Osmo tic pressure

Osmosis is an important process in all living matter. This phenomenon involves aqueous solutions separated by a membrane that is permeable to some solution components but not to others, typically macromolecules and large ions. The cell wall in Fig. 11.4a is a typical semipermeable membrane. If the solution on one side of the membrane contains a macromolecule that cannot pass through the membrane and other side does not, a pressure difference between the two solutions is created as water moves through the membrane to the side containing the macromolecule.

11.7.1 Osmometers

Figure 11.17 depicts a number of natural or man-made devices that exhibit this phenomenon, including osmometers:

(a) laboratory instruments intended for determination of the molecular weight of macromolecules and, at the same time, a parameter reflecting the nonideality of the macromolecule in solution. The movement of water through the membrane is measured by the rise of the level of the macromolecule solution in a capillary tube

(b) instruments serving the same purpose as (a) but in which the volume of the macromolecule solution is kept constant by a rigid container and a gauge detects the rise in pressure.

(c) devices with ligands on one or both sides of the membrane and the macromolecule in one side. The purpose of such a device is to measure the equilibrium constant for ligand binding, as was covered in the preceding section.

(d) dialysis machines for separating urea from valuable proteins using semipermeable membranes.

(e) instruments which, in addition to the macromolecule, contain a completely ionized salt (i.e., an electrolyte) on both sides of the membrane. The addition of charged species to the mix sets up an electrostatic potential between the solutions on either side of the membrane.

The analysis of osmotic pressure is prone to changes in units, so Table 11.1 summarizes them.

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