232 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



chemical investigations have demonstrated that the bulk of the potassium in 

 skin is in the epithelial cells (6ib). 



Recently, electronmicroscopic pictures of frog skin were taken (56). A spe- 

 cial basement membrane has been discovered of 200 to 300 A thickness. This 

 basement membrane was found at a distance of about 200 to 300 A from the 

 cell membranes of the first row of epithelial cells near the corium (fig. 16). From 

 the surfaces of the epithelial cells of the epidermis arise a large number of thin 

 extensions reaching across the extracellular spaces to surfaces of neighboring 

 cells (lig. 17). Each epithelial cell, however, maintains its anatomical individual- 

 ity; the epidermis does not represent a syncytium. 



The discovery of this mesh work of protoplasmic extensions bridging the 

 extracellular spaces is quite important. Looking at an electronmicroscopic 

 picture of the epidermis of frog skin, one is reminded of Scatchard's description 

 of the structure of ion exchange membranes, which he depicts as "a continuous 

 resin network and a continuous aqueous network which are interpenetrating", 

 leading to a spongy structure of the membrane (63). The highly developed sur- 

 faces within the interstitial spaces of the epidermis of frog skin provides an 

 excellent opportunity for chemisorption of potassium and also for Ki <=± Nag 

 exchange, evidence for which has been presented previously. 



The finer details of the interior of the epithelial cells are, as yet, not known. 

 It is almost certain, however, that a maze of most complex surfaces (mitochon- 

 dria; Golgi apparatus) will be found, similar to those which have been discov- 

 ered, e.g., in the tubule cells of the kidney (60). 



MECHANISM OF ACTIVE CATION TRANSPORT. CONWAY'S REDOX PUMP 



A well conceived proposal of a possible mechanism of cation transport is 

 Conway's 'redox pump' (2). The redox theory of bioelectricity is original with 

 Lund (48). This theory implies that electrons are transported across membranes. 

 The strongest proponents for this concept today are Lundegardh and Conway. 

 By making use of the well established facts about the mechanism of biological 

 oxidation involving iron-containing enzymes and by visualizing a definite spa- 

 tial organization of the enzymatic machinery in certain membranes, Conway 

 was able to give explanations for active anion and cation transport under the 

 following assumptions: /) it is assumed that back diffusion of the actively 

 transported ion is prevented, e.g. by a low permeability to the free ion. 2) It is 

 assumed that, of the members of the enzyme family involved in passing on 

 electrons from the substrate to molecular oxygen, the final acceptor, only one, 

 the so-called ion carrier, has a high specificity to the transported ion, forming, 

 with the ion, a complex that penetrates across a barrier layer in the membrane. 

 j) It is assumed that no transfer of electrons takes place from the carrier part of 

 the enzyme complex to electron carriers in the cytoplasm. 4) It is assumed that 



