404 



PLANT METABOLISM 



isms. Azotobacter and Clostridium are important under normal soil 

 conditions, and the blue-green algae function in maintaining the nitrogen 

 supply under the wet conditions encountered in rice paddies. The other 

 organisms probably function most extensively in aquatic habitats. 



Mechanism of N2 Fixation. Although the intermediates arising di- 

 rectly from the fixation of N2 are unknown, these intermediates are 

 quickly converted to ammonia before its incorporation into organic com- 

 pounds. The reduction to ammonia logically should pass through hy- 

 droxylamine as an intermediate, but formation of amino acids from 

 hydroxylamine via their oximes does not appear to be an important path- 

 way of synthesis. Ammonia is the key compound in the sense that it 

 is the compound which combines with carbon chains to yield organic 

 nitrogenous substances in the plants or bacteria. The primary pathway 

 of No fixation involves conversion of No to NH3, through unknown inter- 

 mediates, and the formation of glutamic acid by reductive amination 

 of a-ketoglutaric acid. Glutamic acid in turn can form new amino 

 acids by transferring its amino group by transamination. 



The evidence that ammonia is the key intermediate in biological nitro- 

 gen fixation has been accumulated largely by investigating the fixation 

 with the aid of the stable isotopic tracer, N'^. These studies, and tests 

 with specific inhibitors of nitrogen fixation, indicate a unity in the mech- 

 anism of nitrogen fixation in all the diverse organisms investigated. 



RESPIRATION OF PLANTS 



Although plants are most notable for their photosynthetic capacities, 

 they carry on respiration both in the light and in the dark. Their 

 respiration per unit weight is less intense than that of most animals 

 because many of the structural materials of the plant such as cellulose 

 and lignin are metabolically inert. However, when activity is expressed 

 per unit of nitrogen in the tissues, the respiratory activity of plants and 

 animals is comparable. 



The activity of cytochrome oxidase has been clearly demonstrated in 

 plants, and cytochrome c has been isolated from plant materials. Plant 

 cytochrome c and cytochrome oxidase are very similar to these constitu- 

 ents of the cytochrome system of animal origin. Other oxidases of im- 

 portance in plants are ascorbic acid oxidase and tyrosinase (catecholase, 

 dopa oxidase, and laccase are plant enzymes similar to tyrosinase). It 

 is characteristic of the true oxidases that they use O2,' produce HoO rather 

 than H0O2, and have a heavy metal component. Both ascorbic acid 

 oxidase and tyrosinase contain copper, and though cytochrome oxidase 

 never has been purified, it apparently contains iron. A dominant posi- 

 tion of the true oxidases characterizes plant respiration, in contrast to 



