20 : 2 /Photosynthesis 361 



by the sun. It can best be described in terms of entropy or information, 

 an approach followed in more detail in Chapters 21 and 25. 



Photosynthesis is necessary for life on earth for another quite different 

 reason. The entire chemistry of the surface of the earth has a net 

 reducing property which, in the absence of photosynthesis, would bind 

 all oxygen in the form of oxides. If this happened, protoplasm as we 

 know it, which depends on oxidations to use chemical energy, would 

 not be possible. However, photosynthesis produces sufficient molec- 

 ular oxygen that it actually "controls" the oxygen in our atmosphere, 

 raising it to an equilibrium value of about 20 per cent. Thus, photo- 

 synthesis is necessary for living organisms, as they exist on the surface 

 of the earth, both in supplying the necessary energy-rich organic com- 

 pounds and also in producing the oxygen necessary to use the energy 

 in these compounds. 



The over-all reaction occuring in photosynthesis is the fixation of C0 2 

 and water to form a sugar and molecular oxygen. This may be written 

 symbolically as 



6C0 2 + 6H 2 + nhv^ C 6 H 12 6 + 60 2 (1) 



In this formula, hv represents a photon of visible light, n the number of 

 photons necessary, -^nd C 6 H 12 6 a hexose sugar. At one time, it was 

 believed that the number of photons per C0 2 molecule should be con- 

 stant and various models were built on the size of this constant. The 

 number n may be regarded as a measure of the efficiency of the photo- 

 synthetic process; as shown in Section 6, this is by no means a constant 

 but varies with many different parameters. 



The foregoing stoichiometric equation is deceptive in its simplicity. 

 Actually, many steps and subprocesses occur at the molecular level in 

 the photosynthetic reactions summarized by this equation. The 

 principal aim of this chapter is to describe the current knowledge of 

 these molecular steps. Research in photosynthesis has moved rapidly 

 forward since about 1940 and there is no indication that the process has 

 stopped. Increasingly, it has involved the tools and the ideas of the 

 biophysicist. 



2. A Little Plant Histology 



All green plants and euglena contain organelles called chloroplasts. 

 The chloroplasts can be removed from the cells by suitable fractionation 

 procedures and, when resuspended in media containing the necessary 

 additives, will catalyze photosynthesis at rates comparable to those in 

 the intact plants. The chloroplasts contain the pigments primarily 



