The Nature of Thermodynamic Coupling in Transport Processes

Abstract

In considering problems of transport in biological systems, the definitive role of the numerous membranes as evidenced by electron microscopy (Robertson 1960, Palade 1964) is yet to be elucidated. These membranes appear to delineate not only the limits, of the individual cells, but indeed many of the subcellular structures as well. In an extremely complex fashion, membranes are involved in forming the cytoplasmic structure per se and are a prominent feature of the internal structure of subcellular elements. Attention in the past has been directed largely to those transport processes which are envisaged to occur transmembranally, that is across those barriers whose thickness is about 100 A. However, attention should also be directed to the possibility that such membranes may play a major role in defining and directing cytoplasmic fluid flow processes within the cell. Cytoplasmic circulation, known to occur in living cells, may well represent more than the random mixing and stirring of the cytoplasmic contents. It may be considerably more fundamental in relation to the detailed functioning of the cell. For instance, it is difficult to imagine how m-RNA, synthesized in the nucleus, could in its random-coiled state and moving by simple random diffusion, avoid becoming hopelessly entangled in the nuclear and cytoplasmic substructure. However, with actual fluid flow, a configurational alignment parallel to the lines of streaming is highly probable and, thus, transport could proceed efficiently from the presumed site of synthesis out through the pores of the nuclear membrane and to the sites of transcription on the endoplasmic reticulum. One may also readily envisage how cytoplasmic flow or micro-circulatory processes could play an important role in those cells engaged in the transport of materials from one bounding surface to another, such as renal tubular cells and those of the intestinal mucosa. In fact, as will be indicated below, the flow process could conceivably be intimately related to and involved in the work of concentration associated with active transport.