Photosystem II

Proton pump

Photosystem I

NADP reductase e

Figure 6.27 The Tandem Photosystems of Cyanobacteria and Chloroplasts Radiant energy captured by photosynthetic pigments excites the reaction-center chlorophyll, causing it to emit a high-energy electron, which is then passed to an electron transport chain. In cyclic photophosphorylation, electrons emitted by photosystem I are returned to that photosystem; the path of the electrons is shown in green arrows. In non-cyclic photophosphorylation, the electrons used to replenish photosystem I are donated by radiant energy-excited photosystem II; the path of these electrons is shown in orange arrows. In turn, photosystem II replenishes its own electrons by stripping them from water, producing O2.

energy by the two photosystems allows the process to raise the energy level of electrons stripped from water high enough to be used to generate a proton motive force and produce reducing power.

First we will consider the simplest situation, which occurs when the cell needs to synthesize ATP but not reducing power (NADPH). To accomplish this, only photosystem I is used. Radiant energy is absorbed by this photosystem, exciting the reaction-center chlorophylls, which causes them to emit high-energy electrons. The electrons are then passed to an electron carrier, which transports them to a proton pump; this pump is analogous to complex III in the respiratory chain of mitochondria. After being used to pump protons across the membrane, thus generating a proton motive force, the electrons are returned to photosystem I. As occurs in oxidative phosphorylation, ATP synthase harvests the energy of the proton motive force to synthesize ATP. This overall process is called cyclic photophosphorylation because the molecule that serves as the electron donor, reaction-center chlorophyll, also serves as the terminal electron acceptor; the electrons have followed a cyclical path.

When cells must produce both ATP and reducing power, non-cyclic photophosphorylation is used. In this process, the electrons emitted by photosystem I are not passed to the proton pump, but instead are donated to NADP+ to produce NADPH.

While this action provides reducing power, the cell must now replenish the electrons emitted by reaction center chlorophyll from another source. In addition, the cell must still generate a proton motive force in order to synthesize ATP. Photosystem II plays a pivotal role in this process. When photosystem II absorbs radiant energy, the reaction-center chlorophylls emit high-energy electrons that can be donated to photosystem I. First, however, the electrons are passed to the proton pump, which uses some of their energy to establish the proton motive force. In order to replenish the electrons emitted from photosystem II, an enzyme within that complex extracts the electrons from water, donating them to the reaction-center chlorophyll. Removal of electrons from two molecules of water generates O2. In essence, photosystem II strips the electrons from water molecules and captures the energy of light to raise the energy of those electrons to a high enough level that they can be used to power photophosphorylation. Photosystem I then accepts those electrons, which still retain some residual energy, and again captures the energy of light to boost the energy of the electrons to an even higher level so they can be used to reduce NADPH.

Light-Dependent Reactions in Purple and Green Bacteria

Purple and green bacteria employ only a single photosystem and are unable to use water as an electron donor for reducing power. This is why they are anoxygenic, or do not evolve O2. Molecules used as electron donors by purple and green bacteria include hydrogen gas (H2), hydrogen sulfide (H2S), and organic compounds.

Purple sulfur bacteria use a photosystem similar to the photosystem II of cyanobacteria and eukaryotes. The electrons emitted from the photosystem are passed along an electron transport chain, fueling the formation of a proton motive force, which is then harvested to synthesize ATP. However, the photosystem does not raise the electrons to a high enough energy level to reduce NAD+ (or NADP+), so the purple sulfur bacteria must use an alternative mechanism to generate reducing power. To do this they use a process called reversed electron transport, which uses ATP to run the electron transport chain in the reverse direction, or "uphill."

Green bacteria employ a photosystem similar to photosystem I. The electrons emitted from this photosystem can be used to either generate a proton motive force or reduce NAD+.

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