MINERAL METABOLISM 401 



lutarnic 



-ooc(c;h.2)2Cocoo- + NH4+ + dpnh ^ 



(IchydroKeiiasc 

 a-ketoglutarate 



-OOC(CHo),CHC:00 + DPN^ 



NH3+ 



glutamate 



Again the green plants require NH4+ (or NOs^) from the soil, but 

 the metabolism of animals provides enough in their tissues for such 

 reactions. Undoubtedly complete removal of ammonium ion from the 

 tissues would cause death of the animal. 



ROLES OF ESSENTIAL ELEMENTS 



Many of the special roles played by the elements in animals have 

 been discussed above. Several of the general functions have also been 

 mentioned without any real consideration of the roles themselves and 

 the interplay of the factors involved. One of the physiological func- 

 tions almost invariably depending on the presence of the essential 

 elements is growth. It is clear that failure to grow is in reality due to 

 a failure of some more specific process. Therefore, we are more con- 

 cerned here with the specific problem than with growth, since satis- 

 factory performance of the individual functions leads to the growth 

 of young animals. Some of the biological processes depending prima- 

 rily on one or a few elements have been summarized in connection 

 with the elements concerned. Other processes or systems depending on 

 more elements follow. 



Skeletal Structure 



Although there are several types of bone, most of the structure 

 studies have involved the long bones. It is assumed that other bones 

 are similar. Cells called osteoblasts lead to bone formation by com- 

 plex processes yielding a combination of organic and inorganic com- 

 pounds. 



About 75 per cent of the volume of bone is organic material form- 

 ing a matrix in which the inorganic fraction is embedded. This 

 matrix is largely composed of collagen fibers distributed through the 

 cement substance chondroitin sulfate. The collagen of bone does not 

 seem to differ from that of other fibrous tissues. However, the cement 

 substance of bone is somehow different from that elsewhere. The 

 matrix is interspersed with osteoblasts and gives the bone its form and 

 elasticity. 



