ROHERT HILL AND WALTER I). BONNER, JR. 



431 



can be repeated any lunnber of times. The dark oxidation is due to 

 endogenous respiration, mediated by the mitochondrial oxidase. The 

 reduction ol cytochrome c in this experiment may be viewed as a 

 light-induced (H) transfer against the thermochemical gradient, in 

 other words, an exogenous reaction. 



H,0 + } -> XH + I'OH 



(4) 



The suggestion that (H) transport in the respiratory chain is in 

 the opposite direction to that in the "photosynthetic chain" invites 

 further comparison between these two systems. Fig. 6 shows some 



Plastids from developing 

 etiolated leaves (reduced) 



Cytochrome f in solution 



n solution 



Cytochrome b^ in solution 



-| 1 1 1 1 r 1 1 1 



Fig. (i. Spectra, as observed in a low-dispersion spectroscope of chloroplasts and 

 of some hematin compounds extracted from leaves, compared to the cytochrome 

 spectrum of yeast. Only the o-bands are represented. 



spectra of the hematin compounds present in leaves compared to 

 those present in yeast. Also shown is a chloroplast spectrum, but it 

 should be emphasized that this spectrum is a combined one, derived 

 from our knowledge concerning plastids from etiolated plants or 80 

 per cent acetone-extracted chloroplasts (for cytochromes) and from 

 green chloroplasts (for chlorophyll). Fig. 7 compares the oxidation- 

 reduction potentials of some hematin components in leaves and in 

 yeast. 



Further comparisons between mitochondria and chloroplasts may 

 be made as follows: 



(1) Mitochondria perform oxygen reduction. Illuminated chloro- 

 plasts perform oxygen production. 



