RUBER r HILL AM) WALTER I). BONNER, JR. 429 



investigations sho\\' that photosynthetic structures or organisms con- 

 tain an /, or /-type, cytochrome and a h cytochrome. The direct par- 

 ticijiation of these cytochromes in ilic j)hotosynthetic process remains 

 to be tlemonstrated. 



Before discussing a possil)le role of chloroplast cytochromes in 

 photosynthesis, it is advisable to consider some general properties 

 of electron transport systems and how such properties could apply 

 to the photosynthetic process. 



In respiration, the utilization of oxygen for reactions depending on 

 hydrogen transfer is represented by the following equation: 



4(H) + O2 -^ 2 HoO (1) 



Keilin's classical work (12) showed that the transfer of (H) to 

 oxygen is a stepwise process w'hich is carried out through the cyto- 

 chrome components; the reduction and oxidation of the cytochrome 

 components is a univalent or one-electron process and can be con- 

 sidered to be a series of thermochemical reactions which take place 

 in the dark. The gradient of these reactions is toward oxygen and 

 its reduction. 



Mitochondrial preparations can catalyze the oxidation of a variety 

 of hydrogen donors according to the following equation: 



RHo -f 1/2 Oo -^ i? 4- HoO (2) 



Illuminated chloroplasts have been found to carry out the re- 

 verse of Equation 2. The production of oxygen can occur with a 

 variety of (H) -acceptors; they can be 2 (H) acceptors, such as quinones 

 or dyes, or one-electron acceptors such as ferricyanide. The latter 

 case is illustrated in Equation 3: 



4 Fe(CN)3- = -f 2 HoO -> 4 Fe (CN) 4- 



— _f.4H+ + O2 (5) 



The light-dependent production of oxygen in the presence of vari- 

 ous (H) acceptors is called the chloroplast reaction, because the ac- 

 tive production of molecular oxygen, from the chloroplasts, in the 

 process of photosynthesis has been known for nearly 80 years (6) . 

 Fig. 5 is a reproduction of an old illustration of Engelmann's experi- 

 ments. These experiments, published in 1884, clearly showed that 

 oxygen production was confined to illuminated chloroplasts. The 

 chloroplast reaction is limited to green cells; photosynthetic bacteria 

 do not produce the oxygen molecule. 



