University:
Kentucky State University
Course:
BIO 408 | Cell Biology
Academic year:

2019
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451

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39
Author:

garciavelascogr11

Chapter 9: CELLULAR RESPIRATION & FERMENTATION 1. Overview of Respiration 2. Glycolysis 3. The Citric Acid Cycle 4. Oxidative Phosphorylation 5. Fermentation 1. Overview of Respiration Cellular Respiration Cellular Respiration is essentially the reverse of photosynthesis Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2  H2O Catabolism of energy rich organic molecules is coupled to the endergonic synthesis of ATP Organic  O2 molecules Cellular respiration in mitochondria ATP Heat energy ATP powers most cellular work Important Concepts Before examining the process of Cellular Respiration it is important to review some important concepts that are central to the process: • mitochondrial structure • oxidation/reduction reactions • the central role of HYDROGEN and its electrons (e–), and electron carriers • substrate-level phosphorylation vs oxidative phosphorylation • gradual or incremental release of energy Mitochondrial Structure Mitochondria have 2 membranes (inner and outer) and 2 distinct compartments: • intermembrane space • matrix inner membrane is folded into cristae to increase surface area Oxidation/Reduction There are 2 ways in which are said to be oxidized or reduced: 1 OXIDATION = the loss of electrons REDUCTION = the gain of electrons becomes oxidized (loses electron) becomes reduced (gains electron) becomes oxidized becomes reduced 2 OXIDATION = the partial loss of electrons REDUCTION = the partial gain of electrons Reactants Products becomes oxidized Energy becomes reduced Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water Oxidation/Reduction & Hydrogen The electrons associated with hydrogen atoms are key to the oxidation states of organic molecules in biochemical processes: • electrons (e–) are transferred as part of a hydrogen atom in most biochemical reactions (i.e., with a proton) • when a molecule gains hydrogen it is “reduced” and when a molecule loses hydrogen it is “oxidized” becomes oxidized becomes reduced When e– of H are transferred from a less electronegative atom to a more electronegative atom, energy is released!! Electron Carriers Hydrogens and their e– will be captured by electron carriers such as NADH and delivered elsewhere: NAD NADH Dehydrogenase Reduction of NAD (from food) Oxidation of NADH Nicotinamide (oxidized form) Nicotinamide (reduced form) Dehydrogenase Substrate-level Phosphorylation Enzyme Enzyme ADP P Substrate ATP Product A phosphate group from an organic molecule is transferred directly to ADP to make ATP • an enzyme called a “kinase” catalyzes the reaction involving the transfer of a phosphate from one substrate to another, ADP Oxidative Phosphorylation “Oxidative Phosphorylation” refers to the phosphorylation of ADP to make ATP by a process that depends on oxidation-reduction reactions (“oxidative”). As we shall see this is more complex than it sounds and involves two key processes: 1) A series of oxidation/reduction reactions involving the “Electron Transport Chain” which will produce an electrochemical gradient of H+ ions 2) Coupling the energy stored in this electrochemical gradient of H+ to the endergonic synthesis of ATP Respiration Liberates Energy Gradually H2  1/2 O2  2H 1/ 2 O2 (from food via NADH) Controlled release of energy for synthesis of ATP Explosive release of heat and light energy Free energy, G Free energy, G 2 H+  2 e ATP ATP 2 e 2 1/ H+ H2O (a) Uncontrolled reaction ATP H2O (b) Cellular respiration 2 O2 Cellular Respiration at a Glance Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolysis Glucose Pyruvate Pyruvate oxidation Citric acid cycle Acetyl CoA Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION CYTOSOL ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation ATP Oxidative phosphorylation 2. Glycolysis What is Glycolysis? Glycolysis is a metabolic pathway occurring in the cytoplasm that is essentially “phase 1” of the catabolism of glucose and other monosaccharides. • technically glycolysis is NOT part of cellular respiration, though two of the products of this pathway are used in cellular respiration: 2 ATP 2 NADH* 2 pyruvate* *used in cellular respiration Glycolysis uses ATP to make ATP Energy Investment Phase Glucose 2 ADP  2 P 2 ATP used Energy Payoff Phase 4 ADP  4 P 2 NAD+  4 e  4 H+ 4 ATP formed • 2 ATP must be “consumed” during the catabolism of glucose in order to produce 4 ATP 2 NADH  2 H+ 2 Pyruvate  2 H2O Net Glucose 4 ATP formed  2 ATP used 2 NAD+  4 e  4 H+ 2 Pyruvate  2 H2O 2 ATP 2 NADH  2 H+ net yield of 2 ATP The Energy Investment Reactions Glycolysis: Energy Investment Phase Glucose ATP ATP Glucose 6-phosphate Fructose 1,6-bisphosphate Fructose 6-phosphate ADP ADP Hexokinase Phosphoglucoisomerase Phosphofructokinase 2 3 1 Aldolase Dihydroxyacetone phosphate 4 Glyceraldehyde 3-phosphate Isomerase 5 To step 6 The initial reactions of glycolysis require the hydrolysis (i.e., “consumption” or “loss”) of 2 ATP in order to add 2 phosphates to intermediates of the pathway thus increasing their potential energy (PE). The Energy Payoff Reactions In the subsequent steps of glycolysis, the PE invested is used to accomplish: • the addition of 2 more phosphates • the capture of 2 pairs of energy-rich e– associated with hydrogen (reduction of 2 NAD+ to NADH) • the synthesis of 4 ATP (substrate-level phosphorylation) Glycolysis: Energy Payoff Phase 2 ATP 2 ATP 2 H2O 2 NADH 2 NAD Triose phosphate dehydrogenase 6 + 2 H 2 ADP 2 2 2Pi 1,3-Bisphosphoglycerate 7 Enolase Phosphoglyceromutase Phosphoglycerokinase 2 ADP 2 Pyruvate kinase 9 3-Phosphoglycerate 8 2-Phosphoglycerate Phosphoenolpyruvate (PEP) 10 Pyruvate 3. The Citric Acid Cycle What is the Citric Acid Cycle? The Citric Acid Cycle (CAC) is a metabolic pathway occurring in the mitochondrial matrix that is essentially “phase 2” of the catabolism of glucose: • pyruvate from glycolysis is first catabolized to acetyl-Coenzyme A before entering the CAC • all carbons from the original glucose will be completely oxidized to waste CO2 • more energy-rich e– in hydrogens will be captured by electron carriers • 2 more ATP by substrate-level phosphorylation Oxidation of Pyruvate to Acetyl-CoA MITOCHONDRION CYTOSOL CO2 Coenzyme A 3 1 2 Pyruvate Transport protein NAD NADH + H Acetyl CoA Acetyl CoA Pyruvate CO2 NAD CoA NADH + H Acetyl CoA derived from pyruvate (or the catabolism of fatty acids and other organic molecules) will directly enter the Citric Acid Cycle Acetyl CoA CoA CoA Citric acid cycle 2 CO2 3 NAD FADH2 3 NADH FAD + 3 H ADP + P i ATP Acetyl CoA is combined with the end product of the pathway (oxaloacetate) to produce citric acid to begin the cycle again Acetyl CoA CoA-SH NADH Citric Acid Cycle H2O 1 + H NAD 8 Oxaloacetate 2 Malate Citrate Isocitrate NAD NADH 3 + H 7 Per acetyl group, the cycle yields: H2O CO2 Fumarate CoA-SH -Ketoglutarate 4 6 1 ATP 3 NADH 1 FADH2 2 CO2 CoA-SH 5 FADH2 NAD FAD Succinate NADH Pi GTP GDP ADP ATP Succinyl CoA + H CO2 Keeping Score Up to this point, the original molecule of glucose has yielded the following: GLYCOLYSIS – 2 ATP 2 NADH OXIDATION of PYRUVATE – 2 NADH 2 CO2 CITRIC ACID CYCLE – 2 ATP 6 NADH 2 FADH2 4 CO2 TOTAL – 4 ATP 10 NADH 2 FADH2 6 CO2 • NADH & FADH2 provide energy-rich e– for the synthesis of many more ATP by oxidative phosphorylation… 4. Oxidative Phosphorylation Overview of Oxidative Phosphorylation Oxidative Phosphorylation involves 2 distinct processes: 1) Electron Transport • e- from NADH and FADH2 are passed along the electron transport chain (ETC) via oxidation-reduction reactions • convert energy from e- into energy stored in H+ gradient 2) Chemiosmosis • energy from H+ gradient harnessed to make ATP both processes occur across the inner mitochondrial membrane Electron Transport & Chemiosmosis H H H Protein complex of electron carriers H Cyt c Q I IV III II FADH2 FAD NADH 2 H + 1/2O2 NAD ATP synthase H2O ADP  P i (carrying electrons from food) ATP H 1 Electron transport chain Oxidative phosphorylation 2 Chemiosmosis Basics of Electron Transport Occurs within the inner mitochondrial membrane High energy electrons supplied by: • NADH & FADH2 Electron Transport generates H+ gradient between intermembrane space & matrix • provides proton motive force for ATP synthesis Requires O2 as the final electron acceptor • anaerobic respiration involves other final electron acceptors Electron Transport Chain H INTERMEMBRANE SPACE H H Cyt c Protein complex of electron carriers IV Q III I II FADH2 FAD NADH NAD (carrying electrons from food) 2 H + 1/2 O2 MITOCHONDRIAL MATRIX H2O Basics of Chemiosmosis “The flow of a H+ from high to low concentration” Energy derived from the flow of H+ is used to synthesize ATP (from ADP & Pi) • H+ flows “down” concentration gradient through ATP synthase in the inner mitochondrial membrane • ATP synthase = enzyme complex that catalyzes: ADP + Pi H+ flow ATP Yields up to 28 ATP per glucose molecule! • in addition to 2 ATP in glycolysis, 2 ATP in Krebs cycle INTERMEMBRANE SPACE H ATP Synthase Stator Rotor • as H+ ions flow through ATP synthase, the rotor portion of the enzyme complex and the internal rod actually rotate Internal rod Catalytic knob ADP + Pi MITOCHONDRIAL MATRIX The enzyme complex known as ATP synthase couples the energy released by chemiosmosis to the synthesis of ATP ATP • rotation of the internal rod provides the force needed to to get ADP & Pi bound by the catalytic knob close enough to react and form ATP Summary of Cellular Respiration Electron shuttles span membrane 2 NADH Glycolysis Glucose 2 Pyruvate 2 NADH Pyruvate oxidation 2 Acetyl CoA 6 NADH Citric acid cycle  2 ATP  2 ATP Maximum per glucose: CYTOSOL MITOCHONDRION 2 NADH or 2 FADH2 About 30 or 32 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis  about 26 or 28 ATP Respiration & other Organic Molecules In addition to glucose, other organic molecules “feed” into the process of respiration at various stages Proteins Carbohydrates Amino acids Sugars Glycolysis Glucose Glyceraldehyde 3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation Fats Glycerol Fatty acids Glucose AMP Glycolysis Fructose 6-phosphate  Stimulates  Phosphofructokinase  Regulation of Respiration Fructose 1,6-bisphosphate Inhibits Inhibits Pyruvate ATP Citrate Acetyl CoA Citric acid cycle Oxidative phosphorylation Allosteric regulation of key enzymes such as phosphofructokinase helps keep the entire process in balance. 5. Fermentation The Purpose of Fermentation In the absence of O2, glycolysis is the only source of ATP NAD+ glycolysis fermentation NADH • fermentation ensures sufficient NAD+ is available for glycolysis 2 ADP  2 P Glucose 2 ADP  2 P 2 ATP i Glycolysis Glucose 2 ATP i Glycolysis 2 Pyruvate 2 NAD  2 NADH  2 H 2 NAD  2 CO2 2 NADH  2 H 2 Pyruvate 2 Ethanol Yeast (a) Alcohol fermentation 2 Acetaldehyde 2 Lactate (b) Lactic acid fermentation Animals Fermentation or Respiration? Glucose CYTOSOL Glycolysis Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol, lactate, or other products Acetyl CoA Citric acid cycle If O2 is present, respiration is preferred (more ATP!), however without O2 fermentation is the only option to produce ATP Variety in Fermentation Products Glucose NAD+ NADH Pyruvate Propionibacterium Aspergillus Lactobacillus Streptococcus Saccharomyces Clostridium Escherichia Acetobacter NADH Acetic acid NAD+ Fermentation CO2, propionic acid Lactic acid CO2, ethanol Acetone, isopropanol Fermentation products Swiss cheese Cheddar cheese, yogurt, soy sauce Wine, beer Nail polish remover, rubbing alcohol Vinegar • different organisms produce different fermentation products Key Terms for Chapter 9 • glycolysis, fermentation • Citric Acid Cycle • electron carriers (NADH, FADH2) • oxidation vs reduction • substrate-level vs oxidative phosphorylation, • electron transport chain (ETC), chemiosmosis • ATP synthase • mitochondria: inner & outer membranes, matrix, cristae

Cellular Respiration & Fermentation

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