490 LIGHT AND LIFE 



"The universal struggle for existence in the living world is there- 

 fore not a struggle for raw. materials — all organisms can find these 

 in abundance in air, water and soil . . . but a struggle for entropy 

 that becomes available in the energy transition from the hot sun 

 to the cold earth. To make the most use of this transition, green 

 plants spread the enormous surface of their leaves and, in a still un- 

 known way, force the energy of the sun to carry out chemical syn- 

 theses, before it cools down to the temperature level of the earth's 

 surface. These chemical syntheses are to us in our laboratories com- 

 plete mysteries. The products of this chemical kitchen form the 

 prize for which the animal world struggles." 



To investigators of photosynthesis the core of the energy conversion 

 problem is the identification, by experiment rather than by surmise, 

 of these "improbable energy transition forms" before they become 

 consumed in the "chemical kitchen." Different investigators have 

 followed different experimental approaches in studying this prob- 

 lem. The approach emphasized in my laboratory has been the in- 

 vestigation of photosynthesis in cell-free systems. The main purpose 

 of this article is to simimarize some of our findings and to interpret 

 them from the standpoint of a unified but still tentative theory. No 

 comprehensive coverage of the pertinent literature will be attempted 

 here, since much of it has recently been reviewed elsewhere (10, 74) . 



2. Photosynthesis as an Isolated Process 



In the intact cell the energy conversion in photosynthesis occurs 

 simultaneously with non-photosynthetic energy conversion processes, 

 notably respiration, that are essential to the maintenance of cellular 

 structure and function. The products of photosynthesis become the 

 substrates of respiration; and conversely, the products of respiration 

 become the substrates of photosynthesis. This intertwining of the 

 two processes extends to their intermediate products and to the en- 

 zymes and coenzymes that are concerned in the relevant syntheses 

 and degradations. 



Given the structural and functional complexity of an intact cell, 

 it was apparent that the study of the energy conversion process of 

 photosynthesis would be greatly simplified if it coidd be separated 

 from the degradative energy conversion process of respiration. The 

 most hopeful jjossibility for isolating j)hotosynthesis from other 

 physiological processes seemed to lie in chloroplasts. Isolated chloro- 

 plasts, freed from other cytoplasmic impurities, do not respire — a 



