(A) Arrangement of cofactors involved in the electron-transfer reaction. P700, special pair Chls; Acc, accessory Chl; A0, primary electron acceptor; A1, secondary electron acceptor menaquinone-4; FX, FA, and FB, iron–sulfur clusters. P700 serves as a primary electron donor, and the electron produced is transferred to A1 via A0 (and ACC) in each branch, followed by sequential transfer to FX → FA → FB. However, whether the electrons are transferred through both branches or proceed preferentially through one branch is still under debate. (B–I) The cryo-electron microscopy (cryo-EM) density maps of cofactors and their refined models. (B) Chl a′ in P700; (C) Chl a in P700; (D) Acc; (E) A0; (F) A1; (G–I), FX, FA, and FB. Red arrows indicate structural differences between Chl a and Chl a′ (B, C). The densities for Chls, quinone, and iron–sulfur clusters were depicted at 5 σ, 3 σ, and 15 σ, respectively.