Gradient-Driven Transport

Let us examine how these concepts can be applied in practice by taking up a familiar example of a gradient-driven process, that of the conducHon of heat. 

The general reader knows that heat flows from a high temperature T, which is the driving potential here, to a lower temperature at some other location. The greater the difference in temperature per unit distance, x, the larger the transport of heat i.e., we have a proportionality  

Rate Laws Based on Linear Driving Forces 

Process Flux or Flow Driving Force Resistance 

The minus sign is introduced to convert AT /Ax, which is negative quantity, to a positive value of heat flow q. In the limit Ax 0, the difference quotient converts to the derivative dT/dx. Noting further that heat flow will be proportional to the cross-sectional area normal to the direction of flow and introducing the proportionality constant k, known as the thermal conductivity, we obtain 

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Figure 6.13 The mechanisms for the elimination of Ca2+ from the cytoplasm sequestration into the ER via the Ca2+-ATPase system sequestration into the mitochondria (Mito) via the Ca2+ uni porter transport outside the cell via the Ca2+-ATPase system ion gradient-driven transport outside the cell via the Ca2+/NaH exchanger. Abbreviations ER, endoplasmic reticulum.

Leuthold S, Hagenbuch B, Mohebbi N, Wagner CA, Meier PJ, and Stieger B (2009) Mechanisms of pH-gradient driven transport mediated by organic anion polypeptide transporters. Am J Physiol Cell Physiol 296 570-582. 

Uptake of noradrenaline into the vesicles depends on an electrochemical gradient driven by an excess of protons inside the vesicle core. This gradient is maintained by an ATP-dependent vesicular H+-triphosphatase. Uptake of one molecule of noradrenaline into the vesicle by the transporter is balanced by the counter-transport of two H+ ions (reviewed by Schuldiner 1998). It is thought that either binding or translocation of one H+ ion increases the affinity of the transporter for noradrenaline and that binding of the second H+ actually triggers its translocation. 

This result shows that the most likely rate of change of the moment due to internal processes is linearly proportional to the imposed temperature gradient. This is a particular form of the linear transport law, Eq. (54), with the imposed temperature gradient providing the thermodynamic driving force for the flux. Note that for driven transport x is taken to be positive because it is assumed that the system has been in a steady state for some time already (i.e., the system is not time reversible). 

Secondary active transport (against electrochemical gradient, driven by ion moving down its gradient)... 

Kaback, H.R. (1988). Site-directed mutagenesis and ion-gradient driven active transport On the path of the proton. Ann. Rev. Physiol. 50,243-256. 

Energy is provided, for example, by ATP for pumping sodium ions out of and potassium ions into the cell. Another important example of primary active transport is the proton concentration gradient driven ATP synthesis (Mitchell-hypothesis). 

During the last thirty years, intensive investigations by numerous laboratories converted photophosphorylation from a highly debatable and marginally detectable process to a well-established and well-dissected reaction. We have today a wealth of information about the overall photochemical steps, the electron transport reactions driven by it, the electrochemical gradient driven by the electron transport, and the overall reaction responsible for ATP synthesis by the enzyme-bound ATP... 

Accumulated protons generated within the pores of a catalyst cause the formation of a proton gradient. The extent of such a gradient is predominantly a matter of the proton formation rate, which is dependent on the immobilized enzyme s activity and the mass transfer driven transport of protons to the outside of the catalyst particles. At steady state a mass balance occurs. 

For the BOHLM systems (see Chapter 5) with water-immiscible carriers, the concentration gradient-driven solute-solvent complexation/ decomplexation interactions are the dominant driving forces. For the BAHLM systems, Donnan membrane potential [18-26, 32-36], osmotic pressure gradient [27, 37], and possibly pressure gradient [38-40], have to be added as driving forces. Therefore, the theory should take into account both diffusive and convective transport. 

Electrochemical potential-driven transporters Group of transporters driven by electrochemical gradients using facilitated or (secondary) active transport mechanisms. 

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