INACTIVATION OF CATALYTIC SYSTEM AS CAUSE OF INDUCTION 1417 



The strong dependence of the gush on carbon dioxide pressure is an 

 argument against attributing it to a decomposition of the A • CO2 complex. 

 According to Emerson and Lewis (cf. sect. A3), a gush becomes noticeable 

 only when the cells have been exposed, in the dark period, to a carbon di- 

 oxide concentration of several per cent; it still increases between 5 and 

 10% CO2 — a behavior more similar to that of buffers {cf. fig. 19) than to 

 that of the A -002 complex (which appears to be saturated with carbon 

 dioxide below 0.1% CO2). 



If, despite this apparent difficulty, we follow Franck in attributing the 

 carbon dioxide gush to a decomposition of the complex A • CO2, we have to 

 answer two further questions: Why should this complex decompose in 

 light at all, and why does this decomposition occur even in very weak 

 light? To answer these questions, we would have to assume, first, that the 

 normal photochemical reaction {i. e., the reduction of A -002 to carbo- 

 hydrate) is blocked during the gush, for example, by inactivation of the 

 stabilizing catalyst, Eb, and, second, that the A • CO2 molecules which have 

 undergone the first reduction step (to A -11002) and fail to be "stabilized," 

 react back with the first oxidation product. A' -OH (or with oxidized 

 chlorophyll, X), liberating carbon dioxide, e. g.: 



(33.2) A-HCOo + A'OH * A + A' + CO2 + H2O 



or: 



(32.3) A-HC02 + X > HX + A + CO2 



The assumption that the primary products, not stabilized by catalyst 

 Eb, react back was made before in chapter 28 (sect. B7) in the interpreta- 

 tion of light saturation, and of the relation (or rather lack of systematic 

 relation) between the saturation of photosynthesis and the yield of fluores- 

 cence. The additional assumption made here is that the energy released 

 in the back reaction splits A • CO2 into A and CO2. 



Complete inhibition of the catalytic mechanism somewhere beyond the 

 carboxylation stage can thus explain why light causes the decomposition 

 of the complex A • CO2 ; but it is more difficult to see why the recarboxyla- 

 tion is so slow that the gush begins with a net quantum yield of almost 

 unity, the photostationary state appears to be entirely on the side of de- 

 carboxylation, even in very weak light, and the carbon dioxide uptake 

 after cessation of illumination continues for as long as an hour. We recall 

 that, according to the concept of saturation as a consequence of back 

 reactions, which we considered most plausible in chapter 28, a large propor- 

 tion (of the order of 1/8) of the light quanta not utilized for photosynthesis 

 because of limitation by a finishing catalyst (in saturating light, this may 

 mean >50% of all absorbed quanta) should bring about decarboxylation 



