Redox Systems in the Primary Processes of Photosynthesis

A whole series of different chlorophylls is known, called chlorophyll a, b, c, etc. The basic structure of all of these chlorophylls is a porphyrin system. This porphyrin system is formed from 4 pyrrole rings, which are linked together by methylene groups to form a ring system. A sequence of 

The absorption of light quanta from the blue and red regions of the spectrum causes a transition of the chlorophyll molecule to the excited state and, simultaneously, an energy-rich electron is ejected from the chlorophyll molecule. Thus, the chlorophyll-molecule becomes ionized. The electron can be accepted by certain acceptors (Fig. 28). 

Cytochromes are formally closely related to the chlorophylls. They also possess the porphyrin skeleton which, in their case, complexes iron, not Mg++ (Fig. 29). A whole series of other biologically important substances such as the peroxidases, catalases, and the red blood pigment hemin also contain these same structural components (iron + porphyrin). All of these substances, including the cytochromes, are known collectively as cell hemins. In the living cell the iron-containing porphyrin system is linked to protein. Thus, the cell hemins occur as proteids. 

A series of different cytochromes is known, which can be further subdivided into the groups a, b, and c. Cytochromes of the b and c types are primarily of importance in photosynthesis. 

All cytochromes are redox systems for the reason that the central iron atom can oscillate between the divalent and trivalent states. Electron emission causes it to shift from the divalent to the trivalent state and electron uptake causes a shift in the opposite direction. 

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