[Chap. XIV THE RATE OF PHOTOSYNTHESIS 123 



This plastid surface is really the photosynthetic area of the leaf. Some 

 allowance should be made for the area occupied by the larger veins. 



Enzymes and photosynthesis. In addition to chlorophyll and other 

 pigments, the chloroplasts also contain enzymes. Since enzymes are in- 

 volved in photosynthesis and in other biological processes discussed in 

 subsequent chapters, certain of their general features may be noted at 

 this time. 



Enzymes are organic catalyzers which accelerate chemical reactions 

 that occur in physiological processes ( photosynthesis, digestion, and the 

 like) without becoming a permanent part of the final products of the 

 process, and without being destroyed by the reaction. An enzyme may 

 therefore continue to catalyze a particular chemical reaction until it is 

 destroyed or made inactive by other conditions in the cell. Ultra-violet 

 radiation and high temperatures may destroy enzymes. Synthesis of 

 enzymes undoubtedly occurs in all growing parts of a plant. Without 

 them the chemical changes in cells would be so slow that living organ- 

 isms could not long survive. Enzymes may both initiate and accelerate 

 reactions. 



In order to speed up chemical reactions in laboratory experiments and 

 industrial processes, chemists often subject the materials to very high 

 temperatures. Eventually thev discovered that many of these processes 

 could be made to occur just as rapidly at lower temperatures by the addi- 

 tion of a small amount of some inorganic substance, called a catalyst. A 

 high temperature cannot be substituted for enzymes in living cells, but 

 a temperature increase that is not detrimental to protoplasm does ac- 

 celerate plant processes. 



Some processes may be catalyzed by either organic or inorganic 

 catalysts. For example, if we boil cane sugar (sucrose) in pure water, it 

 hydrolyzes to glucose and fructose slowly. 



Sucrose + Water > Glucose + Fructose 



Ci2H220n+ H2O >C6H,206+ CeHiaOe 



If a very small amount of acid is added, the reaction proceeds rapidly. 

 During the process of boiling, some of the sugar is converted into mucic 

 acid, which further accelerates the process. Even at much lower tem- 

 peratures the reaction is fairly rapid, but for every increase of 10° C. 

 the speed of the reaction increases from 2 to 3 times. Many other chemi- 

 cal processes have this same relation to increase in temperature. 



