14 PHOTOSYNTHESIS 



confined to the grana. The number of grana in a single chloro- 

 plast may vary from about lo to several hundred. Studies 

 with the electron microscope have shown individual grana 

 as disc-like bodies consisting of lamina at right angles to the 

 short axis. The lamina vary in diameter but are commonly 

 about 0-6 /t in diameter corresponding to the diameter of the 

 granum (Granick (1949) ). 



In the blue-green algae and the photosynthetic bacteria 

 no chloroplasts are visible. It is, however, possible to make 

 preparations which show that the pigments are present in 

 small particles some 0-5 to 0-3 [x in diameter, too small to be 

 visible with the light microscope. With the electron micro- 

 scope the particles appear as disc-like structures similar to 

 the grana of other plants (Pardee, Schachman, and Stanier 



(1952)). 



Very thin sections of Euglena have shown under the elec- 

 tron microscope the chloroplast as consisting of about 20 

 continuous layers with no sharp differentiation into indi- 

 vidual grana. In this case it appears that the chloroplast is 

 equivalent to a single granum. 



Thus evidence is accumulating that the chloroplast is a 

 definite structure consisting of differentiated layers. How 

 the constituent pigments are related to these layers has not 

 yet been determined experimentally. Evidence is accumu- 

 lating that the submicroscopic structure of the pigment 

 complex may be an important factor in a physico-chemical 

 interpretation of photosynthesis. 



The plastids show many points of similarity to mito- 

 chondria, but it is not certain that these two cytoplasmic 

 structures are analogous. The chloroplasts probably possess 

 a variety of synthetic mechanisms in addition to those 

 directly concerned with photosynthesis. 



Evidence as to whether the whole of the photosynthetic 

 mechanism is located in the chloroplasts is still lacking. Some 

 suggestion that this is not the case was obtained from 

 experiments with radioactive carbon (Frenkel, 1941). In 

 the dark, carbon fixation occurred almost exclusively in 

 the cytoplasm whereas during illumination four-fifths of 

 the activity was found in the chloroplasts. The processes 



