The electrons for photosystem II come from the splitting of water molecules. This process is carried out by the oxygen-evolving complex (OEC) of photosystem II. The OEC is a cluster of four manganese ions and one calcium ion that are held together by a protein scaffold. When light energy is absorbed by photosystem II, it excites an electron in the chlorophyll molecule of the reaction center. This electron is then transferred to an electron acceptor, and it eventually reaches the OEC. The OEC uses the energy of the electron to split a water molecule into two protons and one oxygen atom. The protons are pumped into the lumen of the thylakoid, while the oxygen atom is released into the atmosphere.