328 



PHOTOSYNTHESIS 



TABLE 30 THE EFFECT OF PHOTOSYNTHESIS UPON THE ABSOLUTE AND RELATIVE QUANTITIES 



OF THE CHLOROPLAST PIGMENTS IN LEAVES OF CHERRY LAUREL {PriWUS laUrOCCraSUs) 

 (PATA OF WILLSTATTER AND STOLL, I918). 



2. Other Protoplasinic Factors. — The chloroplast pigments are not the 

 only constituents of the living cell system which are essential for photosyn- 

 thesis. Although chlorophyll is indispensable in the process it has never been 

 possible to accomplish photosynthesis in vitro by the use of chlorophyll solu- 

 tions or dispersions. There is some evidence that photosynthesis will not 

 occur in the complete absence of oxygen, suggesting that the respiratory sys- 

 tem of the cell plays a part in the process. Apparently, however, other 

 systems than the respiratory mechanism are also involved since photosynthesis 

 ceases in most plants at temperatures between 40 and 50° C. and before 

 respiration is completely inhibited. Furthermore, photosynthesis is inhibited 

 by smaller doses of narcotics than respiration. Since neither temperatures up 

 to 50° C. nor narcotics in sm.all doses are known to exert any effect upon 

 the chloroplast pigments these facts indicate very strongly that certain pro- 

 toplasmic factors other than the pigment system or the respiratory mechanism 

 are essential for the occurrence of photosynthesis. Just how many such pro- 

 toplasmic factors operate as part of the photosynthetic system it is impossible 

 to say, but considerable evidence indicates that at least one enzymatic mechan- 

 ism is involved in the process. 



3. Steps in the Photosynthetic Process. — The experimental evidence at 

 present available indicates with a fair degree of certainty that there are prob- 

 ably at least four distinct steps in the process of photosynthesis: (i) a dif- 

 fusion phase in which dissolved carbon dioxide or carbonic acid molecules 

 migrate from the cell walls to the chloroplasts, (2) at least one strictly 

 chemical reaction, (3) a photochemical reaction, and (4) at least one reaction 

 catalyzed by an enzyme. This last reaction may be identical with either (2) 

 or (3). 



There can be no question about the occurrence of the first of these steps 



