In this chapter we turn from more general thermodynamic considerations to specific models for the transport of substances across cell membranes with the help of special molecules such as carriers. The reason for carriers is obvious since many substances such as sugars and amino acids that are continually needed inside a cell and are continually being used up cannot cross a membrane without special help. In some examples the carrier merely functions as a catalyst or enzyme, and no energy is required. The process is known as facilitated transport and is treated in the first section of this chapter. In other examples, movement of substances is directly or indirectly linked to a chemical reaction which provides a source of energy. This process is known as active transport and is discussed in the second section. The coupling of flows to chemical reactions such as occurs in active transport can be included in the equations of irreversible thermodynamics simply by adding extra terms to the right-hand side of equations such as (2.12) (Kedem, 1961). However, this chapter is directed towards more specialized results, and the last section shows that the inclusion of active transport is sufficient to account qualitatively for the voltage and concentration differences across cell membranes and also for the regulation of cell volume.
KeywordsActive Transport Carrier Transport Steady State Flux Carrier Molecule Rectangular Hyperbola
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