864 EXTERNAL AND INTERNAL FACTORS CHAP. 26 



see there that, even when a "bottleneck" reaction does exist, the relation- 

 ships postulated by Blackman are only approximated, but never exactly 

 fulfilled. 



2. Photosynthesis Not a Homogeneous Reaction 



In the kinetic treatment of photosynthesis, it is necessary to keep in 

 mind that photosynthesis is not a homogeneous reaction. This statement is 

 true in a twofold sense : In the first place, the photosynthetic apparatus is 

 a colloidal system with a definite structure — probably containing rows of 

 oriented pigment molecules adsorbed on interfaces between proteinaceous 

 and lipide layers (c/. the discussion in chapter 12, Vol. I). In the second 

 place, the supply of light energy and of the reactants to different parts of 

 this structure is not uniform, particularly when photosjoithesis proceeds at 

 a high rate. 



Let us consider these two aspects of the problem more closely. 



The reactions involved in photosjaithesis are, at least in part, surface 

 reactions. It is furthermore likely that they can be called topochemical 

 reactions — meanmg that they occur not by encounters of molecules moving 

 about at random in two-dimensional adsorption layers, but by a more pur- 

 poseful mechanism, which requires the reacting molecules to take definite 

 paths past a number of catalytic "reaction centers." The products formed 

 in one reaction center are directed to the next one, so that back reactions 

 of unstable, intermediate products of oxidation and reduction products are 

 prevented by their spatial separation. Equations based on the law of mass 

 action can be applied to this type of reaction only with considerable reser- 

 vations. 



The second aspect of the inhomogeneous character of photosynthesis — 

 the nonuniform supply of reactants and energy to the different regions of 

 the photosynthetic apparatus — was repeatedly discussed in the literature, 

 for example, by Schanderl and Kaempfert (1932) and Katz, Wassink and 

 Dorrestein (1942). Even if the chlorophyll molecules and the catalyst 

 molecules participating in photosynthesis were available uniformly in all 

 parts of a chloroplast — which is in itself uncertain — the supply of carbon 

 dioxide, as well as that of light quanta, is likely to vary from place to place. 

 In strong light, and with a limited supply of carbon dioxide, the chloro- 

 phyll molecules closer to the source of supply may use up most of the avail- 

 able reduction substrate and leave only little for the less favorably situated 

 pigment particles; in other words, the numerical value of the concentra- 

 tion factor [CO2] in kinetic equations may vary from place to place. A 

 similar consideration applies to the light mtensity factor, /: Because of 

 the high optical density of chloroplasts — particularly for red and blue- 



