The diffusion of a solvent, such as water, through a selectively permeable membrane is osmosis. Osmosis is extremely important in determining the distribution of water in the various fluid-containing compartments of the body (cells, blood, and so on). In the clinic, you will encounter patients with swelling due to the abnormal accumulation of fluid in their tissues (edema).
Even though water is highly polar, a small amount of it can "sneak through" the plasma membrane by osmosis because of its small size. Water also moves freely and reversibly through water-specific channels con- structed by transmembrane proteins called aquaporins (AQPS), which allow single- file diffusion of water molecules. The water-filled aquaporin channels are particularly abundant in red blood cells and in cells involved in water balance such as kidney tubule cells.
Osmosis occurs whenever the water concentration differs on the two sides of a membrane. If distilled water is present on both sides of a selectively permeable membrane, no net osmosis occurs, even though water molecules move in both directions through the membrane. If the solute concentration on the two sides of the membrane differs, water concentration differs as well (as solute concentration increases, water concentration decreases).
The extent to which solutes decrease water's concentration depends on the number-not the type of solute particles, because one molecule or one ion of solute (typically) displaces one water molecule. The total concentration of all solute particles in a solution is referred to as the solution's osmolarity. When equal volumes of aqueous solutions of different osmolarity are separated by a membrane that is permeable to all molecules in the system, net diffusion of both solute and water occurs, each moving down its own concentration gradient. Equilibrium is reached when the water (and solute) concentration on both sides of the membrane is the same.
If we consider the same system, but make the membrane impermeable to solute particles, then the water moves and the volume changes. The latter situation mimics osmosis across plasma membranes of living cells.
As water diffuses into living plant cells, the point is finally reached where the hydrostatic pressure (the back pressure exerted by water against the cell wall) within the cell is equal to its osmotic pressure (the tendency of water to move into the cell by osmosis). At this point, there is no further (net) water entry. As a rule, the higher the amount of no diffusible, or nonpenetrating solutes in a cell, the higher the osmotic pressure and the greater the hydrostatic pressure must be to resist further net water entry. In our plant cell, hydrostatic pressure is pushing water out, and osmotic pressure is pulling water in; therefore, you could think of the osmotic pressure as an osmotic "suck."
In living animal cells, such major changes in hydrostatic (and osmotic) pressures cannot occur because they lack rigid cell walls. Osmotic imbalances cause animal cells to swell or shrink (due to net water gain or loss) until either (1) the solute concentration is the same on both sides of the plasma mem- brane, or (2) the membrane stretches to its breaking point.
Many solutes, particularly intracellular proteins and selected ions cannot diffuse through the plasma membrane. Consequently, any change in their concentration alters the water concentration on the two sides of the membrane and results in a net loss or gain of water by the cell.
Tonicity refers to the ability of a solution to change the shape (or plasma membrane tension) of cells by altering the cells' internal water volume.
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