Essig A
Biophys J. 1968 Jan;8(1):53-63. doi: 10.1016/S0006-3495(68)86474-4.
Steady-state concentration gradients across cell membranes have often been attributed to the associated leakage of solute down its electrochemical potential gradient, and active transport at an equal rate in the opposite direction. Several workers have evaluated the minimal energetic requirements of such a "pump-leak" model for sodium in muscle tissue, presuming that influx occurs only via the leak pathway and to no extent by way of the active transport pathway. The high energy requirements so predicted have led to the suggestions that either (a) sodium is not actively transported, being at equilibrium distribution across the cell surface, or (b) substantial sodium movement must be by means of exchange diffusion. The present treatment, based on the consideration that the active transport mechanism is bidirectional, demonstrates that the rates of influx and efflux associated with a given rate of active transport are explicit functions of two parameters: (1) the ratio of the exchange resistance of the active pathway to that of the leak pathway, and (2) the electrochemical potential difference across the cell surface. Lacking precise values for these parameters, the demonstration of a high rate of isotope flux is not compelling evidence either against active transport or for a discrete exchange diffusion mechanism. Various concepts and criteria of exchange diffusion are discussed.
跨细胞膜的稳态浓度梯度通常归因于溶质沿其电化学势梯度的相关泄漏，以及在相反方向以相等速率的主动运输。几位研究人员评估了肌肉组织中钠的这种“泵 - 漏”模型的最低能量需求，假定流入仅通过泄漏途径发生，而绝不是通过主动运输途径。如此预测的高能量需求导致了以下两种建议：(a) 钠不是主动运输的，在细胞表面处于平衡分布；或者 (b) 大量的钠移动必须通过交换扩散进行。基于主动运输机制是双向的这一考虑，目前的处理方法表明，与给定主动运输速率相关的流入和流出速率是两个参数的明确函数：(1) 主动途径的交换阻力与泄漏途径的交换阻力之比；(2) 跨细胞表面的电化学势差。由于缺乏这些参数的精确值，同位素通量高的证明既不是反对主动运输的有力证据，也不是支持离散交换扩散机制的有力证据。讨论了交换扩散的各种概念和标准。
