Osmotic Pressure Causes Water to Move Across Membranes

Water tends to move across a semipermeable membrane from a solution of low solute concentration to one of high concentration, a process termed osmosis, or osmotic flow. In other words, since solutions with a high concentration of dissolved solute have a lower concentration of water, water will spontaneously move from a solution of high water concentration to one of lower. In effect, osmosis is equivalent to "diffusion" of water. Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions (Figure 7-24). In this context, the "membrane" may be a layer of cells or a plasma membrane that is permeable to water but not to the solutes. The osmotic pressure is directly proportional to the difference in the concentration of the total number of solute molecules on each side of the membrane. For example, a 0.5 M NaCl solution is actually 0.5 M Na+ ions and 0.5 M Cl_ ions and has the same osmotic pressure as a 1 M solution of glucose or sucrose.

Pure phospholipid bilayers are essentially impermeable to water, but most cellular membranes contain water-channel proteins that facilitate the rapid movement of water in and out of cells. Such movement of water across the epithelial layer lining the kidney tubules of vertebrates is responsible for concentrating the urine. If this did not happen, one would excrete several liters of urine a day! In higher plants, water and minerals are absorbed from the soil by the roots and move up the plant through conducting tubes (the xylem); water loss from the plant, mainly by evaporation

Water-permeable membrane

Hydrostatic pressure required to prevent net water flow

Water-permeable membrane

Hydrostatic pressure required to prevent net water flow

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