Osmosis and Ofher Mechanisms - 123 



this material sloughs off into the vacuolar 

 cavity as a microscopically visible mass of 

 coagulum (Fig. 6-9). It was possible to follow 

 the translocation of the protein and glucose 

 because, in each case, the molecules were 

 radioactively labeled (p. 142). Thus when the 

 amoebae, after proper fixation, were placed 

 upon a sensitive, fine-grained photographic 

 plate, a localization of the sites of radioac- 

 tivity and an estimation of the intensity, or 

 concentration, could be made. 



Water Transport Mechanisms. Many cells 

 are naturally exposed to a fresh-water en- 

 vironment from which, as a result of steep- 

 ness of the concentration gradient, water 

 tends to enter the protoplasm with consider- 

 able force. Plant cells, of course, possess very 

 strong encasing walls and are able to develop 

 a counteracting force, in the form of turgor 

 pressure. But animal cells must expend en- 

 ergy, either to exclude the water or to pump 

 it out after it has entered. 



Very little is known about the mechanisms 

 of water exclusion, although they must be 

 important in certain cases — as in the skin 

 cells of multicellular fresh water animals, 

 particularly the fresh water fishes and am- 

 phibians. Unicellular animals, on the other 

 hand, employ the contractile vacuole (Fig. 

 6-6) as a water-eliminating device. 



The ability of a contractile vacuole to 

 contract appears to depend upon the forma- 

 tion of a gelated photoplasmic layer immedi- 

 ately surrounding the vacuolar membrane 

 and bordering the confluent channels leading 

 into the vacuole (Fig. 6-6). Thus the pump- 

 ing activity of the vacuole is abolished by 

 agents, such as high hydrostatic pressure, that 

 are known to cause a drastic solation of pro- 

 toplasmic gels generally, as has been shown 

 by D. A. Marsland, of New York University, 

 in America and by J. A. Kitching, of the 

 University of Bristol, in England. The for- 

 mation of such contractile gel structures is 

 an endothermic operation that cannot be 

 performed unless the cell provides energy 

 from its metabolism. Moreover, each poten- 

 tial bonding site in such a gel structure (Fig. 



4-23) appears to be blocked off by a shell of 

 densely packed, closely adsorbed water mole- 

 cules, and this must be dispersed before the 

 site becomes effective in forming a gelational 

 bond. In short, these and other data led to 

 the tentative conclusion that a gelational 

 phenomenon underlies not only the me- 

 chanical pumping action of the contractile 

 vacuole, but also its action in picking up 

 water, during solation, and giving it up, dur- 

 ing gelation, in a continued cycle of activity. 

 But precisely how the gel delivers water into 

 the vacuolar cavity remains an unsolved 

 problem. 



Ionic Transport Mechanisms. As previously 

 stated, a relatively high concentration of cer- 

 tain ions, particularly potassium (K + ), is 

 maintained within the cell, as compared to 

 its surrounding medium; and a relatively 

 low concentration of other ions, particularly 

 sodium (Na+), is likewise maintained. The 

 maintenance of these differentials is very 

 important in the life of the cell. They de- 

 termine the electrical potential of the cell 

 membrane (p. 191), which in turn determines 

 cellular excitability (p. 190). But to create and 

 preserve these important differences in ionic 

 concentration, which usually are some 10- to 

 30-fold in magnitude, the cell must expend 

 considerable metabolic energy in forcing or 

 holding the appropriate ions against the 

 steep diffusional gradients. 



Despite intensive research upon ionic 

 transport mechanisms, not much progress 

 has been made. As a cloak for ignorance, one 

 may speak of a "potassium pump" or a "so- 

 dium pump," but these terms merely desig- 

 nate rather than explain the basic phe- 

 nomena. Among current theories, perhaps 

 the most plausible postulates the existence 

 of one or more "ion carrier molecules" that 

 are energized by metabolism as they shuttle 

 back and forth across the surface membrane. 

 One such theory, proposed by T. J. Shaw, of 

 Cambridge University, assumes that the 

 same organic molecule carries both sodium 

 and potassium ions. At the deep surface of 

 the cell membrane, this carrier is assumed 



