558 LIGHT AND LIFE 



Non-cyclic photophosphorylation enabled green plants to form a 

 CO2 reductant at the expense of light energy with the aid of an 

 ubiquitous substance, water, and in this way to invade and live 

 autotrophically in areas devoid of reduced sulfur compounds or of 

 other electron donors of restricted distribution. The resultant proli- 

 feration of plant growth was responsible for releasing to the atmos- 

 phere the oxygen, locked in the water molecule, by the only known 

 mechanism capable of accomplishing this, the photosynthesis of green 

 plants (117, 107). 



Once molecular oxygen became available, the way was open for 

 biochemical evolution to progress toward aerobic metabolism. The 

 oxygen-independent cyclic photophosphorylation by chlorophyll-con- 

 taining particles could now be paralleled by an efficient biological 

 utilization of the energy of chemical substrates through the mechanism 

 of oxidative phosphorylation of mitochondria. Photosynthesis of 

 green plants now provided both the substrates and oxygen to make 

 oxidative phosphorylation and aerobic life on this planet possible. 



An interesting aspect of the relation between photosynthetic and 

 oxidative phosphorylation in biochemical evolution is the common 

 phylogenetic relationship between proplastids and mitochondria, as 

 it was recently reported by Miihlethaler and Frey-Wyssling (108) . 

 Their observations on proplastid development in embryonic cells 

 suggest that mitochondria followed rather than preceded chloroplasts 

 as functional organelles in cellular metabolism. This is in harmony 

 with the biochemical evidence, since photosynthetic phosphorylation 

 by chlorophyll-containing particles, being independent of molecidar 

 oxygen, could occiu" before oxidative phosphorylation by mitochon- 

 dria, which requires molecular oxygen. 



20. Concluding Remarks 



The concept of photosynthesis to which we were led in the six 

 years since the process Avas first completely localized in isolated chloro- 

 plasts (II, 6, 1) dittcrs from the conventional view of photosynthesis 

 that it is mainly a process of CO2 assimilation. Photosynthesis ap- 

 pears to be first and foremost a process for converting simlight into 

 chemical energy and tiiis conversion is more directly associated with 

 j)hosphorus than \\'\i\\ (arbon assimilation. The trapped light energy 

 is first converted into ATP and reduced pyridine nucleotides, and then 

 may be used by the cell for assimilation of carbon, nitrogen, phos- 

 phorus, or suHui (ompounds, or for any other energy-requiring cellu- 

 lar activity. 



