7: Electron Transport, Oxidative Phosphorylation, and Photosynthesis
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- 7.1: Introduction
- We have seen that glycolysis generates two pyruvate molecules per glucose molecule, and that the subsequent oxidation of each pyruvate generates two Ac-S-CoA molecules. After the further oxidation of each Ac-S-CoA by the Krebs cycle, aerobic cells have captured about 30 Kcal out of the 687 Kcal potentially available from a mole of glucose in two molecules of ATP
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- 7.2: The Electron Transport Chain (ETC)
- All cells use an electron transport chain (ETC) to oxidize substrates in exergonic reactions. The electron flow from reduced substrates through an ETC is like the movement of electrons between the poles of a battery. In the case of the battery, the electron flow releases free energy to power a motor, light, cell phone, etc. In the mitochondrial ETC, electrons flow when the reduced electron (NADH, FADH2) are oxidized.
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- 7.3: Oxidative Phosphorylation
- Oxidative phosphorylation is the mechanism that by which ATP captures the free energy in the mitochondrial proton gradient. Most of the ATP made in aerobic organisms is made by oxidative phosphorylation, rather than by substrate phosphorylation (the mechanism of ATP synthesis in glycolysis or the Krebs cycle).
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- 7.4: Photosynthesis
- If respiration (reaction 1) is the complete oxidation of glucose to H2O and CO2, then photosynthesis (reaction 2) is the reduction of CO2 using electrons from H2O. Photosynthesis is thus an endergonic reaction. During photosynthesis, sunlight (specifically visible light), fuels the reduction of CO2 (summarized below)
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- 7.5: More Thoughts on the Mechanisms and Evolution of Respiration and Photosynthesis
- We can assume that the abundance of chemical energy on our cooling planet favored the formation of cells that could capture free energy from these nutrients in the absence of any oxygen. For a time, we thought that the first cells would have extracted nutrient free energy by non-oxidative, fermentation pathways. And they would have been voracious feeders, quickly depleting their environmental nutrient resources. In this scenario, the evolution of autotrophic life forms saved life from an early e
Thumbnail: The electron transport chain in the cell is the site of oxidative phosphorylation in prokaryotes. The NADH and succinate generated in the citric acid cycle are oxidized, releasing energy to power the ATP synthase. (Public Domain; Fvasconcellos ).