392 PHOTOMECHANICAL CONSIDERATIONS 



trap the quantum. It is easy to see how this trapping could occur in 

 green plants and algae. P700, absorbing at a slightly longer wave 

 length than Chi a, should act as an energy sink through the resonance 

 transfer mechanism (6). Delocalized singlet energy in Chi a can thus 

 be trapped as localized singlet energy in P700. In chromatophores the 

 absorption maximum of P870 or P890 coincides with the long-wave 

 band of BChl, P870 or P890 cannot be expected to act as an energy 

 sink relative to BChl, and other mechanisms should be entertained for 

 the trapping process. A singlet excited state, upon reaching P870 or 

 P890, will be detained there if it is converted efficiently into a localized 

 state of any kind. If the localized state is long lived, its utilization for 

 photochemistry will be facilitated, A localized, long-lived state might 

 be a triplet state, or a highly polarized state such as an nir* singlet, 

 or a bimolecular charge-transfer state, or any sort of ionized state in 

 which electrons or holes are trapped. Entry into a "forbidden" state 

 such as a triplet is facilitated by factors producing asymmetry (47); 

 a reaction center is automatically a point of asymmetry in a photo- 

 synthetic unit. Highly polarized states, including charge-transfer 

 states and other ionized states, might be favored in P870-890 by the 

 proximity of electron-acceptor or donor molecules such as UQ or Cyt. 

 These mechanisms for the trapping of excitation energy and the 

 initiation of photochemistry in photosynthetic tissues are entirely 

 speculative. Nevertheless it is gratifying that concrete mechanisms 

 drawn from defined chemical models can be envisioned and, hopefully, 

 tested. 



CONCLUSION 



Evidence has been presented that certain photochemical reaction 

 centers exist in photosynthetic tissues. Light energy absorbed in a 

 major Chi or BChl component is transferred to the reaction centers 

 and mediates the reactions of specific molecules with high quantum 

 efficiency. Among these specific molecules are cytochromes, P700 

 or P870-890, and perhaps also PQorUQ. Lacking evidence to the con- 

 trary it is reasonable to conclude that these reactions reflect primary 

 photochemical acts of photosynthesis, 9 This conclusion cannot be taken 

 as proved until it has been shown that these reactions initiate the main 

 stream of electron flow for the chemistry of photosynthesis. 



9 The primary reactions of Photochemical System II (mediating oxygen evolu- 

 tion) in greCn plants and algae are a separate and poorly understood consider- 

 ation. 



