For the early photophosphorylation systems no oxygen was generated. These reactions evolved in anaerobic environments, there was very little molecular oxygen available. Two sets of reactions evolved under these conditions, both directly from anaerobic respiratory chains. These are known as the
light reactions because they require the activation of an electron (an excited electron)from the absorption of light energy by bacteriochlorophyll. The light reactions are categorized either as
cyclic or as
noncyclic photophosphorylation. To help you better understand the similarities of photophosphorylation to respiration, figure 9 below is an electron tower that will be useful in our discussion of photosphosphorylation.
Electron tower that has a variety of common photophosphorylation components. PSI and PSII refer to Photosystems I and II of the oxygenic photophosphorylation pathways. For the examples in Figure 8 and Figure 9 P840 is similar in reduction potential as is PSI.
Cyclic photophosphorylation
In cyclic photophosphorylation the bacteriochlorophyll
red molecule absorbs enough light energy to energize and eject an electron forming bacteriochlorophyll
ox . The electron reduces a carrier molecule in the reaction center which in turn reduces a series of carriers via red/ox reactions. These carriers are the same carriers found in respiration. If the change in reduction potential from the various red/ox reactions are sufficiently large, protons, H
+ are translocated across the membrane. Eventually the electron is used to reduce bacteriochlorophyll
ox and the whole process can start again. This is called cyclic photophosphorylation because the electrons make a complete circuit: bacteriochlorophyll is the source of electrons and is the final electron acceptor. ATP is produced via the
F
1 F
0 ATPase . The schematic in figure 10 below demonstrates how cyclic photophosphorylation works.
Cyclic Photophosphorylation. The reaction center P840 absorbs light energy and becomes excited, denoted with an *. The excited electron is ejected and used to reduce an FeS protein leaving an oxidized reaction center. The electron its transferred to a quinone, then to a series of cytochromes which in term is reduces the P840 reaction center. The process is cyclical. Note the gray array coming from the FeS protein going to a ferridoxin (Fd), also in gray. This represents an alternative pathway the electron can take and will be discussed below in non-cyclic photophosphorylation.
NOTE the same electron that leaves the P480 reaction center is not necessarily the same electron that eventually finds its way back to reduce the oxidized P840.
Non-cyclic photophosphorylation
In cyclic photophosphorylation electrons cycle from bacteriochlorophy (or chlorophyll) to a series of electron carriers and eventually back to bacteriochlorophyll (or chlorophyll): there is no loss of electrons, they stay in the system. In non-cyclic photophosphorylation the electrons are removed from the system, they eventually end up on NADPH. That means there needs to be a source of electrons, a source that has a higher reduction potential than bacteriochlorophyll (or chlorophyll) that can donate electrons to bacteriochlorophyll
ox to reduce it. An electron tower is proved below so you can see what compounds can be used to reduce the oxidized form of bacteriochlorophyll. The second requirement, is that when bacteriochlorophyll becomes oxidized and the electron is ejected it must reduce a carrier that has a lower (more negative) reduction potential than NADP/NADPH (see the electron tower). In this case, electrons can flow from energized bacteriochlorophyll to NADP forming NADPH and oxidized bacteriochlorophyll. Electrons are lost from the system and end up on NADPH, to complete the circuit bacteriochlorophyll
ox is reduced by an external electron donor, such as H
2 S or elemental S
0 . This is diagrammed in figure 11 below.
Non-cyclic photophosphorylation. In this example, the P840 reaction center absorbs light energy and becomes energized, the emitted electron reduced a FeS protein and in turn reduces ferridoxin. Reduced ferridoxin (Fd
red ) can now reduce NADP to form NADPH. The electrons are now removed from the system, finding their way to NADPH. The electrons need to be replaced on P840, which requires an external electron donor. In this case, H
2 S serves as the electron donor.
