PHOTOSYNTHETIC MACROMOLECULES OF CHLOROBIUM THIOSULFATOPHILUM 32 1 



however, were prepared to assume that structural organization probably 

 contributes to efficiency and might even be essential for the primary 

 process. Those so inclined inferred that the submicroscopic lamella, 

 present in rudimentary form as the cortex of the chromatophore and 

 elaborated in the grana and stroma of the chloroplast, is a universal 

 architectural characteristic of photosynthetic systems. Since the properties 

 of a lamella fulfill the requirements for the separation of primary reducing 

 and oxidizing components in a photolysis scheme as well as for the con- 

 densed state required in the semiconductor concept of the primary photo- 

 synthetic process, a theoretical basis could be proposed to account for the 

 universality of this characteristic. 



Our approach to the problem has been to try to characterize both 

 structure and function at the lowest level of structural organization capable 

 of supporting photosynthesis. The present study of the structure and 

 function of the photochemical apparatus of Chlorohium thiosiilfatophilum 

 is raising some questions. The data lead tentatively to the following con- 

 cept. The structural unit is a particle which is spherical rather than 

 elongated, has a maximum dimension of about 150 A and a molecular 

 weight of about i -5 million. Such a macromolecule is about one order of 

 magnitude smaller in mass than the simplest photosynthetic unit studied 

 to date; namely, the Chromafiuni chromatophore. Does this particle 

 represent the limiting size of chromatophore or a lower level of organiza- 

 tion ? Although it is possible to construct a sphere 150 A in diameter with 

 a 60 A thick cortex, the area available at the inner surface could accommo- 

 date onlv about one-tenth of the chlorophyll contained in the particle; 

 thus, the type of architecture postulated for the Chnmatiuiu chromatophore 

 [6] is not applicable to this system. In addition, in some properties, the 

 pigmented particle ditfers rather sharply from the chromatophores which 

 have been studied ; for example, in the degree to which accessory pigments 

 separate from the particle and the tendency of the particles to aggregate 

 into progressively larger rod-like structures. For these reasons, we prefer, 

 for the time being, to regard this system as less highly organized than the 

 chromatophore and refer to the pigmented component either as photo- 

 svnthin, a name employed for the pigmented extracts prior to the advent 

 of the chromatophore concept [18] or as holochrome, the term used to 

 designate "a colored substance as it exists in its natural state within an 

 organism, where the colored group is combined or associated with a 

 carrier which alters the physical or physiological properties of the 

 prosthetic group" [36]. 



If we assume that the lamella is not an architectural feature in this 

 system, then the idea that lamellar organization is essential for photo- 

 synthesis is brought into question. One solution to this dilemma is to 

 consider the lamella as the expression of a more fundamental characteristic 



VOL. II. Y 



