CONCEPTS AND TERMS 



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Glycine gradient, mM/kg water 



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Figure 1 1 Water uptake associated with the development of a 

 glycine gradient by Ehrlich ascites tumor cells. The line corresponds to 

 the predicted isoosmotic uptake of water assuming 70 per cent of the 

 accumulated glycine to be osmotically active, and assuming that no other 

 solutes migrate. {From Christensen, H. N. (1955), in W. D. McElroy 

 and B. Glass (eds.), Amino Acid Metabolism, Johns Hopkins Press, 

 Baltimore, p. 82; with permission.] 



cell for amino acids is really concentrative was made by Oxender 

 and Christensen (1959). On filtering a cell suspension, these cells 

 were collected on the filter in the form of a membrane-like layer a 

 few cells thick. When this membrane was exposed to a gradient of 

 pyridoxal, alanine, or potassium ion, glycine— previously distributed 

 uniformly between the two phases separated by the membrane- 

 moved across the cell barrier to form a small gradient opposite in 

 sign to the imposed gradients. These three agents are able to stimu- 

 late or inhibit the uptake of glycine by the Ehrlich cell when placed 

 in its environment. Presumably, transport across the cells occurred 

 in the present instance because this stimulation or inhibition was 

 applied more strongly on one side of the cell than on the other. 

 In this case substantial volumes of extracellular fluid were available 

 for critical examination. The supposition is that the gradients of 

 the other solutes produced an asymmetry in the concentrative 

 activity from the two sides of the cells, so that part of the concen- 



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