314 PLANT PHYSIOLOGY 



Wieland has called attention to the fact that oxidation probably 

 takes place in a series of stages. Even so simple a reaction as the 

 oxidation of CO to CO2 takes place in two stages and seems to 

 occur only in the presence of water. The first of these may be 

 represented as: CO+H 2 = HCOOH. Then, in the presence of 

 oxygen, the formic acid is oxidized to the carbon dioxide and water : 

 HCOOH+0 = C0 2 +H 2 0. The oxygen in this second stage takes 

 up the hydrogen and is thus what is known as a hydrogen acceptor. 

 Such an " oxidation" is consequently seen to be more the removal 

 of hydrogen than the addition of oxygen; but, as the chemists have 

 told us for some time, the one is just as much an oxidation as the 

 other. 



According to Wieland, oxidations in organisms are generally of 

 this type. The enzymes of oxidation activate the hydrogen in the 

 substance oxidized (respired), and this activated hydrogen is then 

 removed by some sort of hydrogen acceptor. These enzymes are 

 the reductases, dehydrases, or oxido-reductases, so-called because 

 they not only remove the hydrogen but also make it accepta- 

 ble to the acceptor. The substance respired is the hydrogen 

 donator. 



Warburg, on the basis of experiments with animal cells and in- 

 organic models, has formulated a theory that emphasizes the 

 physical aspects of the problem. He thinks that respiration is 

 largely a phenomenon of adsorption on the colloid particles of the 

 protoplasm. Using a model of charcoal and a respirable substance 

 composed of cystine and other amino acids, he found that this 

 model would absorb oxygen and give off C0 2 , H 2 0, NH 3 , and SO3. 

 Iron has been known for some time to be important in animal 

 respiration (Chap. V), and Warburg found that if the charcoal in 

 his model was practically free of iron, it " respired" much more 

 feebly. When traces of iron were added, the gaseous exchange was 

 greatly increased. Furthermore, narcotics (ether and chloroform), 

 poisons, etc., affected the respiration model as they do the organic 

 respiration, the intensity of the effect varying with the degree of 

 adsorption. The one exception was HCN, which depressed " respi- 

 ration" exceedingly in very small concentrations although it was 

 absorbed only very slightly. This, however, was later explained, 

 when it was shown that HCN readily unites with the iron and re- 

 moves its influence from the system. Although Warburg tends to 

 minimize the effect of the various enzymes, considered important in 



