3 CHLOROPLASTS 261 



plastin with the molecular weight 5 millions to be really the chromo- 

 protein of the chloroplast, it ought to hold 2500 chlorophyll molecules, 

 but in reality it contains only about 420. This indicates that the chloro- 

 plastin is a mixture with stroma constituents rather than a pure com- 

 pound from the grana. 



On the other hand, physiologists find that a number of chlorophyll 

 molecules as large as calculated above is necessary for the assimilation 

 of one molecule of COg. That number is called unit of assimilation. 



Whereas chemists think of the photosynthetic process as associated 

 with the chlorophyll molecule (Stoll, 1936), physiologists tend rather 

 to regard the pigment merely as an energy trap ajid to attribute the 

 actual chemical action of the gradual hydrogenation to the proteins 

 in the chloroplast (Rabinowitch, 1945). This is inferred partly from 

 Blackman's dark reaction (1905), but mainly from facts established 

 by Emerson and Arnold (1932), according to which a. unit of assimi- 

 lation of roughly 2500 chlorophyll molecules is needed for the re- 

 duction of one CO2 molecule. Gaffron and Wohl (1956) calculate 

 about 1000 molecules for this same unit. This observed fact calls into 

 question all attempts to deduce the mechanism of assimilation from 

 the chemical constitution of the chlorophyll molecule (Willstatter, 

 1933; Franck, 19.35; Stoll, 1936). Gaffron and Wohl state that 

 the pigment acts merely as a specific energy transmitter and that a very 

 large number of chlorophyll molecules would be required to capture 

 the necessary quanta of light for the assimilation of one CO2 molecule 

 (Warburg and Negelein, 1923; Scplmucker, 1930; Eymers and 

 Wassink, 1938; Emerson and Lewis, 1939). It is to be expected that 

 the occurrence of these units of assimilation will be expressed morpho- 

 logically in some way. Heitz (1936a) presumes that the grana may be 

 involved. This, however, cannot be so, for if, as Euler, Bergman 

 and Hellstrom (1934) state, a chloroplast contains 1.65-10^ chloro- 

 phyll molecules, there would have to be something like 10^ or a 

 million grana, whereas the actual number is about 600. In a bi- 

 molecular layer, 2000 chlorophyll molecules would occupy a surface 

 of 1000 X 225 A^ =^ z.i^ X io~^ fj,^. As a square, this surface has a 

 side of only 0.05 /u. Therefore the unit of assimilation is certainly 

 amicroscopical. 



Seeing that a chloroplastin macromolecule in the grana ought to 

 contain about 2500 chlorophvll molecules, the question naturally 



