23. Energy Transformations: Respiration and Photosynthesis   Previous PageNext Page
       The Light Reactions: Trapping Solar Energy

Photosynthesis is simplest in bacteria, and the process in green and purple sulfur bacteria is diagrammed opposite. Light energy is absorbed by various antenna molecules and is passed on to a bacteriochlorophyll molecule (BChl right) in the form of electronic excitation. The chlorophyll molecule uses these excited electrons to reduce NAD+ to NADH, passing them first to a flavodoxin (FD, a nonheme iron protein) and then to a flavoprotein (FP). (Bacteria use NADH, even in photosynthesis.) The chlorophyll molecule then is deficient in electrons, but the shortage is compensated for by an external reducing agent such as H2S. The H2S is oxidized first to elemental sulfur and ultimately to sulfate. Protons are released into solution, and electrons are fed into an electron-transport chain that leads to the bacteriochlorophyll molecule. This chain contains cytochromes b and c, and other components such as quinones.

It resembles the electron-transport chain of respiration in this and in another key property: Some of the energy that is released when electrons run down the free energy scale from H2S to chlorophyll is captured and used to synthesize ATP. Thus photosynthetic bacteria obtain two benefits: energy stored as ATP, and energy and reducing power combined in NADH.

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