Chapter 9 Cellular Respiration and Fermentation

Chemiosmosis the production of atp via a proton

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Chemiosmosis- the production of ATP via a proton gradient. Mitchell's hypothesis was confirmed- a mitochondrial enzyme can use a proton gradient to synthesize ATP. Chemiosmosis is like a hydroelectric dam, where the movement of water makes turbines spin and generate electricity. The electron transport chain is analogous to a series of gigantic pumps that force water up and behind the dam, and ATP synthase is like the turbine inside the dam. When protons move through ATP synthase, the protein spins and generates ATP. How is the Electron Transport Chain Organized? The component of the elctron transport chain are organized into four large complexes of proteins and cofactors. Two of the complexes pump protons. Q and the protein cytochrome c act as shuttles that
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transfer electrons between complexes. Q also carries a proton across the membrane along with an electron. The protons held by Q are released to the intermembrane space. Q shuttles electrons and protons from one side of the membrane to the other. The electrons proceed down the transport chain and the protons released to the intermembrane space contribute to the proton-motive force. The Discovery of ATP Synthase The stalk-and -knob component of the protein was an enzyme that both hydrolyzes and synthesizes ATP. The ATPase knob component is called the F 1 unit; the membrane-bound, proton-transporting base component is the F 0 unit. The F 1 and F 0 units are connected by a rotor ,which spins the F 1 unit, and a stator, which interacts with the spinning F 1 unit. The entire complex is known as ATP synthase. Flow of protons through the F 0 unit causes the rotor connecting the two subunits to spin. As F 1 units rotate along the rotor, its subunits are thought to change conformation in a way that catalyzes the phosphorylation of ADP to ATP. Organisms Use a Diversity of Electron Acceptors Proton pumping during electron transport creates the proton-motive force that drives ATP synthesis. About 29 ATP molecules are produced from each molecule of glucose. Of these, 25 are produced by ATP synthase. Vast majority of payoff from the oxidation of glucose occurs via oxidative phosphorylation. Species that depend on oxygen as an electron acceptor for the ETC use aerobic respiration and are called aerobic organisms. Cells that depend on electron acceptors other than oxygen are said to use anaerobic respiration. Cells that don’t use oxygen as an electron acceptor cant generate such a large potential energy difference. As a result, they make less ATP than cells that use aerobic respiration. Fermentation is a metabolic pathway that regenerates NAD + from stockpiles of NADH. Allows glycolysis to continue producing ATP in the absence of the electron acceptor required by the ETC.
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