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

Stoichiometry and Energetics of Metabolic Conversions
Transport Across Cell Membranes
Passive Diffusion and Coupled Transport

Passive diffusion is The transport of a substance across a membrane from a region of high concentration to a region of low concentration (Fig. 5.17). The rate of diffusion is proportional to the total driving force, which in this case is the concentration gradient across both sides of the membrane. Thermodynamic analysis shows that passive diffusion is spontaneous, since the transport of a substance from a region of concentration c₂ to a region of concentration c₁ is accompanied by A change in Free energy ∆G° equal to

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Since c₁ is less than c₂, ∆C° in the case of passive diffusion is always negative. If the transported substance is charged, equation (5.41) must be modified as follows:

where Z₁ is the charge of the transported molecules; ℱ is the Faraday constant [23.062 kcal/(V∙mol)]; ∆ψ is the transmembrane potential difference, V.

Due to the specific Structure of Plasma Membranes surrounding all Eukaryotic Cells and Organelles, not all compounds penetrate the membrane with equal ease. There is a correlation between the diffusion rate of most relatively large molecules and their solubility in lipophilic media (Fig. 5.18). This is hardly surprising, since The basis of plasma membranes is believed to be primarily a lipid bilayer (recall Fig. 2.2). The anomalously high rate of Diffusion of Water and other very small molecules is explained by another model, which postulates the presence of small-diameter Pores in the membranes.

FIG. 5.17. Various mechanisms of membrane transport: a — diffusion; b — carrier-mediated transport; c — Active Transport (S — transported substance; C — carrier protein; CS — complex).

Ionized and polar substances are characterized by very low solubility in lipophilic Solvents; therefore, such substances are practically not transported across the membrane via passive diffusion. We have repeatedly emphasized The Importance of this circumstance; intermediary metabolites are mostly ionized compounds and are thus retained within The Cell. However, certain polar substances readily overcome the membrane barrier.

One of the mechanisms explaining this anomalous behavior of polar substances is carrier-mediated transport. As shown schematically in Fig. 5.17, according to this mechanism, the substrate on the outer surface of the membrane binds to a carrier molecule; the resulting complex then "diffuses" to the other surface of the membrane and dissociates there, releasing the transported molecule into the cell interior. This mode of biological transport has A number of characteristic features. Thus, the rate of substance transport in this case does not change linearly with the concentration gradient, but reaches a certain maximum (saturation level), beyond which a further increase in the concentration gradient does not alter the transport rate.

FIG. 5.18. The ability of many substances to penetrate the membrane of the alga Chara correlates with their solubility (partition coefficient) in lipophilic solvents. The sizes of the dots are approximately proportional to the sizes of the corresponding molecules. (According to Collander, 1947.)

Knowing the Michaelis–Menten Equation and other Mathematical models of enzyme catalysis kinetics, it is easy to understand why the process depicted in Fig. 5.17,b is called carrier-mediated transport.

Other important properties of carrier-mediated transport also resemble the features of Enzymatic Catalysis; specifically, only certain substances are transported across the membrane, and specific inhibitors slow down the process. The Specificity and kinetic characteristics indicate that the carriers are Proteins. Many of these carriers have been isolated and characterized; they are referred to as permeases. Perhaps the best-known example of carrier-mediated diffusion is the transport of glucose in human erythrocytes (red Blood Cells). Detailed information on this system and Introduction/47.html">Further Reading on carrier-mediated transport can be found in the literature.