PLANT METABOLISM 



391 



molecule contains 4 pyrrole rings (Fig. 15-5) labeled I, II, HI and 

 IV. Third, the molecule contains an atom of ]\Ig held by primary bonds 

 to two nitrogen atoms of the pyrrole nuclei and by secondary bonds to 

 the other nitrogen atoms. Fourth, the pyrrole rings are connected by 



Fig. 15-4. System of conjugated double bonds 

 in chlorophyll shown by bold lines. 



HC- 



-CH 



^W 



I 

 H 



Fig. 15-5. Pyrrole. 



methene bridges (— CH=). Fifth, pyrrole ring I carries a vinyl group 

 (— CH^CHo). Sixth, attached to pyrrole ring III is a homocylic ring 

 bearing a carbonyl group and a carboxyl esterified with methanol. Sev- 

 enth, pyrrole ring IV carries a propionic acid side chain esterified with 

 phytol; phytol is a higher alcohol (C20II39OH) with a double bond be- 

 tween its a and (3 carbons. The phytyl group gives chlorophyll its solu- 

 bility in fat solvents. Eighth, chlorophyll is optically active because 

 of the presence of asymmetric carbon atoms. In chlorophyll b the — CH3 

 of pyrrole ring II is replaced by — CHO. Protochlorophyll is like chloro- 

 phyll a but is dehydrogenated at carbon atoms 7 and 8. 



Primary photochemiral reaction 



Photosynthesis consists, not of a single reaction, but of a sequence of 

 reactions. Much interest attaches to the first reaction involving light, 

 or the primary photochemical reaction. Photosynthesis was described 

 as a sensitized, photochemical oxidation-reduction, and water was desig- 

 nated as the reductant. Water is not normally an effective reducing 

 agent ; somehow, energy must be added to it to make it effective. Much 

 evidence suggests that the primary photochemical reaction is a photolysis 

 of water, a reaction in which light energy is used for the splitting of water 



