4 PHOTOSYNTHESIS IN NATURE CHAP. 1 



systems; these are called "chemo-autotrophic" bacteria. Others, so- 

 called purple bacteria, use light for the synthesis of organic matter from 

 carbon dioxide and organic or inorganic hydrogen donors, for example, 

 hydrogen sulfide or fatty acids. In this variation of photosynthesis 

 (called "photoreduction" by Gaffron, cf. page 129), light energy is utilized 

 mainly for temporary activation and not for permanent conversion into 

 chemical energ3^ The energy of the organic matter produced by purple 

 bacteria is only to a small part converted light energy; most, if not all 

 of it is chemical energy transferred from one unstable chemical sys- 

 tem to another. The existence of these bacteria is possible only because 

 the crust of the earth has not yet settled into a complete chemical 

 equilibrium, and spots of high chemical potential can still be found here 

 and there (particularly in volcanic regions). Conceivably, these peculiar 

 modes of autotrophic life (we will speak more of them in Chapter 5) may 

 have played a greater role in earlier geological ages, when the chemical 

 activity on the surface of the earth was more widespread and violent. 

 They are consequently of considerable interest in speculations as to the 

 origin and development of life on this planet. In the contemporary cycle 

 of the living matter on earth, these processes are of no consequence. 

 Photosynthesis by green plants alone prevents the rapid disappearance 

 of all life from the face of the earth. 



We cannot definitely assert that photosynthesis takes exactly the same 

 course and leads to the same primary product in all organisms from the 

 lowly diatoms to the highly organized flowering plants. Differences in 

 structure and composition of the photosynthetic organs of different species 

 (described in Chapters 14 and 15) make minor variations in the mecha- 

 nism of photosynthesis probable. However, the universal occurrence of 

 chlorophyll in all photosynthesizing plants and the similarities between 

 the kinetic relationships governing photosynthesis in unicellular algae 

 (e. g., Chlorella), and in higher land plants (e. g., wheat) {cf. Vol. II, 

 Chapters 27 and 28) indicate that the general characteristics of the 

 process must be the same throughout the plant world. 



After the completion of photosynthesis, the plants and animals begin 

 to aminate, halogenate, polymerize, oxidize, reduce or dismute the 

 first products of photosynthesis, thus producing fats, proteins, nucleo- 

 proteids, pigments, enzymes, vitamins, cellulose and other structural ma- 

 terials. In due time, before or after the death of the organism, all these 

 compounds will be oxidized and decomposed. This decomposition goes 

 by many different paths. Only one of them, the oxidation of sugars by 

 the respiratory system, appears as a direct reversal of photosynthesis; and 

 even in this case, it is doubtful whether the analog}' extends beyond the 

 over-all result {cf. Chapter 9, section 4). The reservoir of life is fed by a 

 single channel, through which matter is pumped up from the low-lying 



