Reaction centers (RCs) of photosystems I and II (commonly referred to as PSI and PSII) share many structural similarities . They both are formed by two protein subunits and associated photosynthetic co-factors. The latter have nearly-C 2 symmetric spacial arrangements and, in the case of PSII, include (but are not limited to) the pairs of i) inner chlorophyll a (Chl a) co-factors, ii) accessory Chl a pigments, iii) pheophytin a (Phe a) molecules, and iv) plastoquinones. The first three pairs are labeled as P Dn , Chl Dn , and Phe Dn , where Dn is the protein subunit they belong to, i.e., D1 or D2. The plastoquinones are often denoted as Q A and Q B . In PSI, the inner pair of co-factors is formed by Chl a and its C-13 2 -epimer (Chl a ), which are called P B and P A , respectively. Similar to the case of PSII, the pair of accessory PSI pigments is composed of Chl a molecules (denoted as A -1A and A -1B ), whereas pheophytins and plastoquinones are replaced by Chl a cofactors (A 0A and A 0B ) and phylloquinones (A 1A and A 1B ), respectively. In both PSI and PSII, magnesium ions of inner-pair Chls are coordinated by histidine (His) amino-acid residues, whereas those of accessory pigments are coordinated by water molecules. Despite these structural similarities, the redox potentials of the primary electron donors P700 in PSI and P680 in PSII (the numbers within their names refer to the absorption maxima) differ by about 0.7 V and are equal to +0.5 V and +1.2 V, respectively . This makes the donors P700 and P680 the strongest reducing and oxidation agents, respectively, known in living matter. These electron donors are composed of different co-factors : P700 consist of the inner pair Chls, while P680 is a monomeric accessory Chl a. Additionally, it is known that after photo-excitation PSII performs one-sided electron transfer (ET) along the protein subunit D1 , while a two-sided ET is observed in PSI . These facts pose the question, which factors cause the difference in functional symmetry break.