Out of thin air

Out of thin air - Vol 445|8 February 2007 NEWS VIEWS...

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The oxygen that gives us the breath of life is renewed by sunlight falling on plants, algae and a particular class of bacterium called cyanobacteria — all of which produce mol- ecular oxygen (O 2 ) as a waste product of photosyn±thesis±(Box±1) 1 . Biologists agree that cyanobacteria invented the art of making oxygen 2 , but when and how this came about remain uncertain. Oxygenic photosynthesis involves about 100 proteins that are highly ordered within the photosynthetic membranes of the cell. The main players are two molecular machines, photosystem I and photosystem II, that act as electrochemical solar cells. With the help of chlorophyll (the pigment that makes plants green), they transform sunlight into electrical current (Fig. 1). Photosystem II generates an electrochemical potential of + 1.1 volts, enough to remove two electrons from each of two water molecules, making a molecule of O 2 at a cost of four photons — one for each electron moved. Photosystem II performs this remarkable feat only when photosystem I is present to dispose of the electrons. Photosystem I grabs the four electrons and uses four more photons to deposit them, in two pairs, on an electron carrier called NADP + . NADP + ultimately transfers the elec- trons to carbon dioxide, thereby providing the EVOLUTIONARY BIOLOGY Out of thin air John F. Allen and William Martin The invention of oxygenic photosynthesis was a small step for a bacterium, but a giant leap for biology and geochemistry. So when and how did cells first learn to split water to make oxygen gas? –1.4 –1.2 –0.8 –0.4 0.0 Standard redox potential (volts) 0.4 0.8 2H 2 O O 2 +4H + ADP Electron transport chain Electron transport chain NADPH Photosystem II ATP Chlorophyll Chlorophyll Light Light 4e 1.2 Photosystem I Figure 1 | The two photosystems in photosynthesis. Photosystems I and II absorb light energy, convert it into electrochemical potential, and are connected in series electrically. These two ‘light reactions’ of photosynthesis form links in a chain of electron (e ǁ ) transfers that is coupled, by means of proton pumping, to synthesis of the energy-storage molecule adenosine triphosphate (ATP). The electron transport chain of photosynthesis, also known as the Hill and Bendall Z-scheme, ends with photosystem I delivering electrons to NADP + , making NADPH. ATP and NADPH drive the ‘dark reactions’ that transfer the electrons to CO 2 so as to provide the energy to make sugars and the other molecules of life. The chain begins when water is oxidized to oxygen by the very high electrochemical potential of photosystem II. The catalyst of water oxidation (Box 3) is shown here as a cluster of four red spheres and one yellow one. In 1772, Joseph Priestley
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This note was uploaded on 09/06/2009 for the course BIS 103 taught by Professor Abel during the Spring '08 term at UC Davis.

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Out of thin air - Vol 445|8 February 2007 NEWS VIEWS...

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