between two immiscible liquids) behaves something like an air-water surface, but the contrast in pull exerted on the interfacial molecules is naturally much less. The surface tension at such an interface is often near to the difference of the individual tensions of the two liquids at the air-liquid interface. 

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