DANIEL /. ARNON 



515 



TABLE 4 



Infi.i'i-nck of FrRRicvAMnt, (in riii Abskncf of Ciii.oridi ) on 



Cyclic Photophosphorvlation by Spinach Chloropi.asts 



AND Bactf-riai- Chromatophores (Chromatium) 



(MicROMOLES Phosphate Esterified in 30 Minutes) 



(BOVE, BOVE, WlIATLEY, AND ArNON, 30) 



* Sodium ascorbate (5 /um) was tipped in from a sidearm 15 minutes after the 

 beginning of the experiment, and illumination (35,000 Lux) was then continued for 

 30 minutes. 



capacity for cyclic i^hotophosphorylation. The conclusion seemed 

 justified therefore that the inhibitory effect of ferricyanide resulted 

 from the capture by this ion (in its oxidized form) of electrons which 

 would have normally travelled the cyclic electron transport route 

 (Fig. 5) . This conclusion was strengthened by the finding that the 

 inhibition was produced by very low concentrations of ferricyanide. 

 This would be expected if, as demanded by the hypothesis, the quan- 

 tity of ferricyanide needed to capture electrons from the cyclic sys- 

 tem needs only to be sufficient to leave the catalytic components of 

 the system in an oxidized form. 



Light-induced Oxidations of Cytochromes 



Our theory assigns to cytochromes the role of electron carriers in 

 photosynthetic phosphorylation. The initial suggestion that cyto- 

 chromes oxidized by light may act as electron carriers in the electron 

 transport chain of photosynthetic phosphorylation was based on the 

 observation of Lundegardh (99) that the cytochrome peculiar to 

 chloroplasts, cytochrome / (71), is oxidized upon illiunination. The 

 oxidation of cytochromes upon illumination has also been observed 

 in intact algae and photosynthetic bacteria by Duysens (43, 44) and 

 by Olsen and Chance (116). Of special relevance to our theory (9) 

 is the recent finding of Chance and Nishimura (36a) that, in whole 

 Chromatium cells, a light-induced oxidation of cytochrome Co is 

 independent of temperature. This accords with the main postulate of 

 our theory (9) that the primary photochemical act in photosynthesis 

 consists of electronic excitation and is thus independent of a thermal 

 reaction. 



