6_Cellular_Respiration - Cellular Respiration Home Page...

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1 Cellular Respiration © 2006: D. Julian Home Page Catabolic Pathways © 2006: D. Julian What is catabolism? © 2006: D. Julian Carbohydrates, fats and proteins are “combusted” in metabolic pathways controlled by enzymes in order to replenish the ATP (i.e., supply the energy) needed by cells/organisms: Organic compounds + O 2 + ADP +P i ATP (energy) + CO 2 + H 2 O
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2 Redox reactions © 2006: D. Julian Campbell and Reece, Figure 9.3 Transfer or relocation of electrons ( e ) through oxidation-reduction (redox) reactions releases energy stored in organic molecules. An electron loses energy when it shifts from a less electronegative atom to a more electronegative atom. Oxidation of organic fuels © 2006: D. Julian Carbohydrates, fats and proteins are reservoirs of “high energy” (“hilltop”) electrons associated with hydrogen. Once the activation energy barrier is overcome (via enzymes), these electrons release some of their energy as they are transferred to (reduce) oxygen. But instead of reacting explosively, the electrons move through pathways that allow ~40% of the energy to be captured. NAD + and NADH © 2006: D. Julian Electrons are shuttled from the catabolic substrate (e.g. a sugar molecule) via coenzymes (NAD + and FAD) and typically travel with a proton (i.e., as hydrogen atoms). Dehydrogenase enzymes catalyze the transfer of a pair of hydrogen atoms from the catabolic substrate to NAD + , which gains the two electrons and one of the protons*, reducing NAD + to NADH. The remaining proton (H + ion) is released into the surrounding solution. *Recall that H atoms consists of an electron and a proton.
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3 Overview: glycolysis © 2006: D. Julian Campbell and Reece, Figure 9.6 Glycolysis is an evolutionarily ancient, cytosolic pathway.
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