114 TEXTBOOK OF BOTANY 



statements are based will become clearer as we proceed with the study 

 of other processes that occur in living organisms. 



What becomes of the sugar made by plants? The answer to this ques- 

 tion will become more evident as we proceed with the next six chapters. 

 For the present we may be brief. Some of the sugar is transformed to 

 other types of foods; some is transfomied directly into substances of 

 which cells are composed, particularly the substances in cell walls; some 

 of it is oxidized in the plant; and some of it accumulates within the plant. 



The accumulated sugar of course merely represents the part that has 

 not been used within the plant or converted to other kinds of food 

 within it. Animals and non-green plants get some of it. The amount of 

 sugar that accumulates depends partly upon the rate of photosynthesis 

 and partlv upon a number of other processes and conditions. Some plants, 

 such as sugar cane and the sugar beet, are noted for the large amounts 

 of sugar that accumulate in them in a suitable environment. Under good 

 growing conditions, sugar is usually present in varying amounts, as 

 glucose, fructose, and sucrose, in the cells of all green plants. 



In plant cells these three sugars are intraconvertible; i.e., one sugar 

 may be formed from another by certain rearrangements and combina- 

 tions of atoms: 



glucose =F=^ fructose 

 glucose + fructose :^=^ sucrose + H2O 



The rearrangements of atoms involved in the transformation of glucose 

 to fructose and vice versa may be inferred from the structural formulas 

 represented in the footnote on page 104. The formation of glucose and 

 fructose from sucrose is merely a process of hydrolysis; the converse is a 

 process of condensation. 



Summary. The primary food of all living organisms is sugar. It is the 

 basis of all other food syntheses and is made in green plants by a process 

 of photosynthesis. In this process sugar is made from carbon dioxide and 

 water in the chloroplasts of green plants exposed to the radiant energy, 

 light. An insignificant amount of sugar is also made photosynthetically 

 by purple bacteria that contain a chlorophyll-like pigment, and chemo- 

 synthetically in the dark by certain colorless bacteria that obtain energy 

 by oxidizing reduced iron, sulfur, and nitrogen. In every case the raw 

 materials are carbon dioxide and water, and the end products are sugar j 

 and free oxygen. The intermediate steps in the process are not fulh' 

 known. j 



