THE METABOLISM OF THE SULPHUR BACTERIA 



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formation of which precedes the building up of the most 

 complex molecule of all, namely, that of living matter. As 

 however, there is no primary source of energy other than that 

 of the sun, it must be presumed that more carbohydrate is 

 formed by the green plant than is required for its immediate 

 purposes, so that a secondary source of energy is available 

 to effect the further synthesis of the carbohydrate into pro- 

 teins by a dissociation of a part of the surplus carbohydrate 

 into simpler substances. Heterotrophic plants have the ad- 

 vantage of being able to assimilate substances that already 

 contain a potential source of energy, which may be utiHzed 

 for the building up of protoplasm and also for the carrying 

 out of such vital processes as reproduction, cell division, 

 motility, etc. 



All processes which originate from the vital activity of an 

 organism are classed as metabolic. If they are such as result 

 in the production of more complex substances, they are termed 

 anabolic ; if they result in the breaking down of substances, 

 they are termed katabolic. All the vital processes in a living 

 organism are thus to be regarded as consisting of two main 

 streams, one leading upwards and culminating in the formation 

 of protoplasm; and the other leading downwards and cul- 

 minating in the formation of completely oxidized substances, 

 such as nitrates, sulphates, phosphates, etc. 



A closer examination of the various katabolic processes 

 shows that the relationship of the reaction to the protoplasmic 

 molecule is not of the same intimate nature in all cases. Three 

 kinds of katabolic processes may be distinguished : 



1. Processes that result from the direct operation of the 

 protoplasmic molecule itself, that is, those in which there 

 are no special secretions for the purpose. 



2. Processes that are activated by the agency of a special 

 secretion of the protoplasm ; here the secreted material is in 

 such intimate relationship to the protoplasm that injury to 

 the latter results in the destruction of the former. 



3. Processes that are activated by a special secretion which, 

 unlike that of the second class, can be retained in an active 

 condition after the organism is killed. 



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