The redox Z-scheme is a graph showing the change in redox potentials (in mV) through the photosynthetic reactions. The excitations from light on P680 in photosystem II and P700 in photosystem I are the two excitation events, with the oxidation processes being shown as losses in potential. In photosystem II, the redox potential increases on excitation of P680 to P680*, then decreases as the energy flows through chlorophyll A, pheophytin A, plastoquinone A, plastoquinone B, and then remains constant to the plastoquinone pool. Plastoquinone is converted to plastocyanin by cytochrome b6f, passing its redox potential on to P700. P700 is excited by light, to P700*, and then the energy flows through chlorophyll A1, chlorophyll A2, phylloquinone A, the iron-sulfur cluster, before ultimately reaching ferredoxin.
After the excitation of P700 to P700*, the most energetically favourable reaction would be a direct charge recombination event, returning back to P700 via chlorophyll A1. This does not happen, due to the large energy change required - larger changes are generally less favourable than small ones. This is stated by Marcus theory, where a greater $$-\Delta\text{G}$$ results in a slower reaction rate. Therefore, the smaller energy step in the electron transfer from chlorophyll A1 to chlorophyll A2 is considerably faster, and thus favoured by energetics. The Marcus inverted region endorses this observation, where the reorganisation energy becomes greater the larger the $$\Delta\text{G}$$.