36 FOUNDATIONS OF BIOLOGY 



potential energy. Thus the plant in sunlight is continually 

 separating the carbon from the oxygen of carbon dioxide. 

 The oxygen is liberated as free oxygen while the carbon 

 which has been separated from the oxygen is combined with 

 molecules of water to form carbohydrates grape sugar 

 (glucose) and fruit sugar (fructose). 

 The conventional equation for this reaction is: 

 6 C0 2 + 6H 2 O = C 6 Hi 2 Ofi + 6O 2 



(carbon dioxide) (water) (glucose or fructose) (free oxygen) 



It should be emphasized, however, that the processes in- 

 volved are by no means so simple as is implied above; but 

 since there is little conclusive data in regard to the details, 

 the equation as stated affords a formal explanation which is 

 adequate for the present discussion. 



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

 sugar, having been accomplished, the green plant trans- 

 forms 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 photosynthesis. 



We have seen that the 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 such simple combinations as 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. By the addition, then, of 

 nitrogen to the carbohydrate basis a very simple nitrogen 

 compound, such as an amine (e.g., asparagine = C4H 8 N 2 O 3 ), is 

 built up, which may be transformed into a protein by the ad- 

 dition of sulfur and other elements secured from sulfates, 

 phosphates, etc. Again, we do not know how this is done, 



