SALTS 431 



Calcium. — The role of calcium is important though not always 

 definitely known. It serves, with phosphorus, in bone produc- 

 tion. The calcium-phosphorus balance is of prime significance 

 to the growing child (see page 515). Calcium seems to have 

 other influences on the animal body, for when the supply is 

 deficient man becomes irritable. Its supply to the blood appears 

 to be controlled by the parathyroid glands. 



Blood serum in health contains about 10 mg. of calcium per 

 100 cc. of serum, of which about one-fifth exists as free ions, the 

 remainder being bound. Nearly half of the total calcium is 

 presumed to be bound to proteins and lipins. The calcium 

 content of the blood is kept very constant, so that the body 

 mechanism must be an accurately controlled one in order to 

 maintain so perfect a balance between the absorption, use, and 

 elimination of calcium. The normal calcium requirements of a 

 healthy adult (0.5 gram per day) are increased during growth, 

 pregnancy, and lactation. 



The role of calcium in plant life has long been assumed to be a 

 catalytic one, but it may, so Hansteen-Cranner believes, be 

 utilized directly in the formation of the cell wall (of root hairs). 

 It is apparently the only constituent of the cell wall not synthe- 

 sized in the plant. R. True also assumes that calcium is needed 

 for cell-wall formation, possibly as a constituent of the wall 

 in the form of calcium pectate or as a catalyst in cellulose 

 synthesis. 



Phosphorus. — The part played by phosphorus in the living 

 world is great. Its prime role in plant life is as phosphates in the 

 soil, where the plant obtains these salts for its nutritional require- 

 ments. Phosphorus occurs in organisms as phosphates, phos- 

 phoric acid, and phosphoproteins. Tricalcium phosphate, 

 Ca3(P04)2, is the principal mineral salt of bone. Phosphorus is 

 often bound to sugars; as hexose phosphate it occurs in the 

 blood, where it is also the chief ingredient of the enzyme phos- 

 phatase. The role of phosphorus in muscle metabolism is of 

 recent discovery (page 458). 



Magnesium. — The function of magnesium, long known to be 

 a necessary element in metabolic processes, is only just coming 

 to light chiefly through the experiments of McCollum, which 

 indicate that a diet lacking in this element causes convulsions 

 and death in the majority of cases (in rats). 



