Osmosis and Oiher Mechanisms - 121 



water in the protoplasm is reduced to the 

 level of the medium, the shrinking ceases. 

 But later, the slow penetration of glucose 

 begins to be significant. Glucose is the only 

 solute present in the outside medium, and 

 the concentration of glucose in the solution 

 is greater than in the protoplasm. Conse- 

 quently glucose slowly enters the cell. This 

 disturbs the equilibrium that was reached 

 when the plotoplasm stopped shrinking. As 

 glucose enters, the water concentration in- 

 creases in the outer solution and decreases in 

 the protoplasm. Consequently, the entrance 

 of glucose is continuously accompanied by 

 an entrance of water. Gradually, therefore, 

 deplasmolysis occurs, and finally the plant 

 cell regains its normal turgor. 



Animal cells, crenated in hypertonic solu- 

 tions of slowly penetrating solutes, likewise 

 do not remain crenated. Gradually such cells 

 also regain their original volume. In fact, an 

 animal cell, lacking the protection of a strong 

 external wall, may continue swelling, and 

 may undergo cytolysis in a solution that orig- 

 inally was distinctly hypertonic. 



ACTIVE TRANSPORT MECHANISMS 



Diffusion and osmosis do not by any means 

 account for all exchanges between cells and 

 their surrounding fluids. Many cells display 

 a remarkable capacity to accumulate certain 

 substances and to exclude other substances 

 against the natural tides of diffusion. For ex- 

 ample, many marine algae accumulate iodine 

 up to a concentration that is more than a 

 million times greater than that of the sea. 

 Also, as was mentioned previously, the cyto- 

 plasm generally is exceedingly rich in potas- 

 sium (K.+) and poor in sodium (Na + ), com- 

 pared to the lymph, or sap, or other fluid 

 around the cell; and fresh-water protozoan 

 cells force water to pass into the contractile 

 vacuole, from the cytoplasm, where the water 

 concentration is considerably lower. Such 

 phenomena do not "contradict" the laws of 

 diffusion and osmosis any more than lifting 

 a weight "contradicts" the law of gravity. 



They merely mean that the cell is expending 

 energy in forcing the molecules or ions of a 

 particular substance to move against a con- 

 centration gradient. But when a substance is 

 moved "against the diffusional tide," some 

 other kind of energy, derived from metab- 

 olism, is being expended by the cell. Thus if 

 metabolism is temporarily suspended or de- 

 pressed, as by asphyxiation or by poisoning, 

 the cell loses its capacity to work against the 

 tide; and now the laws of simple diffusion 

 and osmosis hold full sway. 



Precisely how a cell manages to "pull in" 

 some substances and to "push out" others 

 represent questions that cannot be answered 

 very precisely at present. However, intensive 

 research is currently being devoted to these 

 problems and some progress can be reported. 



Pinocytosis and Phagocytosis. Pinocytosis 

 and phagocytosis, the active processes of cellu- 

 lar ingestion, were described briefly in Chap- 

 ter 2. Essentially they are similar, since each 

 involves an inpocketing of the cell membrane 

 and an internal pinching off of a fluid-filled 

 vesicle, or vacuole, which may then be car- 

 ried to other regions of the cytoplasm. How- 

 ever, phagocytic vacuoles are relatively larger 

 and they always contain one or more micro- 

 scopically visible particles, such as bacteria, 

 coagulated organic matter, or other solid 

 food materials. Phagocytosis represents the 

 standard method by which protozoans and 

 certain other cells ingest solid materials — 

 as will be described more fully in Chapter 7. 



Pinocytic vacuoles display no visible par- 



PSEUDOPODIUM 



PINOCYTIC 

 CHANNEL 



CELL 



MEMBRANE 



PINOCYTIC 

 VACUOLES 



Fig. 6-8. Pinocytosis in a common Amoeba. Based on 

 the observations and records of S. O. Most and W. L. 

 Doyle, at Johns Hopkins University in 1934. 



