The mechanism of natural photosynthesis can conveniently be divided into several distinct steps : i) light harvesting in antenna proteins, ii) a fast primary electron-transfer event in the reaction center (RC), iii) protein stabilization due to charge separation, and iv) subsequent steps involving the synthesis of new chemical compounds. In the first step, the photosynthetic system does not undergo any chemical transformations and simply transfers the absorbed energy to the RC. The RC contains (bacterio)chlorophyll [(B)Chl] molecule(s), which constitute the primary electron donor. For the cases of green plants, algae, and cyanobacteria, two primary electron donors are known: P700 in photosystem I (PSI) and P680 in photosystem II (PSII) . Bacterial reaction centers (BRCs) containing the electron donors P870 and P960 can be found in Rhodobacter sphaeroides and Rhodopseudomonas viridis, respectively . Note that the numbers correspond to the absorption maxima of those complexes. The primary electron donors of PSI and the BRC are pairs of inner (B)Chl a , which are called the special pair (SP) in cases of BRCs (for an overview on RC structures, see Sec. 2). In case of PSII, the primary electron donor is composed of an accessory Chl a co-factor . Being excited by the delivered energy, these primary electron donors donate an electron forming short-lived radical pairs with neighboring co-factors (for more detail, see Refs. ). In the third step, a chain of secondary electron-transfer reactions is initiated preventing the recombination of radical pairs back to neutral compounds and stabilizing the separation of charges. Thus, the absorbed energy is converted into electro-chemical potential, which is further used for synthesis of chemical compounds.