456 FURTHER EVOLUTION 



mixture, the production of a high-voltage spark to explode 

 the mixture, cooling, lubrication, transmission, the regula- 

 tion of speed, etc. If the engine is to run smoothly, not only 

 must each of these systems function well, they must also 

 be well co-ordinated in both time and space. The spark from 

 the plugs must occur when the piston is in a particular 

 position in the cylinder ; the mixture must enter the cylinder 

 at the appropriate moment, etc. 



Similarly, in the photosynthetic apparatus of a plant, we 

 are not dealing with one single chain of chemical transforma- 

 tions but with a number of cycles of biochemical reactions, 

 whole aggregates of catalytic and photochemical systems. 

 Only when they are highly co-ordinated, when they are 

 continually interacting, can their proper effect be obtained. 

 This is achieved not only by a definite accurate co-ordination 

 of the separate reactions in time, but also by their spatial 

 localisation, the existence of a certain structure in the 

 photosynthetic apparatus. The photosynthetic enzymes are 

 ' assembled ' on this structure and the products of the photo- 

 synthetic cycle move over it. Nobody has yet succeeded in 

 reproducing photosynthesis outside the living cell, in contrast 

 to alcoholic fermentation which may be observed in a solu- 

 tion if this contains all the necessary enzymes. This, in itself, 

 indicates the extreme complexity of the photosynthetic 

 system. 



In the chloroplasts of plants the chlorophyll is concentrated 

 in minute granules which take the form of flattened cylinders 

 having a diameter of 05 /a and a thickness of 02 /x. The 

 granules consist of plates of protein combined with a chloro- 

 phyll-containing lipid layer, like a sandwich made of two 

 slices of bread with butter inside."* According to this view 

 the polar, magnesium-porphyrin nucleus of the chlorophyll 

 is associated with the protein, while its hydrophobic phytyl 

 tail is directed towards the lipid layer of the granule. 



On such a protein-lipid aggregate there occurs, first of all, 

 the initial photochemical act which may be provisionally 

 designated as the ' photolysis of water '. However, in addi- 

 tion to this ' photolytic ' system, and in parallel to it, there 

 must be, as in the motor-car engine of which we spoke, other 

 systems or aggregates taking part in the process of photo- 



