Movement of Water
■ Most biological membranes are semipermeable, more permeable to water than to ions or most other solutes. Water moves by osmosis across membranes from a solution of lower solute concentration to one of higher solute concentration.
■ Animal cells swell or shrink when placed in hypotonic or hypertonic solutions, respectively. By maintaining the normal osmotic balance inside and outside cells, the Na+/K+ ATPase and other ion-transporting proteins in the plasma membrane control cell volume.
■ The rigid cell wall surrounding plant cells prevents their swelling and leads to generation of turgor pressure in response to the osmotic influx of water.
■ In response to the entry of water, protozoans maintain their normal cell volume by extruding water from contractile vacuoles.
■ Aquaporins are water-channel proteins that specifically increase the permeability of biomembranes for water (see Figure 7-26). Aquaporin 2 in the plasma membrane of certain kidney cells is essential for resorption of water from the forming urine; its absence leads to a form of diabetes.
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