162 - The Cell 



tion or it may be transferred to an electron 

 acceptor — if such a molecule occupies a 

 strategic position, in proximity to the locus 

 of excitation. If no acceptor is present, the 

 excitation energy returns to the environment, 

 often in the form of re-emitted light, as in 

 the case of substances that display fluores- 

 cence or phosphorescence subsequent to an 

 exposure to light. But if an acceptor com- 

 pound is on hand, the excitation energy is 

 conserved as chemical energy. 



The acceptor, after it has been reduced, 

 possesses more energy than before. Chloro- 

 phyll, isolated from the cell in pure form, 

 displays vigorous phosphorescence after it 

 has been illuminated. But in the chloroplast 

 the energy transformation is devoid of radia- 

 tion and much of the energy is conserved as 

 chemical potential energy for subsequent use 

 by the cell. 



As might be deduced from its color, chloro- 

 phyll displays a maximum absorption of light 

 in the red-orange region of the spectrum. 

 More specifically stated, the peak of absorp- 

 tion by chlorophyll is for photoradiations 

 with wavelengths close to 660 millicrons 

 (ni/z ). The energy content of photons of this 

 wavelength (13 Cal per mole of excited sub- 

 strate) is not as great as for photons of shorter 

 wavelength (for example, blue light: wave- 

 length 410 ni/x; 64 Cal per mole of excited 

 substrate). However, the photons absorbed 

 by chlorophyll are exactly suited to the ex- 

 citation energy requirements of this com- 

 pound. Photons are units that must be used 

 as a whole or not at all; and the photons ab- 

 sorbed by chlorophyll provide exactly the 

 proper energy for the excitation of one elec- 

 tron per photon absorbed at the proper locus. 



Photolysis of Water. A very important dis- 

 covery was made by Samuel Ruben and co- 

 workers at the University of California, in 

 1940, shortly after heavy oxygen (O 18 ) became 

 available. Ruben showed that all of the 

 oxygen liberated in photosynthesis is derived 

 from H 2 0; none of it from CO ; . Plant cells 

 supplied with FLO 18 liberated 0\ s ; but when 

 the cells were supplied with CO 1 , 8 , no heavy 



oxygen was formed. Thus it can be said with- 

 out equivocation that the photochemical de- 

 composition of water (photolysis) accounts 

 for the entire production of oxygen during 

 photosynthesis. Part of the hydrogen, how- 

 ever, is transferred to a primary acceptor 

 (TPN) during the process of photolysis, as is 

 shown in Figure 9-4. 



A major unsolved problem in photosyn- 

 thesis is how light energizes the splitting of 

 H 2 0. Chlorophyll, TPN, and the cyto- 

 chromes are essential components in the re- 

 action, as has been shown by Arnon. But just 

 how the photons absorbed by chlorophyll are 

 employed to bring about a transfer of elec- 

 trons from hydroxyl (OH~) ions to the cyto- 

 chrome system remains an open question 

 (Fig. 9-4). It is plain, however, that four 

 hydroxyl ions must be discharged of their 

 electrons in the formation of each one mole- 

 cule of oxygen: [4(OH~) — \e~ -* Q ± -f- 

 2H 2 Oj. 



Photophosphorylation. As is clearly shown 

 by the work of Arnon and his collaborators, 

 chloroplasts can utili/e light energy for the 

 formation of ATP from ADP and inorganic 

 phosphate. There are, however, two path- 

 ways. These are called cyclic and noncyclic 

 photophosphorylation, respectively. 



If isolated chloroplasts are provided with 

 ample light, ADP, and inorganic phosphate, 

 but no COo and no TPN, they still are capa- 

 ble of generating ATP. Without CO, they 

 cannot synthesize carbohydrates, of course, 

 and without TPN (to accept hydrogen) they 

 cannot split H 2 (and liberate O.,). But they 

 can form ATP, in a manner indicated in 

 Figure 9-4. The photon-excited electrons 

 from chlorophyll are transmitted via FMN 

 (or vitamin K) to the cytochrome svstem and 

 thence they are passed bach to the chloro- 

 phyll. The flow of each electron along this 

 channel provides energy lor charging up two 

 molecules of ATP: one as electrons drop from 

 FMN to the cytochromes, and the other as 

 they flow through the cytochrome system, 

 back to chlorophyll (Fig. 9-4). This pathway 

 is referred as cyclic because the photon- 



