2 PHOTOSYNTHESIS IN NATURE CHAP. 1 



i^low coml)U.stion of sugars or other organic compounds. A certain part 

 of the chemical energy contained in organic matter, is thus utihzed for 

 the maintenance and manifestation of hfe, while the larger residue is lost 

 in the decay of dead bodies, fallen leaves, and excretions. Sooner or later, 

 all living matter reverts into carbon dioxide, water and nitrogen, and all 

 the energy which it once contained becomes converted into heat. Only 

 a small fraction of organic matter and of its energy escapes rapid dissipa- 

 tion, and is preserved for millions of years, in the half-decomposed forms 

 of coal, peat or mineral oil, under a protective layer of silt and rock. 



The continuous renewal of life on earth requires that its chemical 

 elements, scattered by the decomposition of organic matter, be brought 

 back into combination; and that the energy which was converted into 

 heat be replaced. The regeneration of organic matter can occur by a cyclic 

 process — the same carbon dioxide, water and nitrogen which were liber- 

 ated by organic decay, can be used again for sj-nthesis. The liberated 

 energy, however, is lost beyond recovery. Living organisms, no less than 

 inanimate nature, are subject to the laws of thermodynamics. These 

 laws decree that once the energy liberated by oxidation has been dissi- 

 pated in the vastness of the atmosphere and hydrosphere, it has become 

 practically unavailable for the reverse conversion into chemical energy. 

 Thus, the energy required for organic synthesis must come from an ex- 

 ternal source — and the only external energy which continuously reaches 

 the surface of the earth is sunlight. No thermodynamic restrictions 

 stand in the way of a complete conversion of light into chemical, electric 

 or mechanical energy; but it requires a mechanism able to intervene im- 

 mediately after the light strikes the absorbing surface, and to prevent the 

 energy of this impact from being converted into heat. Man has not yet 

 solved this engineering problem; but he would not be here if other or- 

 ganisms had not solved it for him long ago. These organisms are the 

 green plants. (Green, as used here, means chloro'phyll hearing plants, 

 even though the green color of chlorophyll may sometimes be masked bj^ 

 j^ellow or red pigments.) The process by which these plants synthesize 

 organic compounds from carbon dioxide and water, with the help of 

 sunlight, is, beyond doubt, the most fundamental of all biochemical 

 reactions. 



Chemically speaking, the green plants are the only productive section 

 of the earth's population; they are "self-supporting" ("autotrophic," to 

 use the technical term), and they alone enable animals and "hetero- 

 trophic" plants (fungi, most bacteria) to subsist. They accumulate 

 chemical energy, while other organisms dissipate it. 



Of course, organic synthesis of one kind or another is carried out by 

 all organisms; but the green plants alone start from scratch. All other 

 plants and animals use presynthesized materials, which they transform 



