BIOLOGICAL TRANSPORT 



Transport into cells 



Concern about the way in which cells gain their components 

 from the environment goes far back in the history of biology. Tis- 

 sue analyses were made over a century ago that were eventually 

 understood to show potassium and magnesium to be the typical 

 cations of the cell, whereas sodium and calcium ions are typically 

 extracellular ions (cf. Manery, 1954). Until isotopes of the alkali 

 metals became available, the magnitude of the problem of maintain- 

 ing this biological situation was widely underestimated by assuming 

 a static placement ("fossilization") of the cations in either one com- 

 partment or the other. In 1939 Cohn and Cohn showed that ionic 

 Na 24 rapidly entered the erythrocytes in the intact dog, and in 

 1940 and 1941 Harris (see also Danowski, 1941) showed that a net 

 exchange of sodium for potassium ion occurs when erythrocytes are 

 cooled, or soon after they become depleted of glucose. Since these 

 ions do steadily cross the membrane and since gradients of their 

 concentrations are nevertheless maintained, a continuous transport 

 activity must operate to maintain the asymmetric distribution of 

 these two alkali metal ions. 



In the paper introducing the term pump (without mechanistic 

 implications) for an uphill-transport mechanism, Dean commented 

 in the same year (1941): ". . . to assume that muscle fibers are 

 normally impermeable to sodium and only excrete sodium when 

 some gets in is not as simple as to assume that the fiber is permeable 

 to sodium all the time and is continuously excreting it." (Perhaps 

 we may also say that to assume that only secretory cells arranged 

 as membranes are able to concentrate solutes is not as simple as to 

 assume that all cells can do so.) 



Packed cells contain such small amounts of characteristic ex- 

 tracellular solutes (sodium ion, chloride ion) that they obviously 

 cannot admit the environmental solution to any great average depth. 

 Instead, one finds present in the cell quantities of potassium and 

 magnesium ions that presumably are nearly equivalent to the total 

 negative charges represented by the anionic structure and machinery 

 of the cell, plus a small amount of bicarbonate and even smaller 

 quantities of other small anions. The relationship between the quan- 

 tities of the osmolites and of water suggests an average osmotic pres- 

 sure for a predominant portion of the cell interior close to that of 

 the extracellular fluid. Such a relationship would hold if water could 



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