120 - The Cell 



the cell must expend energy in performing 

 this work. This energy comes from metab- 

 olism, and if a protozoan is treated with a 

 metabolic poison, the contractile vacuole 

 stops working. Then the cell begins to swell 

 and eventually it will burst, unless the ac- 

 tivity of the contracile vacuole is restored 

 in time. 



Hypertonic Solutions. A hypertonic solu- 

 tion, compared to the protoplasm of the cell 

 that it surrounds, contains a relatively high 

 concentration of nonpenetrating solute. Since 

 the solute concentration is high, the concen- 

 tration of water in a hypertonic solution is 

 relatively low. Therefore cells placed in hy- 

 pertonic solution tend to lose water and 

 shrink. 



Plant and animal cells behave differently 

 when they undergo shrinkage in hypertonic 

 solution. The pellicle of many animal cells, 

 being llexible, becomes wrinkled as the cell 

 loses volume. This wrinkled, shrunken ap- 

 pearance, which can be seen in the red blood 

 cells of Figure 2-18, is described as crcnation. 

 The lesser volume of the crenated cells is due 

 mainly to a loss ol water. Water is by far 

 the most prevalent of the substances capable 

 of penetrating the membrane, anil conse- 

 quently equilibrium is established largely by 

 the escape of water. The shrinking of the 

 cell continues until finally the water con- 

 centration in the protoplasm is reduced to 

 the level of the outside medium. In some 

 cases, when the loss of water is not drastic 

 enough to destroy the living structures, the 



cell may regain its original volume, if it is 

 returned to an isotonic medium. But if the 

 cells are severely damaged by the loss of 

 water, hypertonic shrinkage is irreversible. 



The appearance of plant cells in a hyper- 

 tonic medium is quite different. The cell 

 wall, being more rigid than the pellicle, 

 maintains its original form, while the proto- 

 plasm continues to shrink. The resulting 

 condition, which is shown in Figure 6-7, is 

 called plasmolysis. The protoplasm of the 

 plasmolyzed cell occupies only a part of the 

 space enclosed by the cellulose wall. Plas- 

 molysis is likewise reversible, provided that 

 the plasma membrane is not damaged in the 

 process. 



Deplasmolysis. Instead of using a strong salt 

 solution to plasmoly/e a plant cell, one can 

 use a hypertonic solution of glucose. Com- 

 pared to the ions of a salt, glucose molecules 

 can penetrate the cell more rapidly. Never- 

 theless the rate of penetration of the glucose 

 is so slow compared to water that an initial 

 plasmolysis occurs as soon as the cell is placed 

 in the hypertonic solution. In a moderately 

 hypertonic glucose solution, however, the 

 plasmolysis does not endure indefinitely. 

 Within about 10 minutes the protoplasm 

 shows signs of swelling, and within half an 

 hour the protoplasm again occupies all the 

 space within the encompassing cell wall. 



The original plasmolysis is due to a very 

 rapid equalization of the water concentra- 

 tion inside and outside the plasma mem- 

 brane; and as soon as the concentration of 



Fig. 6-7. Plasmolysis as seen in the cells of Spirogyra. Note that the cytoplasm 

 (and chloroplast) has shrunk away, leaving a gap between the plasma mem- 

 brane and the cell wall. Photograph retouched slightly to show outlines of the 

 cytoplasm and cell walls more clearly. (Copyright, General Biological Supply 

 House, Inc.) 



