Osmosis and Other Mechanisms - 119 



membranes around the cell are ruptured, 

 and the cell as a whole is destroyed. When 

 human red blood cells are put in a solution 

 containing only 0.2 percent NaCl (instead of 

 the 0.9 percent salt present in an isotonic 

 solution), the corpuscles swell and burst so 

 quickly that no opportunity is afforded for 

 examining them with a microscope. 



Plant and animal cells are very different 

 in their capacity to tolerate submersion in 

 hypotonic solutions. Pond water, since it con- 

 tains only very small quantities of the inor- 

 ganic salts and other nonpenetrating solutes, 

 is extremely hypotonic to all cells. Neverthe- 

 less pond water is the normal habitat of a 

 great variety of unicellular forms. In the case 

 of aquatic plants, such as Spirogyva, the great 

 strength of the cellulose wall prevents the 

 cells from swelling unduly. As water enters 

 by osmosis from the hypotonic medium, the 

 protoplasm of the plant cell is forced out- 

 ward against the unyielding cell wall. When 

 a sufficiently high internal pressure is gener- 

 ated, the further entrance of water is pre- 

 vented. This high internal pressure, which is 

 called turgor, is characteristic of plant cells 

 generally. The turgor pressure of a normal 

 plant cell may rise to several atmospheres 

 before a further influx of water is stopped. 

 Typically the medium surrounding a plant 

 cell remains hypotonic to the protoplasm, 

 but turgor constitutes a counterforce that 

 prevents more water from entering the cell. 



If a plant is deprived of water, the indi- 

 vidual cells lose turgor and the tissues be- 

 come wilted. Such a wilted tissue (for exam- 

 ple, a limp lettuce leaf) may regain its 

 normal crispness and turgidity if it is re- 

 turned soon enough to fresh water. But 

 should the cells be killed from loss of water, 

 the wilted tissue never regains turgor. At 

 death, the plasma membrane becomes indis- 

 criminately permeable to virtually all solutes, 

 and consequently the normal osmotic be- 

 havior of the cells is lost. 



Unlike plant cells, most animal cells can- 

 not tolerate exposure to drastically hypo- 

 tonic solutions. The pellicle is not sufficiently 



strong to generate much turgor. Consequent!) 

 most animal cells continue to swell and will 

 burst if the influx of water is excessive. This 

 phenomenon is called osmotic cytolysis. Ani- 

 mal cells can withstand moderately hypo- 

 tonic solutions, however, since the incoming 

 water may dilute the protoplasm sufficiently 

 to establish equilibrium. When the water 

 concentration on either side of the mem- 

 brane becomes equal, no further swelling 

 will occur. 



Many of the Protozoa constitute a special 

 case. These animal cells have become adapted 

 to live in fresh water. To counteract the con- 

 stant influx of water into the protoplasm, 

 these species have developed contractile vac- 

 uoles (Fig. 6-6). Such vacuoles prevent the 

 cells from swelling by collecting water from 

 the protoplasm and pumping it back into 

 the environment. Just how the water is forced 

 to move against its osmotic gradient in pass- 

 ing from the protoplasm into the vacuole is 

 not clearly understood. But it is known that 



-' A 



B 



acts*' 



Fig. 6-6. Greatly magnified view of a contractile 

 vacuole in a Paramecium; A, filled to capacity; and B, 

 just emptied. Note that the radiating canals, which 

 conduct the fluid into the vacuole, can be seen more 

 clearly when the vacuole is empty. 



