CONCEPTS AND TERMS 



cell (Bennett, 1956). Goldacre (1952) had suggested that the plasma 

 membrane participates in the contraction-relaxation cycle of proto- 

 plasmic streaming. The membrane was supposed by Bennett to 

 carry with it certain solute molecules fixed to specific acceptor sites. 

 Observations with ameba have contributed to this concept (Holter, 

 1959). All measurements of the frequency of occurrence of acceptor 

 sites on the cell surface have indicated such astonishingly low values, 

 however, that enormous rates of flow of the membrane would be 

 necessary to account, for example, for the potassium-ion taken up 

 by the red blood cell. Apparently a section of membrane no less 

 than 1,000 by 1,000 by 70 A thick would be necessary for each 

 potassium ion taken up or exchanged. One finds it hard to believe 

 that the cells have access to enough energy to exchange their cations 

 as rapidly as they do in this way, even if all the membrane taken 

 into the cell is preserved to become exterior again. Furthermore, 

 many cytologists now question the earlier concept that deep in- 

 foldings of plasma membrane into the cell are a common or gen- 

 eral occurrence. 



Engulfment by invagination and vacuolization may very well 

 apply to particulate matter and also to large molecules such as the 

 proteins that have been shown to be able to enter and leave cells. 

 Solutes that are otherwise excluded from the cell may also gain 

 access whenever particulates are being engulfed; this behavior oc- 

 curs for erythrose diphosphate when polystyrene particles are en- 

 gulfed by macrophage cells (Karnovsky and Sbarra, 1959). 



Group translocation 



Figure 17 shows a scheme proposed by Mitchell and Moyle 

 (1958a, 1958b) to show how a phosphoryl group could be donated 

 by ATP, by way of a phosphokinase, to phosphorylate a substrate 

 S. In this scheme, the enzyme is pictured as part of a membrane, so 

 that the chemical transfer of the phosphoryl group also translocates 

 this group from one phase to another. The active site of the enzyme 

 is visualized as lying at a cleft in the barrier, so that it is accessible 

 from both sides; but steric features lead at least one of the substrates 

 to tend to approach the site more readily from one side than from 

 the other, thus establishing net translocation in one direction. 

 Mitchell has discussed also net translocation of the succinyl group 

 and of amino-acyl residues by similar hypothetical systems. 



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