ELECTRON MICROSCOPY 1735 



Fully green chloroplasts contain, on the average, 10-15% chlorophyll 

 and other pigments (in relation to dry weight). This corresponds to about 

 1-2 X 10^ pigment molecules per chloroplast, or an average pigment con- 

 centration of 0.05-0. 1 mole/liter. 



In a single granum, the number of pigment molecules should be of the 

 order of 1-2 X 10', and the chlorophyll concentration in them must 

 be 0.2-0.4 mole/liter. Grana probably are denser and contain less water 

 than the predominantly proteinaceous phases of the cell (the stroma and the 

 cytoplasm) ; nevertheless, for the chlorophyll concentration in them to 

 reach 0.2-0.4 M, the proportion of chlorophyll by weight in the dry matter 

 of the grana must be very high — perhaps as high as 20 or 30%. 



With a concentration of chlorophyll in the granum of the order of 0.2- 

 0.4 mole/liter, the distance between the centers of two nearest chlorophyll 

 molecules (and also the average distance from a molecule of a carotenoid, 

 or phycobilin, to the nearest chlorophyll molecule) should be about 20 A — 

 if these molecules are distributed at random. Because the pigment mole- 

 cules probably are arranged in layers, the distance between the centers of 

 two nearest chlorophyll molecules in a layer probably is much shorter than 

 20 A. 



If one considers the granum as a disc 0.6 /x in diameter, its two large sur- 

 faces have a total area of 0.5 fj.-. According to surface film measurements of 

 Hanson (Vol. 1, p. 449), chlorophyll molecules, stacked like books on a half- 

 filled shelf, require an area of about 1 m/x^ apiece. That means that not 

 more than 5 X 10^ chlorophyll molecules could find place in the two surface 

 layers of the granum, and that at least 20 parallel layers of these molecules 

 must be present in each of them. The thin discs into which grana have been 

 observed to disintegrate, might then contain one or two layers of chloro- 

 phyll molecules each — perhaps one layer on each side of the disc (as sug- 

 gested in Frey-Wyssling's and Steinmann's model, cf. fig. 37A.15). 



Wolken (1954) made similar estimates for the continuous laminae in the 

 non-granulated chloroplast of Euglena and Poteriochromonas. Each 

 Euglena chloroplast contained 18-24 bands, 18-25 m/x thick, separated by 

 layers of "stroma" 30-50 m^u thick. The chlorophyll content of a chloro- 

 plast was estimated by Wolken as 0.88-1.23 X 10» molecules (average 

 chlorophyll concentration in Euglena, 0.025 mole/liter). In Poterio- 

 chromonas, the number of bands was 9-11 per chloroplast, the band thick- 

 ness 21-50 m/x, their separation 25-51 m/i, and the chlorophyll concentra- 

 tion 2.2 X 10^ molecules per chloroplast, or 0.016 mole/liter for the cell 

 as a whole. (In both flagellates, the average chlorophyll concentration is 

 considerably lower than in green plant cells.) 



From the average length and width of the lamina, Wolken estimated 

 that a surface area of 222 A- is available per chlorophyll molecule in 



