Catalysts in the chloroplasts of photosynthesizing plants help split water by binding water molecules and separating protons and electrons.

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The process of photosynthesis in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce organic compounds and oxygen. Early in the process, molecules of chlorophyll pigment are excited by solar energy and donate their electrons to start a flow of energized electrons that play a key role in the photosynthetic process (see the related strategy).

The chlorophyll’s donated electrons need to be replaced, and these electrons come from the splitting of water. In a process called photolysis (‘light’ and ‘split’), light energy and catalysts interact to drive the splitting of water molecules into protons (H+), electrons, and oxygen gas. The electrons go to the chlorophyll, the protons contribute to a proton gradient that is used to power synthesis of the energy-carrying molecule, ATP, and the oxygen is a byproduct.

The enzyme complex that catalyzes the water-splitting reaction (known as the oxygen-evolving complex) contains manganese and calcium, and is located in photosystems embedded in thylakoid membranes within the chloroplast. Researchers are still uncovering details about the exact mechanism by which the enzyme works, but it appears that the enzyme binds water molecules in place while separating the protons and electrons and forming oxygen bonds.

Many researchers are synthesizing various bio-inspired catalysts in the hopes of developing efficient and cost-effective means to generate alternative forms of energy (for example, hydrogen fuel) from the splitting of water.

Check out these related strategies on the rest of the photosynthetic process:
Pigment molecules absorb and transfer solar energy: Cooke’s koki’o
Photosynthesis converts solar energy into chemical energy: plants
Photosynthesis makes useful organic compounds out of CO2: plants

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“To replicate one of the important steps in natural photosynthesis, Brookhaven chemists James Muckerman and Dmitry Polyansky have turned to molecular complexes containing metals such as ruthenium that can drive the conversion of water into oxygen, protons, and electrons. These ruthenium catalysts hold water molecules in place to make oxygen bonds while the protons and electrons are transferred among the molecules and the catalyst, providing the charges necessary to continue the photosynthesis process.” (ScienceDaily 2007)

Web page
Artificial Photosynthesis: Inspired By Nature, Scientists Explore Pathways To Clean, Renewable Solar FuelScienceDailyMarch 28, 2007
DOE/Brookhaven National Laboratory

Journal article
Artificial photosynthesis: water cleavage into hydrogen and oxygen by visible lightAcc. Chem. Res.January 1, 1981
Graetzel M

“Photosynthetic water oxidation, where water is oxidized to dioxygen, is a fundamental chemical reaction that sustains the biosphere. This reaction is catalyzed by a Mn4Ca complex in the photosystem II (PS II) oxygen-evolving complex (OEC): a multiprotein assembly embedded in the thylakoid membranes of green plants, cyanobacteria, and algae.” (Pushkar et al. 2008:1879)

Journal article
Structural changes in the Mn4Ca cluster and the mechanism of photosynthetic water splittingProc. Natl. Acad. Sci. USAJanuary 1, 2008
Pushkar Y; Yano J; Sauer K; Boussac A; Yachandra VK

“Photosynthesis uses light energy to drive the oxidation of water at an oxygen-evolving catalytic site within photosystem II (PSII)…[In this study] we are able to describe details of the binding sites for cofactors and propose a structure of the oxygen-evolving center (OEC). The data strongly suggest that the OEC contains a cubane-like Mn3CaO4 cluster linked to a fourth Mn by a mono-μ-oxo bridge.” (Ferreira et al. 2004:1831)

Journal article
Architecture of the Photosynthetic Oxygen-Evolving CenterScienceJanuary 1, 2004
Ferreira K; Iverson TM; Maghlaoui K; Barber J; Iwata S

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