SALTS 433 



patient's condition is improved when extra common salt is 

 taken with the food. 



Iron. — Before the recent work on sulphur raised it to pre- 

 eminence as a catalyst in vital reactions, iron held this position. 

 Iron is necessary in plants for the production of chlorophyll. 

 Without it, leaves become chlorotic, i.e.^ suffer from chlorosis, 

 or lack of green color. While it is true that iron deficiency is a 

 cause of chlorosis, there is usually ample iron in the soil to satisfy 

 the modest needs of the plant. The chlorotic condition is most 

 often due to other factors (alkalinity) which make it impossible 

 for the plant to use the available iron (see page 322). 



Iron is present in animals as a constituent of blood, where it 

 presumably serves as a carrier of oxygen. Otto Warburg regards 

 respiration as a cycle in which molecular oxygen reacts with 

 bivalent iron, whereby iron in a higher state of oxidation is 

 formed. Respiration, pure and simple, takes place within cells, 

 but in its entirety it involves oxygenation within the lungs, 

 where molecular oxygen unites with hemoglobin without atomic 

 cleavage; there is no transfer of electrons or hydrogen atoms. 

 The reaction is consequently not comparable to ordinary oxida- 

 tion. (In distinction to oxyhemoglobin, to which we have just 

 had reference, methemoglobin is a true oxidation product of 

 hemoglobin. Methemoglobin cannot take up molecular oxygen 

 and serves no purpose in respiration.) Acting thus simply as a 

 carrier of molecular oxygen, hemoglobin enters the tissues of the 

 body, and here the oxygen that it carries reacts with the divalent 

 iron of a respiratory enzyme (a ferment, pigment, or oxidase), 

 converting the Fe++ into the trivalent Fe+++. 



The amount of iron in different types of cells has been deter- 

 mined and found to be 0.1 to 0.01 mg. per gram of cell substance. 

 Warburg considers the possibility of other heavy metals func- 

 tioning as the respiratory catalyst. Copper and manganese 

 would apparently serve equally well. These elements occur in 

 protoplasm, but their amounts are too minute to account for the 

 rate of oxygen consumption in respiration (for example, there is 

 1 gram of cell substance to 0.0001 mg. of manganese — a hundred 

 to a thousand times smaller than the iron content.) 



Iron has long been known to be beneficial in cases of anemia. 

 Now it is evident that copper must also be present, presumably 

 as a catalyst. 



