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How Cells Harvest Energy: Cellular Respiration Chapter 7 Energy Flow Sunlight energy Photosynthesis uses solar energy to produce glucose and O2 from CO2 and H2O Cellular respiration extracts energy from glucose by oxidation (using O2) to make ATP and produces CO2 and H2O Redox Reactions (oxidation and reduction simultaneously occur in a chemical reaction) ECOSYSTEM Photosynthesis in chloroplasts CO2 + Glucose H2O O2 + Cellular respiration in mitochondria ATP (for cellular work) Heat energy The human body uses energy from ATP for all its activities Glycolysis Cellular Respiration Cytosol Cells need ATP Glucose and other fuel molecules Pyruvate Substrate-level phosphorylation Pyruvate oxidation ! ATP powers • Active transport • Mechanical work (movement) • Anabolic (endergonic) reactions Cellular Respiration occurs in three stages Mitochondrion Acetyl-CoA Citric acid cycle Substrate-level phosphorylation Electrons carried by NADH and FADH2 Electron transfer system and oxidative phosphorylation Glycolysis ATP Glucose Pyruvate oxidation Citric acid cycle Oxidative phosphorylation In glycolysis, glucose is oxidized to pyruvate. Cytosol ATP ATP The electrons from that reaction reduce NAD+ to NADH. NADH carries the electrons to be used in the oxidative phosphorylation Oxidation 2 Pyruvate Oxidative phosphorylation Electron Carrier NAD+ Substrate Level Phosphorylation Enzyme ADP P Substrate Substrate Enzyme + ATP Product Product Glycolysis Summary Glycolysis ! Converts 1 glucose (6 carbons) to 2 pyruvate (3 carbons) ! 10-step biochemical pathway ! Occurs in the cytoplasm ! Net production of 2 ATP molecules by substratelevel phosphorylation ! 2 NADH produced by the reduction of NAD+ The Fate of Pyruvate is Dependent on Oxygen Availability NADH must be recycled ! For glycolysis to continue, NADH must be recycled to NAD+ by either: 1. Aerobic respiration • • Oxygen is available as the final electron acceptor Produces significant amount of ATP 2. Fermentation • • Occurs when oxygen is not available Organic molecule is the final electron acceptor Fermentation occurs when oxygen is not available Glucose Fermentation Glucose Glycolysis Glycolysis Ethanol Lactate Pyruvate Pyruvate Products of pyruvate oxidation Glycolysis • For each 3 carbon pyruvate molecule: Pyruvate oxidation Mitochondrial matrix – 1 CO2 • Decarboxylation by pyruvate dehydrogenase – 1 NADH – 1 acetyl-CoA which consists of 2 carbons from pyruvate attached to coenzyme A Citric acid cycle • Acetyl-CoA proceeds to the Krebs cycle Oxidative phosphorylation 18 Mitochondria, Sites of Cellular Respiration Pyruvate Acetyl group Acetyl-CoA Krebs Cycle (Citric Acid Cycle) ! For each Acetyl-CoA entering: • • • • • Release 2 molecules of CO2 Reduce 3 NAD+ to 3 NADH Reduce 1 FAD (electron carrier) to FADH2 Produce 1 ATP Regenerate oxaloacetate Pyruvate At this point • Glucose has been oxidized to: Acetyl-CoA – 6 CO2 – 4 ATP – 10 NADH – 2 FADH2 These electron carriers proceed to the electron transport chain Citric acid cycle 24 Electron Transport Chain Glycolysis ! ETC is a series of membrane-bound electron carriers ! Embedded in the inner mitochondrial membrane ! Electrons from NADH and FADH2 are transferred to complexes of the ETC ! Each complex Inner mitochondrial membrane Pyruvate oxidation Citric acid cycle • A proton pump creating proton gradient • Transfers electrons to next carrier Oxidative phosphorylation Oxidative Phosphorylation: Electron Transport Chain & Chemiosmosis Why Electron Transport? H2 + 1/2 O2 + 2H 1/ 2 O2 1/ 2 O2 (from food via NADH) Free energy, G n chai port rans on t Explosive release of heat and light energy +2 e– tr Elec Free energy, G 2 H+ Controlled release of energy for synthesis of ATP ATP ATP ATP 2 e– 2 H+ H2O Uncontrolled reaction H2O Cellular respiration Overview Oxidative Phosphorylation Chemiosmosis ! Accumulation of protons in the intermembrane space drives protons into the matrix via diffusion ! Membrane relatively impermeable to ions ! Most protons can only reenter matrix through ATP synthase H+ H+ + H H+ + H H+ H+ H+ ATP H+ Synthase • Uses energy of gradient to make ATP from ADP + Pi FADH2 FAD NADH 1 O2 + 2 H+ 2 NAD+ + H H+ H+ H2O ADP+ P Electron Transport Chain Certain poisons interrupt critical events in cellular respiration – Various poisons • Block the movement of electrons • Block the flow of H+ through ATP synthase • Allow H+ to leak through the membrane Cyanide, carbon monoxide Rotenone H+ H+ + H Oligomycin H+ + H H+ H+ H+ ATP H+ Synthase DNP FADH2 FAD NADH 1 O2 + 2 H+ 2 NAD+ + H H+ H+ Electron Transport Chain H2O ADP+ P ATP Chemiosmosis Overview Cellular Respiration ATP Chemiosmosis Regulation of Respiration ATP Yield ! Example of feedback inhibition ! 2 key control points 1. In glycolysis • Phosphofructokinase is allosterically inhibited by ATP and/or citrate 2. In pyruvate oxidation • Pyruvate dehydrogenase inhibited by high levels of NADH • Citrate synthetase inhibited by high levels of ATP 33 34 Cells use many kinds of organic molecules as fuel for cellular respiration (catabolic processes) Food, such as peanuts Carbohydrates Fats Sugars Proteins Glycerol Fatty acids Amino acids Amino groups Glucose G3P Pyruvate GLYCOLYSIS 35 Acetyl CoA ATP CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis)