lllfi THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



carbon (Chapter 36). From the latter experiments, Calvin drew the same 

 conclusion as Kok — that the rate of respiration in strong light is only about 

 one half of that in darkness. (The remaining one half may represent the 

 proportion of the total cell respiration taking place outside the chloro- 

 plasts and therefore not affected by light.) 



Kok suggested that photosynthesis substitutes for respiration by produc- 

 ing energy' carriers (such as high energy phosphates or "HEP" molecules) 

 which the organism rcfiuires for its metabolic activity, and which it ordi- 

 narily derives from respiration. More specifically, Kok postulated that 

 the primary light reaction in photosynthesis has a twofold function: (1) 

 to produce reducing and oxidizing agents (HX and Z, cf. Vol. I, scheme 

 7. IV) capable, respectively, of reducing CO2 to C'HoO and of oxidizing Ho(J 

 to O2; and (2) to produce HEP-molecules by transphosphorylations coupled 

 with back reactions between these primary products: 



HX + Z + phosphate > HZ + X + HEP 



Until respiration is fully compensated — or, rather, suspended as unneces- 

 sary (at least, in the chloroplasts) — the absorbed light is used only or 

 mainly to produce HEP molecules. Respiration of one ICH2O} group 

 has been reported to produce six HEP molecules (Ochoa, Lippman) ; Kok 

 suggested that the same number can also be obtained by recombination of 

 six (HX -f Z) pairs, and that these six pairs can themselves be produced 

 by three cjuanta. Thus, two HEP molecules, containing about 20 cal./mole 

 disposable energy, are fonned by one quantum of red light (about 40 cal./ 

 einstein). 



Above the compensation point, Kok assumed a quantum requirement 

 of 6; he postulated that here, too, each c^uantum produces two (HX + Z) 

 pairs, and that out of twelve such pairs (produced by six quanta), four 

 (produced by two quanta) react further to reduce { CO2 } to { CH2O } and to 

 oxidize H2O to O2, and eight (produced by four quanta) react back, con- 

 verting eight low energy phosphates into eight HEP molecules (which, in 

 turn, are utilized as "boosters" in the reduction process). The quantum 

 requirement would then be 3 for the reversal of respiration and 6 for true 

 photosynthesis. (No explanation was given by Kok why eight HEP 

 molecules are needed in the latter case, as against only 6 in the first one.) 



This, obviously highly arbitraiy scheme made to fit the (supposedly) 

 experimentally indicated l/7-values of 3 and 6, is closely related to the 

 "energy dismutation" schemes (such as scheme 9. HI) proposed (among 

 other possible reaction schemes of photosynthesis and chemosynthesis) in 

 chapter 9. The assumption that one third of all quanta are used in photo- 

 synthesis to provide oxidation and reduction agents, and two thirds for the 

 formation of energy boosters (HEP molecules), imitates the mechanism of 



