Photolysis of water

By | October 12, 2021
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  • Plants and some other organisms like Algae and Cyanobacteria are involved in a process of Photosynthesis, which is a multi step process involving many reactions.
  • In Photosynthesis, inorganic substances such as Carbon di oxide, water are used and convert them to organic molecules ( carbohydrates / food) and release oxygen in presence of sunlight.
  • During photosynthesis, relatively low-energy electrons are removed from a donor compound and converted into high-energy electrons using the energy absorbed from light.
  • In the beginning of the photosynthesis, special chlorophyll – a dimer in reaction center or Photosystem II, are excited by solar energy and donate their electrons to begin a flow of energized electrons that plays a vital role in photosynthetic process.
  • The lost electrons from the PSII needs to be replaced so that they can start a fresh round of excitation electrons upon receiving solar energy in the form of photons, to begin a flow of electrons to form ATP and NADPH.
  • These electrons comes from splitting of water.
  • This process is called Photolysis of water ( Photo- light , lysis – break or split) where light energy and catalysts interact to help in the splitting of water molecules into protons (H+), electrons, and oxygen gas.
  • All the three components formed upon splitting of water are crucial for photosynthesis process. Electrons are supplied to chlorophyll molecule in PS II to replenish its lost electrons, H+ contribute to the proton gradient which is essential for ATP generation. Oxygen is byproduct and absolute essential for aerobic respiration and survival of all the organisms including plants.
  • In Algae under some conditions ( anaerobic conditions ), Photolysis of water will also produce small amount of hydrogen gas, in addition to usual oxygen gas.

Enzyme catalyzing The reaction – splitting of water

  • The enzyme which catalyzes this reaction – Photolysis or splitting of water is known as Photosystem II (PSII).
  • PSII is a pigment binding multi protein subunit protein complex found in thylakoid membrane of chloroplast.
  • The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the cofactor of the photosystem II enzyme.
  • Photosystem II (PSII) along with its coenzyme – OEC, fuels the oxidation of water important for oxygenic photosynthesis. The active site of water oxidation is the oxygen-evolving complex (OEC), a Mn4CaO5 cluster.
  • Manganese was apparently evolutionarily selected for this role because of its ability to exist in multiple oxidation states (Mn2+, Mn3+, Mn4+, Mn5+) and to form strong bonds with oxygen-containing species.
  • To produce one O2 molecule, electrons extracted from two water molecules along with four protons are required.
  • The molecular machinery for splitting water needs to store energy from the first three photons before the fourth provides enough energy required for oxidation of water. 
  • Later, the OEC transfers four electrons, to photosystem II via a tyrosine residue in the reaction center.
  • In 1970 Bessel Kok proposed a model for this complex – the complex can exist in five states: S to S4, S4 being the most oxidized and S the most reduced.
  • Photosystem II which absorbs the photons, move the system from state S to S4.
  • S4 is unstable and reacts with water producing free oxygen.
  • For the complex to reset to the lowest state, S, it uses two water molecules to pull out four electrons.
  • The exact mechanism underlying the photolysis is not clear known. However the understanding has improved significantly after the deduction of structural models of the biological oxygen-evolving complex
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PS II with OEC

Importance of Photosystem II enzyme

  • With the emergence of Photosystem II enzyme some 3 millions years ago, has dramatically altered the chemical composition of our planet.
  • The ability of this enzyme to split water using solar energy is remarkable, as this reaction is thermodynamically and chemically very demanding.
  • In lab conditions, splitting of water requires temperatures close to 2000oC as water is very stable molecule held tightly by hydrogen and oxygen atoms.
  • However plants are able to split water even on a snowy mountain by using just visible light.
  • PSII converts CO2 into organic molecules important for life and also produce oxygen, which eventually transformed our atmosphere from an anaerobic to aerobic state, leading to explosion in biological activity on our planet.
  • PS II and OEC are indispensable for all life forms on the Earth.
  • If photosynthesis were to cease, all higher forms of life would be extinct in about 25 years.
  • This is termed as Photosynthetic catastrophe . A milder version of this which occurred 65.1 millions ago due to a large asteroid struck the Yucatan Peninsula of Mexico resulted in ending Cretaceous period. Due to this asteroid attack, photosynthesis capacity was greatly diminished, which could have been one of the reasons behind disappearance of the dinosaurs from earth.

Experiment proving the role of water in the formation of molecular oxygen

  • In Photosynthesis, CO2 is reduced in presence of water and sunlight to give carbohydrates and release Oxygen.
  • 6 CO2 + 12 H2O in presence of light gives C6H12O6 + 6 H2O + 6 O2
  • Initially it was thought that the molecule of oxygen is derived from CO2, but it was experiment performed by Samuel Ruben and Martin Kamen in 1941, which showed beyond doubt, the role of water in releasing oxygen during photosynthesis.
  • In their experiments on suspension of green algae, they used specially labeled isotope (radioactive) of oxygen, 18O, as a replacement for the common isotope, 16O.
  • One sample of algae was exposed to radioactive or labeled C[18O2] and unlabeled water.
  • The other sample was exposed to unlabeled carbon dioxide and labeled H2[18O].
  • Then they wanted to see – which sample will release labelled 18O .
  • The algae given labeled water produced labeled oxygen – indicating that O2 produced during photosynthesis is derived from H2O.
  • The algae given labeled carbon dioxide produced unlabeled oxygen, confirming that O2 was not being produced by a chemical splitting of CO2.
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Image Credit : Wikipedia

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