Figure 1. The photosystem as it appears in purple bacteria carrying out anoxygenic photosynthesis. Light is harvested and the excitation energy is channeled to the reaction center in which electrons (red) are released. They are pushed through a cycle of carriers and, driven by this cycle, protons (yellow) are translocated from inside to outside. The electrons return to the reaction center (cyclic electron flow). The resulting proton gradient is then used for ATP synthesis. The driving force is the high redox energy of the electrons released, which is lost stepwise in the cycle.

Figure 2. Oxygenic photosynthesis requires the interplay of two photosystems. PS I generates electrons (red) that travel through the carrier cycle but are also used to provide reducing power for the reduction of CO ₂. This results in a lack of electrons that no longer reach the reaction center of PS I. The pool of electrons is replenished by the action of PS II. Ultimately, these electrons come from water, which is cleaved by a manganoprotein complex. As mentioned in Figure 1, the redox energy is important. At the reaction center of PS II, it is so high that water can be oxidized to oxygen and the electrons are pulled all the way through the carrier system to reduce NADP ⁺.