58 



Cellular Structure and Activity 



the protein complex of the membrane re- 

 sponds dynamically to changes in chemical 

 environment, for example to the presence of 

 ATP. This possibility may repay inquiry, for 

 a dynamic system of this sort might well be 

 capable of altering permeability (through 

 change in state of aggregation of the protein 

 fabric of the membrane) in response to 

 physiological changes in the cytoplasm. In 

 such considerations, it is of course difficult 

 to rule out the possible role of the cortical 

 cytoplasm immediately underlying the mem- 

 brane, for it too contains macromolecular 

 protein complexes which are known to be re- 

 sponsive to changes in the environment. 



That the membrane is indeed highly re- 

 active is shown by the fact that the per- 

 meability for sodium ions, which must de- 

 pend in some way on the molecular lattice 

 of the membrane, may be increased several 

 hundred fold in a ten-thousandth of a second 

 in response to the passage of cvirrent. This 

 has been clearly demonstrated in the mem- 

 brane of nerve and muscle fibers; it might 

 also be demonstrable, with different time 

 constants, in other types of cells if adequate 

 techniques were available. 



The surface membrane is intimately con- 

 cerned with the establishment of the char- 

 acteristic electrolyte pattern and with bio- 

 electric phenomena which occur at this dis- 

 continuity. The facts in this area of physiol- 

 ogy bear importantly on the concept of the 

 surface membrane as a structure in dynamic 

 relationship with metabolic reactions in the 

 underlying cytoplasm. For example, certain 

 ions, such as Na*, are present in much lower 

 concentration inside the cell than outside. 

 This concentration gradient is maintained 

 by an ion "pump" capable of moving Na% 

 which enters the cell at a low rate, back 

 across the membrane into intracellular space 

 (see Steinbach and Moog in this volume, 

 Section III, Chapter 2). 



The energy necessary for this osmotic work 

 derives from metabolic reactions but the 

 coupling of the reactions with membrane 

 structure is not known. One possibility is 

 that the "pump" may be located in the mem- 

 brane itself, the ability of a carrier molecule 

 to combine with or release Na* being deter- 

 mined by its reaction with molecules in- 

 volved in intermediary metabolism. A redox 

 carrier mechanism proposed by Conway ('51) 

 encounters certain theoretical difficulties but 

 serves to illustrate the idea (see also Ussing, 

 '49; Rosenberg and Wilbrandt, '52). Bio- 

 electric phenomena depend upon such ion 

 regulatory mechanisms, as do ion secretory 



processes such as the secretion of hydro- 

 chloric acid, Ch, etc. 



The phenomena which occur at or near 

 the surface of the egg cell when activation 

 (natural or artificial fertilization) occurs 

 are very complex (see Runnstrom, '49). In 

 a not too literal sense they resemble the ac- 

 tivation of the irritable membrane in nerve 

 and muscle though of course the structural 

 and chemical impedimenta of this cytoplas- 

 mic system differ markedly from those of 

 nerve and muscle. 



Enzymes form an important part of the 

 chemical complement of the surface mem- 

 brane. Cholinesterase, catalase and several 

 other enzymes occur in the red cell envelope. 

 Alkaline phosphatase has been demonstrated 

 in the surface of the cells of the intestinal 

 epithelium and of the proximal convoluted 

 tubules of the kidney. A variety of hydro- 

 lytic enzymes concerned with phosphate and 

 sugar metabolism (phosphatase, invertase, 

 lactase, sucrase, trehalase and ATP-ase) are 

 thought to be located at the surface of yeast 

 cells; enzymic phosphorylation of sugar is 

 generally believed to be necessary for en- 

 trance of the sugar into the cell (cf. Roth- 

 stein, Meier, and Hurwitz, '51; Brown, '52; 

 Rosenberg and Wilbrandt, '52). 



Living cells are usually resistant to the 

 action of tryptic enzymes. This may be due 

 in part to the presence in the cell surface of 

 certain polysaccharides which are powerful 

 tryptic inhibitors (Runnstrom, '49). 



Some difference of opinion exists as to 

 whether the cell membrane is in fact an or- 

 ganized, complex molecular lattice as indi- 

 cated in the above discussion or whether it 

 is merely surface film which forms by ad- 

 sorption of solutes from the underlying cyto- 

 plasm. It has long been known that when 

 the cell wall is ruptured, a film forms very 

 quickly over the naked protoplasm and this 

 film may have some of the semipermeable 

 characteristics of the normal membrane 

 (Naegeli). Chambers points out that Ca** 

 is necessary for this reaction. However, pro- 

 ponents of this view must demonstrate that 

 such a spontaneously formed interfacial film 

 has more of the properties of the normal cell 

 membrane than mere water immiscibility 

 and impermeability to certain colloidal dyes. 



On the other hand, too much preoccupa- 

 tion with the "fixed" lattice of the mem- 

 brane at the expense of study of the dynamic 

 aspects is also undesirable. We have stressed 

 certain of these dynamic aspects above. An- 

 other aspect of the molecular ecology (to 

 borrow a phrase from Paul Weiss) of the 



