Note, in this paper read "D" as delta.
To understand photosynthesis and respiration, you have to see the relationship between moving electrons through a potential difference (volts) and a free energy change (kilocalories or kiloJoules). Generally the "high energy" end of an electron transport chain will be the cofactor NADH or NADPH. The point of photosynthesis is to promote electrons to higher energy so that these can be made. Once a cell is supplied with NADPH, it can then run reactions which "stick together" molecules using the electrons of the hydride ion on this cofactor. Conversely, respiration is a process which starts with NADH/NADPH and runs the electrons down to a low-energy acceptor. Like water running downhill, this provides energy to drive various processes including ATP synthesis. A cell provided with both NADH and ATP can do anything it "wants," chemically speaking. In the most familiar examples, green plants use H2O as an electron donor, producing NADH and O2, and mitochondria take electrons from NADH and "run them downstream" to oxygen as the electron acceptor, producing water. Anaerobes can't tolerate oxygen and thus use older and simpler electron transport chains.
DGo' = - nFDEo'
Photosynthesis of the sort found in chloroplasts or Cyanobacteria runs this same process in reverse. Obviously, reversing a "spontaneous" process means that the process has to go "uphill" and would be impossible without addition of outside energy. The outside energy comes from light, or photons. A visible light photon at the frequencies absorbed by chlorophyll has an energy of 1.82 electron volts. If life processes were perfectly efficient this would be enough to promote an electron the necessary -1.14 volts, but in actuality this can only be done by the successive absorption of two photons. In other words, water is "too low" in energy to be utilized as an electron donor by the photosystem which reduces NADP+ to NADPH, Photosystem I.
It is thought-provoking, from an evolutionary point of view, to realize that Photosystem I must be older than Photosystem II which takes electrons from water. It follows that there must have been organisms that use only Photosystem I, and take electrons from some substance which is higher in energy than water. In fact these organisms are still easy to find, including the Green Sulfur Bacteria and the Green Filamentous Bacteria, which start with H2S or other substances including hydrogen gas. The half reaction reduction potential for S -> H2S is +0.14 volts, so the potential difference for the reaction H2S+ NAD+ -> NADH + S is only -0.46 volts, making photosynthesis much easier (starting with water we needed -1.14 volts). Remember that a negative voltage change is non-spontaneous, just like a positive Free Energy change.
Photosystem II is thus a later addition which enabled photosynthesis to use water, which is much more widely distributed on the Earth than hydrogen sulfide. Photosystem II starts with water as the electron donor. Taking the electrons away involves running them down to a slightly lower energy level than water (about +1.0 volts) and then promoting them 1.8 volts to a level of -0.8 volts. This is much higher than the eventual destination of the electrons in this photosystem, Plastocyanin (a terpenoid molecule which resembles Ubiquinone/Coenzyme Q) at +0.4 volts. The electrons go through a transport chain which amounts to a kind of respiration in the middle of photosynthesis, in other words the energetically favorable transfer from -0.8 V to +0.4 V drives the synthesis of some ATP. Then, from Plastocyanin, the electrons move into the older Photosystem I. The promotion from +0.4 V up to -1.4 V puts the electrons much higher than necessary, and then they again run down an electron transport chain through high energy molecules including the protein Ferredoxin, eventually winding up on NADPH at -0.32 volts.
Mitochondrial Respiration also is revealed as a "recent" process when we realize that the use of molecular oxygen as an electron acceptor would not have been possible during the period between the formation of Earth 4.5 billion years ago, and the appearance of oxygen in the atmosphere roughly 2 billion years ago. Once again, there have to be other ways of doing this, using other molecules as electron acceptors. And once again sulfur comes into play, because Sulfate Respiration is well known in certain anaerobes such as Desulfovibrio. Sulfate (SO4) is reduced to H2S plus several H2O, and the transfer of electrons from high (NADH) to low (Sulfate) provides enough energy to synthesize ATP.