Osmosis

Osmosis14 (oz-MO-sis) is the diffusion of water through a selectively permeable membrane, from the "more watery" to the "less watery" side. Cells exchange a tremendous amount of water by osmosis. For example, red blood cells

,4osm = push, thrust + osis = condition, process

Chapter 3 Cellular Form and Function 107

pass 100 times their own volume in water through the plasma membrane every second. Water moves through plasma membranes by way of channel proteins, especially those called aquaporins. Cells can regulate the rate of osmosis by adding aquaporins to the plasma membrane or removing them. Certain cells of the kidneys, for example, install or take away aquaporins to regulate the rate of water loss from the body in the urine.

It is important to note that a solution with a high solute concentration has a low water concentration, and vice versa, since solutes take up some of the space that would otherwise be occupied by water molecules. Therefore, the direction of osmosis will be from a more dilute solution (where there is more water) to a more concentrated one (where there is less water). In figure 3.15a, for example, we see a chamber divided by a selectively permeable membrane. Side A contains a solution of large particles that cannot pass through the membrane pores—a nonpermeating solute such as albumen (egg white protein). Side B contains distilled water. Since albumen takes up some of the space on side A, water is more concentrated in B than in A, and it diffuses down its concentration gradient from B to A (fig. 3.15b). This is because more water molecules encounter the membrane per second on side B than they do on side A, where water is less abundant, and many of those that encounter the membrane pass through it.

Under these conditions, the water level on side B would fall and the level on side A would rise. It might seem as if this would go on indefinitely until side B dried up. This would not happen, however, because as water accumulated on side A, it would become heavier and exert more hydrostatic pressure on that side of the membrane. This would cause some filtration of water from side A back to B. At some point, the rate of filtration would equal the rate of osmosis, water would pass through the membrane equally in both directions, and net osmosis would slow down and stop. At this point, an equilibrium (balance between opposing forces) would exist. The hydrostatic pressure on side A that would stop osmosis is called osmotic pressure. The more solute there is on side A, the greater its osmotic pressure will be.

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