404 ANIMAL BIOCHEMISTRY 



Gas Exchange 



The soluble ions of the cells and fluids affect the utilization and 

 elimination of gases. Aside from the elaborate system of gas exchange 

 in the blood (see page 297) on which other tissues depend, several gen- 

 eral influences are exerted by the ions present. First, electrolytes alter 

 the solubilities of oxygen and carbon dioxide just by their presence. 

 Since different salts have different effects on the solubilities, the com- 

 position of the solvent will make some difference. It will be recalled 

 that blood and interstitial fluid contain different proportions of ions 

 from those in intracellular fluid. Furthermore, physiological processes 

 may change the composition of the liquid as in the diffusion of sodium 

 and potassium ions in opposite directions through the cellular mem- 

 brane during nerve function. Although these effects do occur, their 

 importance to the organism is unknown. 



Gas exchange is markedly affected by the pH, and pH is controlled 

 by the dissolved ions. Primary and secondary phosphate ions 

 (H2PO4- and HP04=) are probably the most important for this pur- 

 pose. Since chloride ion participates in the displacement of bicarbo- 

 nate (a buffer ion) from cells, CI- is indirectly important also. 

 Furthermore, formation of red cells depends upon the presence of 

 several elements, iron, copper, cobalt, and zinc, which thus become 

 indirectly necessary for gas transport since they are essential for the 

 main gas-transferring mechanism. 



Enzyme Action 



Many of the essential elements are prosthetic groups of enzymes. 

 Others function as less firmly bound cofactors but are still required 

 for enzyme action. Although often unknown, the specific roles played 

 are probably diverse. Some metals must actually participate in reac- 

 tions as electron carriers and shuttle from one valence state to another 

 as the electrons move through the substrate-enzyme-product system. 

 In other cases, metals may serve to link enzyme and substrate, bring- 

 ing the two together close enough and long enough for reaction. It 

 is also suspected that the activating element may accelerate the reac- 

 tion by helping to lower the energy of activation. Since a number 

 of systems involve both cationic activators and organic coenzymes, 

 the activator could be important in producing complexes that include 

 the coenzyme and the enzyme. Thus the essential elements may be 

 required by animals for one or more of the enzymatic functions as 

 well as for skeletal, osmotic, or gas-transport roles. It is quite likely 

 that most of the necessary elements have a multiplicity of functions. 



