PLANT LIFE. CELLULOSE, STARCH AND SUGAR 397 



(2) Formaldehyde molecules quickly combine together, or 

 polymerize, to give simple sugars with the formula C 6 Hi 2 6 : 



6H.CHO - C 6 H 12 O 6 . 



(3) These sugars lose a molecule of water, and polymerize 

 further to form starch and cellulose : 



rcH 2 + (C 6 Hi O 5 ) n . 



Reactions (1) and (2) have been carried out in the laboratory 

 with ultra-violet light as a catalyst. Chlorophyll thus appears 

 to act in the r61e of a promoter (compare p. 339), in the presence 

 of which the reactions are able to proceed in visible light. 



Reverse reactions also take place in the plant. Thus starch, 

 which first accumulates in the leaves, is later turned back into a 

 sugar soluble in the sap, and is thus able to pass to parts of the 

 plant requiring new material. Some carbon dioxide is also 

 liberated from plant surfaces by oxidation of sugars. 



The Thermochemistry of the Reaction. In the combina- 

 tion of carbon and oxygen, during combustion of wood or coal, 

 much heat is liberated. Hence, when oxygen is taken out of 

 carbon dioxide again, heat or energy hi some form must be sup- 

 plied. When this takes place in a plant, the energy is evidently 

 furnished by the sunlight, for the action proceeds more slowly in 

 the shade, and ceases in the dark. 



The energy required can be measured, and may be expressed in 

 calories. The energy required to produce one simple formula- 

 weight of cellulose (C 6 Hi O 5 = 6 X 12 + 10 X 1 -f 5 X 16 = 162 

 g.) is 671,000 calories. The whole may be represented in a rough 

 equation, hi which the unknown intermediate steps are left out, 

 and only the starting substances and the final products are shown: 



6C0 2 + 5H 2 + 671,000 cal. -> C 6 Hi O 5 + 60 2 . 



Cellulose (CeHioOs),,. This substance, named cellulose be- 

 cause it forms the walls of the cells, composes much of the frame- 



