H. G.\FFRON 



substraies are e>diausted. If such an easy reversibilirs^ existed e\er\- 

 where in the reaction chain leading firom carbon dioxide to carbo- 

 hNxlrate, an accumulation of photos^•nthetic products would be un- 

 likely. In fact, it is quite essential for the efficiency of the s^Tithetic 

 jMXXXSS that the carboh\tlrate finally formed is not broken dowTi 

 again in the dark by the same catalytic s^^stems which helped to btiild 

 it up. One of the difficulties we shall encounter in tr\-ing to reconstruct 

 the phota5>-nthetic mechanism tn vitro with enz\-mes isolated fix)m the 

 plant cell will probably be the reversibLlir>- of partial reactions, tinless 

 we succeed in remoxing and thereby stabilizing the intermediate just 

 fcHined. At least, the way "doxsTi" is the normal course of events as 

 demonstrated in respiration and fermentation. Now, save for attack 

 by the respiratory or glycoh-tic s\"5tems, the carbohydrate sxTithesized 

 in the light is stable (a ver\- slow reversion is postulated for theoretical 

 reasoDS not discussed here). The solution of the problem, therefore, 

 may be hidden at either or both ends of the photosynthetic s%"stem. 

 Tlie primar\- tinstable carbohNxirate may pohTnerize with the release 

 of some free energ\-. One might even think of a reduction up to the 

 alcc^ol ^OT h\-drocarbon) level and an oxidation by another path back 

 to an aldeh\-de (or alcohol) . At the other end of the line, the ox\-gen- 

 liberating s\"3tem should have prop>erties making it rather ineffective 

 as an oxidase. At any rate, all attempts to cause an accimiulation of 

 intomediates by mistreating the irradiated cell .with narcotics or 

 specific poisons have failed. Hydrogen donors capable of reducing 

 carbon dioxide afterwards in the dark do not survive a period of 

 illxnnination. There is no easily detectable formation of partiy re- 

 duced substances. Xe\"erthelesS; there must be at least three inter- 

 mediate steps in the process of reduction, since four hydrogen atoms 

 have to be transferred firom water to carbon dioxide. This particular 

 problem ^ill probably be solved \sith the aid of carbon isotopes in 

 continuation oi tbe \*T>rk oi Ruben, Kampn^ and Hassid {cf. 14). 



Several iod^jendent investigations have shov*"n that nine or ten 

 l%hi qu2^:i l:z r.rcessary for this process. The TnaYi'mum ntmiber 

 of initial 5 erre, could be ten. For reasons of stoichiometiy it 



is soKible uj assun^ that there are only eight photochemical reactions, 

 a pair for each release and recovers- of a hydrogen atom by the mole- 

 cule transforming electronic into chemical energy (4) . This molecule 

 may be chlorophvll itself. The difference between the theoretical 



38 



