Nutrition of Green Plant Cells - 161 



thesis did not begin until about 1945, when 

 two important isotopes, heavy oxygen, O 18 , 

 and radioactive carbon, C' 4 , had become 

 available (Table 8-1). These tracer isotopes 

 made it possible to discover the source of the 

 oxygen that is liberated during photosyn- 

 thesis and to follow the path of carbon, from 

 the time it is absorbed in the form of CO.,, 

 until it is incorporated into various organic 

 compounds in the cell. 



Much recent work has dealt with various 

 unicellular green algae, which can be grown 

 in test tubes under strictly controlled condi- 

 tions; corroborative results have been ob- 

 tained from higher plants. Moreover, Daniel 

 Arnon, working at the University of Cali- 

 fornia, has succeeded in observing photo- 

 synthesis in chloroplasts removed from the 

 cells of macerated spinach leaves. Studies on 

 such isolated chloroplasts have been very 

 fruitful. The isolated chloroplasts carry on 

 photosynthesis in a medium of known com- 

 position. Accordingly it is possible to vary 

 the components of the medium in systematic 

 fashion and thus to ascertain how each ful- 

 fills its role during the complex processes of 

 photosynthesis. 



The essence of photosynthesis, as deduced 

 from the large body of recent evidence, is the 

 conversion of radiant energy (light) into 

 chemical energy. The conserved energy goes 

 into the formation of: (1) ATP (from ADP 

 and inorganic phosphate); and (2) TPN-H 2 , 

 FMN-H 2 and other reduced primary accep- 

 tors in the chloroplasts. Electrons, necessary 

 for the reduction of the primary acceptors, 

 are derived partly from chlorophyll and 

 partly from water; and in each case the dis- 

 charge of electrons is energized by light. 

 Eventually many of the electrons flow back 

 to the chlorophyll, restoring this unique 

 molecule to its original uncharged state. But 

 while the electrons are in flux, energy is 

 generated and this is utilized for the build-up 

 of the high-energy phosphate reserves of the 

 cell. Equally important is the fact that H 2 0, 

 when it is forced to give off electrons, under- 

 goes pliotolysis. This light-energized splitting 



of H 2 molecules liberates free 2 , while 

 hydrogen (H+) undergoes transmission to 

 one of the primary acceptors of the cell 

 (TPN + 2e- + 2H+-^TPN-H 2 ). The net 

 effect of light, accordingly, is to increase the 

 reserves of chemical potential energy in the 

 cell, mainly by the build-up of ATP and 

 TPNH 2 . 



The evidence that has led to this current 

 view of how light energy is trapped and 

 stored by the chloroplasts is very extensive. 

 Outstanding contributions have been made 

 by C. B. van Niel (Stanford University), S. 

 Ruben, D. Arnon, and M. Calvin (University 

 of California), H. Gaffron (University of 

 Chicago), E. J. Rabinowitch (University of 

 Illinois), S. Ochoa and W. Vishniac (New 

 York University), and many others. This evi- 

 dence will be presented in briefest outline 

 only. But first it is necessary to consider some 

 basic concept in photochemistry. 



Photochemical reactions are those that are 

 energized by light. To be effective, the light 

 must be absorbed by one or more of the re- 

 acting substances; and, of course, chlorophyll 

 is the principal light-absorbing molecule in 

 the light-driven reactions of photosynthesis. 

 When light falls upon a metal, photons 1 

 (units of photoradiation) are absorbed and 

 the electrons of the metal atoms, excited by 

 the absorbed energy, tend to be ejected from 

 the surface. The potential thus generated 

 may be measured, as in a photographic light 

 meter or other type of photoelectric cell. 

 Likewise when photons are absorbed by a 

 molecule, an electron in one or more of the 

 constituent atoms, picking up energy, tends 

 to shift outward from the atomic center into 

 an orbit of higher potential. Such an excited 

 electronic configuration is unstable, however. 

 Within some fraction of a second, the elec- 

 tron either drops back into its original posi- 



1 A photon represents one quantum of light energy. 

 Photons, however, vary in their energy content, ac- 

 cording to the wavelength of the light. This is shown 

 in the fundamental equation: E = lic/\ in which 

 E — energy of the photon; c = velocity of light in 

 cm/sec; X = wavelength in millimicrons; and /) = the 

 Planck energy frequency constant. 



