INTRODUCTORY REMARKS 



ordinary, chemical, temperature-dependent reaction, whereby dvjdi = and 



dvldT > 0. 



From what has been said it follows that photosynthesis comprises two great 

 problems: energetics and chemistry. The chemical part of photosynthesis 

 begins where the energetical part finishes. When light has exerted its 

 photochemical action — photolysis of water or of CO. — chemical reactions 



Fig. 1. An assimilation curve. Rates of 

 photosynthesis v plotted against light in- 

 tensities /. 



set in which enable the plant to build up organic substances. As far as the 

 energetical part of photosynthesis is concerned, a critical and impartial 

 survey of all the work done — in which the problem of the quantum require- 

 ment occupies the most important place — shows that Warburg and Burk 

 found the solution. The chemical part of photosynthesis, however, still 

 remains largely obscure despite important experimental findings. 



§ 2. Chlorophylls and Carotenoids 



Today seven types of chlorophyll may be distinguished : the chlorophylls 

 a, b, c, d and e, bacteriochlorophyll and bacterioviridin.* Chlorophyll a, 

 which is blue-green, is the most abundant and occurs in all autotrophic 

 organisms except pigment-containing bacteria. Chlorophyll b is yellow- 

 green and is found in higher plants and certain algae. The other chloro- 

 phylls only occur, together with chlorophyll a, in algae. Bacteriochloro- 

 phyll and bacterioviridin are pigments of certain bacteria. 



As can be seen from Figure 2, chlorophyll is a magnesium complex of a 

 porphyrin. Porphyrins are composed of four pyrrole nuclei which together 

 form a ring system. Chlorophylls a and b have, at the C7 atom, a carboxyl 

 group which is esterified with the high-molecular alcohol phytol. They 

 differ with respect to the C3 atom. This is linked with a methyl group in 

 chlorophyll a and with an aldehyde group in chlorophyll b. In earlier work 



* Literature on plant pigments: 6, 7, 12, 35, 37, 47. 



