54 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



We prefer the second alternative, because the probabiHty of one water 

 molecule losing both its hydrogen atoms in succession seems to be smaller 

 than that of two different water molecules contributing one hydrogen 

 atom each (and the remaining hydroxyl radicals reacting to water and 

 oxygen). One of the two water molecules which enter reaction (3.13), 

 and three of the four which enter reaction (3.14), are recovered at the 

 end, and could be cancelled out in the equations, if it were not desirable 

 to underline that hydrogen atoms from several water molecules partici- 

 pate in the reduction of one molecule of carbon dioxide. 



If we add the " intermolecular hydrogen transfer" theory as scheme 

 (d) to the three schemes listed on page 52, and consider the origin of 

 oxygen according to all four of them, we find that only the oldest scheme, 

 (a), suggests that all oxygen comes from carbon dioxide; schemes (6) and 

 (d) predict that all of it should come from water; while according to scheme 

 (c), one part of oxygen must come from carbon dioxide and another part 

 from water. The last conclusion is based on the consideration that, since 

 the two hydroxyl groups in H2CO3 are equivalent, they must contribute 

 equally to the production of oxygen. After the hydration 



(3.15) CO2 + H2O . 0=C(0H)2 



one-half the oxygen atoms in the hydroxyl groups have their origin in 

 water and one-half in carbon dioxide. If the hydration is followed im- 

 mediately by decomposition into H2CO and O2, the proportion of oxygen 

 which originated in water can be one-half (if all oxygen comes from the 

 hydroxyl groups), one-third (if all three O atoms in H2CO3 contribute 

 equally to the formation of oxygen), or one-fourth (if one oxygen atom in 

 O2 must come from the C=0 group). However, reaction (3.15) is, 

 usually, not a single transformation, but a series of repeated hydrations 

 and dehydrations, and as a result the ratio of oxygen atoms in H2CO3 

 which originally belonged to water or carbon dioxide, gradually ap- 

 proaches the ratio of these atoms in all available molecules of these two 

 compounds. Since water is present in large excess, practically all oxygen 

 atoms in H2CO3 will ultimately be contributed by water. However, the 

 hydration and dehydration of carbon dioxide are slow reactions (c/. 

 Chapter 8, page 175) and since plants apparently do not contain the 

 enzyme (carbonic anhydrase) which accelerates them (c/. Chapter 15, 

 page 380), the equilibration of CO2 and H2CO3 takes a measurable time, 

 and the contribution of carbon dioxide to the oxygen production accord- 

 ing to scheme (c) must onlyjgradually^drop^from an initial maximum 

 (H, %, or 34) to zero. 



The existence of a heavy isotope of oxygen, 0^^, made possible a direct 

 check of these predictions — a striking example of the possibilities inherent 

 in the method of "isotopic tracers." Ruben, Randall, Kamen and Hyde 



