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Ch. 6: Cellular Respiration Harvesting Chemical Energy Cellular Respiration: An Overview A cellular process that breaks down nutrient molecules with the production of ATP Consumes oxygen and produces carbon dioxide (CO2) ◦ Cellular respiration is an aerobic process. Usually involves the complete breakdown of glucose to CO2 and H2O Occurs in 3 steps Overview Cellular respiration is an exergonic ◦ produces up to 32 ATP molecules from each glucose molecule ◦ captures only about 34% of the energy originally stored in glucose Other foods (organic molecules) can also be used as a source of energy. Figure 6.2 O2 Breathing CO2 Lungs CO2 Bloodstream O2 Muscle cells carrying out Cellular Respiration Glucose O2 CO2 H2O ATP Cellular Respiration: An Overview Cellular Respiration: An Overview Terms to Know… ◦ Oxidation = the loss of electrons Compound becomes more positive ◦ Reduction = the gain of electrons Compound becomes more negative ◦ Electrons and protons (H+) travel TOGETHER Oxidation C6H12O6 + 6O2 6CO2 + 6H2O + energy glucose Reduction Electrons are removed from substrates and received by oxygen, which combines with H+ to become water. Glucose is oxidized and O2 is reduced Important enzymes NAD+ ◦ A coenzyme of oxidation-reduction. ◦ Each NAD+ molecule is used over and over again ◦ Reduced into NADH Accepts 2 electrons plus a hydrogen ion (H+) FAD ◦ Also a coenzyme of oxidation-reduction ◦ Sometimes used instead of NAD+ ◦ Reduced into FADH2 Accepts two electrons and two hydrogen ions (H+) Respiration Respiration is a cumulative function of 3 metabolic stages ◦ Glycolysis Only reactions that takes place outside of mitochondria and doesn’t require O2 ◦ Citric acid cycle (Krebs Cycle) ◦ Oxidative Phosphorylation Electron transport chain (ETC) Cellular Respiration Glucose Glycolysis Oxygen Absent ATP Oxygen Present Anaerobic Respiration Aerobic Respiration (Fermentation) (Krebs Cycle & ETC) ATP Glycolysis “glucose-splitting” Big Picture: ◦ Glucose (6-C) is broken down into 2 molecules of pyruvate (3-C) Occurs in the cytoplasm Occurs without oxygen Oxidation results in NADH and 2 ATP Made up of 2 phases: ◦ Energy investment phase ◦ Energy yielding (payoff) phase Glycolysis: Energy Investment Phase Glucose is converted into 2 G3P (Glyceraldehyde-3phosphate) Requires 2 ATP Glycolysis: Energy-Yielding Phase ◦ 2 G3P are converted into 2 Pyruvate (3C) molecules. ◦ Dehydrogenase enzymes remove H from intermediate compounds and attach them to 2 NAD to produce 2NADH Substrate-Level Phosphorylation An enzyme transfers a phosphate group directly from an organic molecule to ADP to form ATP The ATP produced in Glycolysis & the Krebs Cycle is produced by this method. enzyme ADP BPG ATP 3PG Net Gain in Glycolysis 2 ATP - 2 ATP (Energy investment phase) + 4 ATP (Energy yielding phase) + 2 ATP 2 NADH ◦ Electron carriers ◦ Will be used to make ATP later Choices, Choices! If oxygen is absent, anaerobic respiration occurs ◦ Fermentation Yeast & some bacteria alcoholic fermentation Animal muscle lactic acid fermentation If oxygen is present, aerobic respiration occurs ◦ Krebs Cycle and Electron Transport Chain Cellular Respiration Glucose Glycolysis Oxygen Absent Anaerobic Respiration ATP Oxygen Present Aerobic Respiration (Fermentation) ATP Fermentation Fermentation is an anaerobic process that reduces pyruvate to either lactate or alcohol and CO2 2 major types: ◦ Alcoholic and lactic acid fermentation NAD+ acts as a hydrogen acceptor during glycolysis ◦ If the supply of NAD+ runs out, then glycolysis would have to stop. ◦ Fermentation occurs as simply a means of recycling the NAD+, so that glycolysis can occur again. Alcoholic Fermentation Occurs in some BACTERIA and YEAST 2 step process: ◦ Carbon dioxide is released from pyruvate (3-C), forming acetaldehyde (2-C) ◦ Acetaldehyde is reduced by NADH forming ethanol ◦ NAD+ is regenerated Used to produce beer and wine Lactic Acid Fermentation Occurs in ANIMALS 1 step process: ◦ Pyruvate is reduced by NADH forming lactic acid NAD+ is regenerated Occurs in muscle cells, causing muscle pain and fatigue Used to make yogurt and cheese Cellular Respiration Glucose Glycolysis Oxygen Absent Anaerobic Respiration ATP Oxygen Present Aerobic Respiration (Fermentation) ATP Aerobic Respiration After glycolysis, most of the energy from glucose remains “locked” in 2 molecules of pyruvate If oxygen is present, the pyruvate enters the mitochondrial matrix to complete the Krebs Cycle Preparatory Phase: Pyruvate (3-C) is converted to Acetyl CoA (2-C) ◦ CO2 is released as a waste product ◦ NADH is produced The Krebs Cycle Yield per pyruvate molecule (two turns): ◦ 3 NADH ◦ 1 FADH2 ◦ 1 ATP (produced via substrate level phosphorylation) ◦ 2 CO2 CO2 released as a waste product ◦ We exhale this Figure 6.9A Acetyl CoA CoA CoA 2 CO2 Citric Acid Cycle 3 NAD FADH2 3 NADH FAD 3 H ATP ADP P Electron Transport Chain (ETC) Collection of cytochrome molecules embedded in the cristae membrane ◦ 4 reactions plus ATP synthase Occurs in inner membrane of mitochondrion Proton pump that produces a proton gradient that will be used to create ATP ETC Electrons from NADH and FADH2 from glycolysis and the Krebs Cycle lose electrons, proton gradient The energy in each NADH molecule moves enough protons (H+) into the mitochondrial matrix to create 3 ATP 1 FADH2 2 ATP ETC The electrons from NADH and FADH2 are passed from one electron acceptor molecule to another. Each electron acceptor is more electronegative than the last. Oxygen is the final electron acceptor, producing water e- oxygen Steps of the ETC I- NADH reductase oxidizes NADH to NAD+ resulting in high energy electron II- high energy electron transfers through coenzyme Q to cytochrome reductase III- travels through the cytochrome c IV- travels into cytochrome oxidase where it is now low energy and binds to oxygen to form water Chemiosmosis the energy the electrons lose along the way moves H+ out of the matrix and into the intermembrane space of the mitochondria As H+ ions diffuse through the membrane, ATP synthase uses the energy to join ADP and a phosphate group ATP Oxidative Phosphorylation: ETC & Chemiosmosis Figure 6.12 CYTOPLASM Electron shuttles across membrane 2 NADH Mitochondrion 2 NADH or 2 FADH2 6 NADH 2 NADH Glycolysis 2 Pyruvate Glucose Pyruvate Oxidation 2 Acetyl CoA Citric Acid Cycle 2 FADH2 Oxidative Phosphorylation (electron transport and chemiosmosis) Maximum per glucose: 2 ATP by substrate-level phosphorylation 2 ATP by substrate-level phosphorylation about 28 ATP by oxidative phosphorylation About 32 ATP