Biochemical Engineering Fundamentals: Part 1 - Bailey, J., & Ollis, D. 1989

Stoichiometry and Energetics of Metabolic Pathways
Transport Across Cell Membranes

Normal cellular activity is only possible under the condition of regulated transport of ions and molecules between The Cell and its environment. Cell membranes possess selective permeability to various components and constituent PARTS OF THE cell, thereby regulating transport processes that perform A number of important Functions. First, through transport processes, the pH and COMPOSITION OF THE intracellular environment are maintained at a constant level within a sufficiently narrow range, ensuring the necessary enzymatic activity. In addition, membrane transport regulates cell volume, ensures access and the required concentration of nutrients, and removes toxic compounds.

At least three modes of Transport Across Cell Membranes are known: passive diffusion, coupled transport, and Active Transport. Regardless of the mechanism, the ability of a given membrane to transport a given substrate is often expressed as the membrane permeability K, which is calculated using the equation

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where V is the cell volume; A is the outer surface area of the cell; c_e is the external Substrate Concentration; c_i0 is the initial intracellular substrate concentration; c_i(t) is the intracellular substrate concentration at time t. It follows from equation (5.40) that permeability has the same dimension as velocity (centimeters per second). As we will see later in the course of solving exercises, the assumptions underlying this equation limit its applicability.