Photosystem II (PS II) is the first protein in the electron transport chain (ETC) of oxygenic photosynthesis light-dependent reactions in algae, plants, and cyanobacteria. PS II is found in the thylakoid membranes of these organisms and it catalyzes the most thermodynamically demanding reaction in biology — splitting water into dioxygen and reducing equivalents. It requires two inputs in order to carry out this


This image of the Light Harvesting protein shows the association it has with chlorophyll, xanthophyll and cratenoids, on the upper right complex.

reaction: water and energy in the form of light. When light hits Photosystem II it gets absorbed by the antenna complexes that suround the protein. These proteins contain ligands of chlorophyll a, Xanthophylls, and cratanoids. Once photon energy is absorbed by these pigments it transfers the energy via resonant energy transfer. As the energy is passed from one molecule to the next some of it it lost to the environment as heat, however, this is not detrimental to the system as each different pigment in the complex will absorb a lower amount of energy and continue the transfer to the next. Ultimately, this resonant energy is finally transferred to a pair of chlorophyll molecules called P680 (referring to the frequency of light energy in which they respond to). These molecules are at the heart of the reaction center of photosystem II. As these molecules get excited they donate a pair of electrons to a molecule of pheophytin embedded in the complex. Pheophytin is now a negatively charged radical molecule and readily reduces a second molecule called Plastoquinone. Plastoquinone will sequester two electrons that it received from pheophytin before it transfers them to the electron transport chain and pumps
Full system

PS II along with the other proteins involved in the electron transport chain of the light-dependent reaction of photosynthesis

two protons from the thylakoid stroma to the thylakoid lumen. The two P680 molecules are lacking a pair of electrons and in order to be restored to their original state they must gain a pair of electrons. The Oxygen Evolving Complex (OEC) is responsible for splitting water into two H+ ions, elemental oxygen, and two electrons per molecule of water. The electrons will go back on to the P680 molecules and the cycle will start over agian with each incoming photon.

Importance of Photosystem II

This process is a crucial part of the first step in the ECT. It helps to set up the proton gradient that is used to drive an ATP synthase protein and provides electrons that are harnessed to pump more H+ ions to increase the gradient. The electrons ultimately end up reducing a molecule NADP+ to NADPH, which will then be used in the carbon-fixing Calvin cycle on the chloroplast.

  • A Molecule of Chlorophyyl a. Notice the central Mg2+ ion in the center of the chlorin ring.
  • PLastoquinone before it gets reduced
  • PHeophytin is essentially a Molecule of Chlorophyll a with out the Mg2+ ion
  • The Oxygen Evolving Complex (Mn4CaO5). Also called the Manganese center
Essential ligands

Here are the essential Ligans and molecule that are involved in PS II. The red sphere is a atom in Iron 2+


  1. Rogelio: Photosystem II: Introduction
  2. Rogelio: Photosystem II: Biological function
  3. Rogelio: Photosystem II: Biosynthesis
  4. Rogelio: Photosystem II: Gene sequence
  5. Rogelio: Photosystem II: Amino acid sequence and composition
  6. Rogelio: Photosystem II: Secondary and tertiary structure
  7. Rogelio: Photosystem II: Domains and structural motifs
  8. Rogelio: Photosystem II: Interactions with macromolecules and small molecules
  9. Rogelio: Photosystem II: Molecular biodiversity and evolution
  10. Rogelio: Photosystem II: Literature overview
  11. Rogelio: Photosystem II: Useful online resources