378 PHOTOMECHANICAL CONSIDERATIONS 



quantum-one electron transfer" sort, or whether they involve the con- 

 certed action of two light quanta. There are compelling arguments (11) 

 favoring a two-quanta primary reaction. However, the most concrete 

 spectrophotometric observations dealing with System 1 and the bac- 

 terial system (see later) suggest that the primary reactions involve 

 single quanta driving single electron transfer acts. For the present 

 the observable reactions will be discussed in "one quantum" terms. 



On a "one quantum" basis, the flashing light experiments of Emer- 

 son and Arnold should be interpreted as follows: The photosynthetic 

 unit in green plants and algae consists of a set of about 400 Chi mole- 

 cules plus a reaction center. The energy of one light quantum, ab- 

 sorbed anywhere in the unit, is delivered to the reaction center. There 

 it promotes a reaction in which one electron is transferred so as to 

 produce a stable reductant separated from an oxidant. In photosyn- 

 thetic bacteria the unit, as defined by flashing light experiments and 

 interpreted on a one-quantum basis, consists of about 50 BChl mole- 

 cules plus a reaction center (12). 



A reaction center, functioning in a photosynthetic unit, can be 

 delineated by identifying the primary photochemical events and any 

 closely coupled "dark" reactions. Ideally the significance of observable 

 reactions should be established by showing that these reactions are 

 part of a sequence leading to complete photosynthesis. In most cases it 

 has not been possible to apply this important test, and several less 

 decisive criteria have been accepted by default. A primary photo- 

 chemical reaction for photosynthesis should occur reversibly, in vivo, 

 with high quantum efficiency. It should occur (not necessarily re- 

 versibly) under conditions, such as low temperature, that eliminate 

 secondary enzymatic reactions. As for the dark reactions that follow 

 the primary event, these should lead rationally into proven metabolic 

 pathways. The light- induced formation of NADPH2 (13), of oxidized 

 Cyt (14), and of ATP (15) fall into this category. 



A PHOTOCHEMICAL REACTION CENTER IN 

 GREEN PLANTS AND ALGAE 



In System I of green plants and algae the central position in a re- 

 action center appears to be occupied by P700, a pigment having an 

 absorption band at 700 to 705 m/u. The characterization of this pig- 

 ment has been pursued principally in Kok's laboratory (6,16,17,18). 

 P700 in chloroplasts and algae is bleached reversibly by light absorbed 

 by Chi a. In this reaction fewer than five quanta suffice to bleach one 

 P700 molecule (17). The bleaching results from oxidation of the pig- 

 ment; chemical titration indicates that P700 is a one-electron trans- 

 ferring agent of oxidation potential 430 mv (16). In chloroplasts the 



