Environmental Biotechnology - Theory and Application

Microbes and Metabolism 39

Figure 2.10 Oxygenic photosynthesis, photorespiration and the Hatch–Slack pathway

to NADPH. Both the cyclic and the noncyclic pathways produce a proton gradi-
ent which drives the synthesis of ATP, but only the cyclic route has the facility
of producing NADPH. A combination of cyclic and noncyclic pathways are used
by the organism to produce the required amounts of NADPH and ATP used in
the dark reactions for the synthesis of carbohydrate. So far the description has
been of photosynthesis in eukaryotes and cyanobacteria.

Photosystems in purple and green bacteria

Looking at the general equation for the chemical reactions in photosynthesis
shown in Figure 2.11, it may be seen that water is the electron donor dur-
ing oxygenic photosynthesis while several molecules may play the same role
in anoxygenic systems. Suitable molecules are listed in the figure, from which
several interesting observations may be made. If the electron donor is hydrogen