Purdue professor identifies proton pathway in photosynthesis

The cristae increase the surface area for packing in many electron transport
systems and ATPase molecules - the mitochondria is specialised for electron transport and uses oxygen to drive the ETS and
uses the resultant proton gradient to make ATP for the whole eukaryotic cell.

Quinone-DependentProton Transfer Pathways in the Photosynthetic Cytochrome Complex
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All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight.


Team identifies proton pathway in photosynthesis - …

C. Photosynthesis
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By the middle of the 20th century, most of the steps of photosynthetic electron transport had been worked out. A persistent question was how electron transport resulted in the phosphorylation of ADP, yielding ATP. The answer was provided by the chemiosmotic hypothesis formulated by Peter Mitchell in 1961. It states that electron transport serves to generate an H+ gradient across the chloroplast thylakoid and mitochondrial inner membranes. The gradient is then used to make ATP. Mitchell received the Nobel Prize for his work in 1978.


LabBench Activity Plant Pigments and Photosynthesis

Although a mechanism for energy storage involving transferof protons across biological membranes was the subject of the 1978 Nobel Prizein chemistry and advances had been made in its understanding, the amino acidsinvolved and how they are connected for proton transfer in the photosyntheticprotein complex was unknown, Cramer said.

Cellular Respiration Animation - Sumanas, Inc.

Understanding details of the process of photosynthesisaids work toward the development of artificial photosynthesis, which couldallow for the conversion of solar energy into alternative environmentallyfriendly sources of biofuels.

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The light-dependent reactions starts within Photosystem II. When the excited electron reaches the special chlorophyll molecule at the reaction centre of Photosystem II it is passed on to the chain of electron carriers. This chain of electron carriers is found within the thylakoid membrane. As this excited electron passes from one carrier to the next it releases energy. This energy is used to pump protons (hydrogen ions) across the thylakoid membrane and into the space within the thylakoids. This forms a proton gradient. The protons can travel back across the membrane, down the concentration gradient, however to do so they must pass through ATP synthase. ATP synthase is located in the thylakoid membrane and it uses the energy released from the movement of protons down their concentration gradient to synthesise ATP from ADP and inorganic phosphate. The synthesis of ATP in this manner is called non-cyclic photophosphorylation (uses the energy of excited electrons from photosystem II) .

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If the light intensity is not a limiting factor, there will usually be a shortage of NADP+ as NADPH accumulates within the stroma (see light independent reaction). NADP+ is needed for the normal flow of electrons in the thylakoid membranes as it is the final electron acceptor. If NADP+ is not available then the normal flow of electrons is inhibited. However, there is an alternative pathway for ATP production in this case and it is called cyclic photophosphorylation. It begins with Photosystem I absorbing light and becoming photoactivated. The excited electrons from Photosystem I are then passed on to a chain of electron carriers between Photosystem I and II. These electrons travel along the chain of carriers back to Photosystem I and as they do so they cause the pumping of protons across the thylakoid membrane and therefore create a proton gradient. As explained previously, the protons move back across the thylakoid membrane through ATP synthase and as they do so, ATP is produced. Therefore, ATP can be produced even when there is a shortage of NADP+.