METABOLISM OF ORGANISMS 33 



regard to the details of the intermediate stages, the equation stated 

 affords a satisfactory expression of the end result of photosynthesis 

 that is adequate for the present discussion. 



The first great step in food synthesis, the formation of a sugar, 

 having been accomplished, the green plant usually transforms the 

 sugar and stores it as starch for future use as fuel or as the basis of 

 further synthesis. Starch is the first visible product of photo- 

 synthesis. 



We have seen that the chief characteristic of proteins as com- 

 pared with carbohydrates (sugars, starches) is the presence of 

 nitrogen, and this element must be added to the CHO basis already 

 constructed as the next step toward protein synthesis. The green 

 plant not only can, but must employ nitrogen in simple combina- 

 tions, chiefly nitrates, and this is a fact of prime importance, for 

 typically, as will appear later, animals and most colorless plants 

 require nitrogen in more complex combinations. Thus by the 

 addition of nitrogen to the carbohydrate basis relatively simple 

 nitrogenous compounds, amino acids, are built, which in turn form 

 the foundation for the synthesis of the immense variety of proteins. 

 Little is known of the complex chemical processes involved in 

 protein construction, and nothing about the actual incorporation 

 of the proteins as an intrinsic part of the architecture of the living 

 matter itself. But it is evident that synthesizing enzymes play a 

 crucial role. These are special proteins which are known only as 

 products of living protoplasm and are the activating agents 

 (catalytic agents) for chemical transformations in which, however, 

 they themselves take no integral part. 



Protococcus thus takes the raw elements, so to speak, of living 

 matter and by the radiant energy of sunlight, which its chlorophyll 

 traps, constructs carbohydrate, protein, protoplasm. In other 

 words, the green plant is a synthesizing agent, building up highly 

 complex and unstable molecular aggregates brimming over with 

 the energy received from the Sun. So the green plant, whether 

 Protococcus or Elm, by this autotrophic method manufactures 

 its own food for itself — and incidentally, as it were, for the living 

 world in general, including Man. 



3. Respiration 



As we have already stated, protoplasm is always at work — to 

 live is to work — and this means expenditure of energy : the same 



