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section5_ak - MIT Department of Biology 7.014 Introductory...

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MIT Department of Biology 7.014 Introductory Biology, Spring 2005 Recitation Section 5 February 16-17, 2005 Biochemistry—Photosynthesis and Respiration A. Photosynthesis background 1. Why do we consider O 2 a booster of evolution? The organisms doing only glycolysis were slow and small. The net gain of 2ATP molecules from a molecule of glucose is not enough to fuel more complicated organisms that populate the biosphere today. For that, photosynthesis and respiration needed to develop. The waste product of non-cyclic photosynthesis is O 2 , and its presence allowed respiration to develop, since O 2 could be used as an electron acceptor. 2. Why was it advantageous for a cell to develop photosynthesis? When abiotic sources of energy started to decline, it became advantageous to have the ability to make ATP. While glycolysis allows an organism to make ATP, it requires a source of organic carbon. Developing photosynthesis allowed an organism to become an autotroph—make everything it needs from CO 2 , NH 3 , PO 4 , H 2 O. Thus, in the environment low in abiotic sources of energy and carbohydrates photosynthetic organisms have selective advantage. 3. Photosynethesis has two phases—light and dark. What does each phase accomplish? The light phase is so named because it is dependent on the presence of light to drive its reactions. Light phase reactions use the energy of the excited chlorophylls to make ATP and NADPH, while producing O 2 as a waste product. The dark phase is so named because it is independent from the presence of light. Dark phase is also known as a biosynthetic phase—the phase in which atmospheric CO 2 is fixed into organic carbon. B. Photosynthesis—light reactions 1. Below are the schematics for cyclic and noncyclic photophosphorylation. Identify each schematic and discuss the order of steps in each process. PS1 PS2 Electron acceptor Electron acceptor A2 Acceptor 3 Stroma Thylakoid membrane ATP synthase H + H + H + H + H + H + H + H + H + H + H + H + ADP+ Pi ATP
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NADPH NADP + PS1 PS2 Electron acceptor Electron acceptor A2 Acceptor 3 Stroma Thylakoid membrane ATP synthase H + H + 2H + H + H + H + H + H + H + H + H + H + ADP+ Pi ATP A2’ H 2 O 2e ± (O 2 ) First schematic is of cyclic photophosphorylation. Steps:
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This note was uploaded on 05/02/2009 for the course BIOL 7.014 taught by Professor Walker during the Spring '05 term at MIT.

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section5_ak - MIT Department of Biology 7.014 Introductory...

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