INTRODUCTORY REMARKS 



model of chloroplast 



protein layer 



^n 



porphyrin 

 head 

 iso-cyclic ring 

 phytol tail 





lipid layer 



chlorophyll 

 molecule 



schematic molecular network 

 carotenoid chlorophyll and carotenoids 

 molecule 



Fig. 5. The structure of the chloroplast (Wolken and Schwertz, J. Gen. Physiol.). 



the algae Euglena gracilis and Poteriochromonas stipitata. The figures obtained 

 are not vahd for other plants. According to Hill and Whittingham (20), 

 the molecular ratio chlorophyll/carotenoids in higher plants may be as high 

 as 5 to 1 (weight ratio : 8 to 1 ) . 



Stoll et al. (37) found that chlorophyll is bound to protein. This may be 

 the reason for the pigment's highly specific activity. The molecular weight 

 of the chlorophyllproteid, which has been called chloroplastin,* is about 

 5 X 10". As the molecular weight of chlorophyll is about 900, the stoichio- 

 metric ratio between the protein component and the chlorophyll is 5000, 

 if one molecule of chloroplastin contains only one molecule of chlorophyll. 

 However, chloroplasts contain about 56% protein and 7 to 8% chlorophyll; 

 it may therefore be assumed that one molecule of chloroplastin contains 

 nearly 700 chlorophyll molecules. Taking the chloroplastin molecule to be 

 a globular protein, Frey-Wyssling (14) calculated its diameter. This was 

 found to be 100 A, so that such a molecule has a surface area of about 32000 

 A2. As the surface area of a porphyrin head is 225 A- (see Fig. 5), only 140 

 chlorophyll molecules can be present on the surface of one chloroplastin 

 particle. However, the numerous side-chains of the polypeptides may also 

 be linked with chlorophyll molecules, so that it may well be that one chloro- 

 plastin particle contains as many as 700 chlorophyll molecules. If only part 

 of the protein in the chloroplast is used for the formation of chloroplastin, 

 this number would, of course, be much higher. Though these considerations 

 remain more or less speculative and exact figures are still lacking, there seems 



* Lubimenko (24) was the first to prepare aqueous solutions of chlorophyll. He assumed that in 

 these solutions as well as in the leaves chlorophyll is iDound to protein. Chloroplastin contains 

 chlorophylls a and b and the carotenoids in the same proportion as in the leaves. Wolken and 

 Schwertz (50) prepared chloroplastin by extracting chloroplasts with digitonin. 



