a, Extended schematic of the hydrogen bonding network connecting the OEC to the solvent-exposed surface of PSII and identification of several channels for either possible water movement or proton transfer. Top right, locations of four selected channels in the PSII monomer. b–e, Movements within the water networks across monomers. Coloured spheres are shown for each ordered water or chloride ion across the four metastable states, 0F through 3F, and for both monomers, with the stronger colour matching the first (A) monomer and the lighter colour matching the second (a) monomer. For ordered solvent, residue number is shown; for OEC atoms, the atom identifier is shown; and for the Cl2 site, the Cl− 680 label is shown. b, The O1 water chain. Positional disagreement between monomers is visible especially near waters 77 (2F) and 27 (3F) and is on the same scale as changes between illuminated states, both of which may indicate a more dynamic water channel. c, The O4 water chain. With the notable exception of water 20, most water positions are stable across monomers and illuminated states. Water 20 is highly unstable in position in the two states (0F, 3F) in which it is modelled, and there is not sufficient density in the remaining states to model a water 20 position. d, The Cl1 site water channel with no notable movements. e, The Cl2 site water channel with no notable movements. f, Indication of a split position of W3 in the S 0 state. mF obs − DF calc difference density (green mesh) in the 3F state suggests an alternate position near W3 (W3b in Fig. 4d). g, h, Possible access to W3/Ox side from the Cl1 or the O1 channel. The surface of the protein is shown in grey to visualize the extent of the cavities around the OEC, and Van der Waals radii are indicated for selected residues or atoms by dotted spheres. Shown are two different views for each channel. The direction of the Cl1 channel is indicated by a green arrow and the O1 channel by a pink arrow. Water W2 is shown in purple, W3 in cyan and Ox in orange. Yellow spheres indicate other waters. Mn are shown in magenta, other bridging oxygens as red spheres.