Feher J J, Ford G D
Department of Physiology, Medical College of Virginia, Richmond 23298-0551, USA.
Am J Physiol. 1995 Jun;268(6 Pt 3):S10-20. doi: 10.1152/advances.1995.268.6.S10.
Osmosis is usually taught from the point of view of the osmotic pressure developed when solutions of different concentrations of solute are separated by an ideal semipermeable membrane. The osmotic pressure is defined at equilibrium when there is no net flow, and it takes some time to reach this equilibrium. Although the osmotic pressure is certainly important, teaching only this point of view implicitly diminishes the importance of osmotic flow, which begins almost instantaneously across a membrane. A device was constructed with which students could measure the flow across a model membrane (dialysis tubing) as a function of concentration for solutes of different sizes. The device produced flows that were linearly proportional to the concentration, providing a confirmation of van't Hoff's law. Separate student groups repeated these experiments using both different solutes and different dialysis membranes. The combined results of four student groups showed that the flow across these nonideal membranes depends on the solute and membrane as well as the concentration of solute. Given a value for area times filtration coefficient (A x Lp) for the membranes (determined beforehand by their instructor), the students could calculate the reflection coefficient (sigma) for three solutes and two membranes. The results showed that large solutes had large sigma and that less porous membranes had larger sigma. A concurrent demonstration using this device and membranes showed that the osmotic flow can generate large pressures. These experiments and demonstration provide a balanced view of osmotic flow and pressure.
当不同溶质浓度的溶液被理想的半透膜隔开时所产生的渗透压。渗透压是在没有净流动达到平衡时定义的，而达到这种平衡需要一些时间。虽然渗透压当然很重要，但仅从这个角度进行教学会在无形中降低渗透流的重要性，因为渗透流几乎能瞬间穿过膜。构建了一种装置，学生可以用它测量不同大小溶质穿过模型膜（透析管）的流量与浓度的函数关系。该装置产生的流量与浓度呈线性比例关系，证实了范特霍夫定律。不同的学生小组使用不同的溶质和不同的透析膜重复了这些实验。四个学生小组的综合结果表明，穿过这些非理想膜的流量取决于溶质、膜以及溶质的浓度。对于这些膜，在给定面积乘以过滤系数（A×Lp）的值（由他们的导师事先确定）后，学生可以计算三种溶质和两种膜的反射系数（σ）。结果表明，大分子溶质的σ值较大，孔隙较少的膜的σ值也较大。使用该装置和膜进行的同步演示表明，渗透流可以产生很大的压力。这些实验和演示提供了对渗透流和压力的平衡观点。
