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• Chapter 9 Cellular Respiration: Harvesting Chemical Energy Main Objective is discovering how cells use the energy stored in food molecules to make ATP. Catabolic pathways do not directly move anything, they are linked to work by a chemical drive shaft: ATP. Cellular Respiration • Cellular respiration is a cellular process that breaks down nutrient molecules with the concomitant production of ATP • Consumes oxygen and produces carbon dioxide (CO2) Cellular respiration is an aerobic process. • Usually involves the breakdown of glucose to CO2 and H2O Energy is extracted from the glucose molecule: • Released step-wise • Allows ATP to be produced efficiently Oxidation-reduction enzymes include NAD+ and FAD as coenzymes 2 Cellular Respiration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 3 ATP • Remember that ATP is like a loaded spring. It has three negatively charged Phosphate groups attached and that the 3 P’s close together are unstable. • Cells must recycle the inorganic P that comes off ATP in order to regenerate its supply of ATP • Muscle cells recycle the ATP at a rate of 10 million molecules per second Cellular Respiration NAD+ (nicotinamide adenine dinucleotide) A coenzyme of oxidation-reduction. It can: • Oxidize a metabolite by accepting electrons • Reduce a metabolite by giving up electrons Each NAD+ molecule is used over and over again • FAD (flavin adenine dinucleotide) Also a coenzyme of oxidation-reduction Sometimes used instead of NAD+ Accepts two electrons and two hydrogen ions (H+) to become FADH2 6 Why does the decomposition of Glucose lead to energy? • Transfer of Electrons! • Relocation of e- releases energy stored in food molecules and this energy is used to synthesize ATP • Activation energy of Glucose is high, much higher than body temperature. Enzymes help to lower activation energy. Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 8 Cellular Respiration • Cellular respiration includes four phases: Glycolysis is the breakdown of glucose into two molecules of pyruvate • Occurs in the cytoplasm • ATP is formed • Does not utilize oxygen Preparatory (prep) reaction • Both pyruvates are oxidized and enter the mitochondria • Electron energy is stored in NADH • Two carbons are released as CO2 (one from each pyruvate) 10 Cellular Respiration Citric acid cycle • Occurs in the matrix of the mitochondrion and produces NADH and FADH2 • In series of reactions, it releases 4 carbons as CO2 • Turns twice per glucose molecule (once for each pyruvate) • Produces two immediate ATP molecules per glucose molecule Electron transport chain (ETC) • Extracts energy from NADH & FADH2 • Passes electrons from higher to lower energy states • Produces 32 or 34 molecules of ATP 11 Outside the Mitochondria: Glycolysis • Glycolysis occurs in cytoplasm outside mitochondria • Energy Investment Step: Two ATP are used to activate glucose Glucose splits into two G3P molecules • Energy Harvesting Step: Oxidation of G3P occurs by removal of electrons and hydrogen ions Two electrons and one hydrogen ion are accepted by NAD+ resulting in two NADH Four ATP are produced by substrate-level phosphorylation Net gain of two ATP (4 ATP produced - 2 ATP consumed) Both G3Ps converted to pyruvates 12 Outside the Mitochondria: Glycolysis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glycolysis inputs outputs 6C glucose 2 NAD+ 2 2 (3C) pyruvate 2 NADH 2 ADP ATP 4 ADP + 4 P 2 4 ATP ATP total net gain 13 Glycolysis QuickTime™ and a Cinepak decompressor are needed to see this picture. Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 16 Animation D:\ImageLibrary1-17\09-CellularRespiration\09-09Glycolysis.mov Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Outside the Mitochondria: Fermentation • Pyruvate is a pivotal metabolite in cellular respiration • If O2 is not available to the cell, fermentation, an anaerobic process, occurs in the cytoplasm. During fermentation, glucose is incompletely metabolized to lactate, or to CO2 and alcohol (depending on the organism). • If O2 is available to the cell, pyruvate enters the mitochondria for aerobic respiration. 18 Outside the Mitochondria: Fermentation • Fermentation is an anaerobic process that reduces pyruvate to either lactate or alcohol and CO2 NADH transfers its electrons to pyruvate Alcoholic fermentation, carried out by yeasts, produces carbon dioxide and ethyl alcohol • • • Lactic acid fermentation, carried out by certain bacteria and fungi, produces lactic acid (lactate) • Used in the production of alcoholic spirits and breads. Used commercially in the production of cheese, yogurt, and sauerkraut. Other bacteria produce chemicals anaerobically, including isopropanol, butyric acid, proprionic acid, and acetic acid. 19 Fermentation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. glucose –2 ATP 2 ATP E1 2ADP G3P 2NAD; E2 2 NADH BPG 4ADP E3 +4 ATP 4 ATP pyruvate E4 or 2 ATP 2CO 2 (net gain) 2 lactate or 2 alcohol Animals Plants 20 Fermentation Helps Produce Numerous Food Products Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © The McGraw Hill Companies, Inc./Bruce M. Johnson, photographer 21 Fermentation Helps Produce Numerous Food Products 22 Fermentation Helps Produce Numerous Food Products 23 Outside the Mitochondria: Fermentation • Advantages Provides a quick burst of ATP energy for muscular activity. • Disadvantages Lactate and alcohol are toxic to cells. Lactate changes pH and causes muscles to fatigue. • Oxygen debt Yeast die from the alcohol they produce by fermentation • Efficiency of Fermentation Two ATP produced per glucose of molecule during fermentation is equivalent to 14.6 kcal. Complete oxidation of glucose can yield 686 kcal. Only 2 ATP per glucose are produced, compared to 36 or 38 ATP molecules per glucose produced by cellular respiration. 24 Outside the Mitochondria: Fermentation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fermentation inputs outputs glucose 2 ADP + 2 2 lactate or 2 alcohol and 2 CO2 P 2 ATP net gain 25 ½ Step between Glycolysis and Kreb’s 1. Carboxyl Group is broken off and expelled as CO2 2. A two Carbon fragment is left and oxidized into acetate. An enzyme transfers the extracted e- to NAD+ reducing it to NADH 3. CoEnzyme A will attach to acetate and make Acetyl CoA 4. This only happens if Oxygen is present Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle Mitochondrion Structure and Function Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cristae: location of the electron transportchain (ETC) Matrix: location of the prep reaction and the citric acid cycle outer membrane inner membrane cristae intermembrane space matrix 45,0003X © Dr. Donald Fawcett and Dr. Porter/Visuals Unlimited 28 Inside the Mitochondria • Citric Acid Cycle Also called the Krebs cycle Occurs in the matrix of mitochondria Begins with the addition of a two-carbon acetyl group (from acetyl-CoA) to a four-carbon molecule (oxaloacetate), forming a six-carbon molecule (citric acid) NADH and FADH2 capture energy rich electrons ATP is formed by substrate-level phosphorylation Turns twice for one glucose molecule (once for each pyruvate) Produces 4 CO2, 2 ATP, 6 NADH and 2 FADH2 per glucose molecule D:\ImageLibrary1-17\09CellularRespiration\09-12-KrebsCycle.mov 29 Citric Acid Cycle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. NADH e– NADH e– e– e– e– e– NADHand FADH2 e– Glycolysis Preparatory reaction glucose Electron transport chain and chemiosmosis Citric acid cycle pyruvate Matrix 2 ATP 2 ADP 4 ADP 2 4 ATP total ATP net 2ADP 2 ATP 32ADP or 34 32 or 34 ATP CoA 1. The cycle begins when a C2 acetyl group carried by CoA combines with a C4 molecule to form citrate. acetyl CoA C2 citrate C6 NAD NADH oxaloacetate C2 NADH 5. Once again a substrate is oxidized, and NAD+ is reduced to NADH. 2. Twice over, substrates are oxidized as NAD+ is Reduced to NADH, and CO2 is released. Citric acid cycle NAD+ CO2 fumarate C4 ketoglutarate C5 NAD+ succinate C4 FAD 4. Again a substrate is oxidized, but this time FAD is reduced to FADH2 CO2 NADH FADH ATP 3. ATP is produced as an energized phosphate is transferred from a substrate to ADP. Figure 9.11 A closer look at the Krebs cycle (Layer 4) h Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 33 Inside the Mitochondria Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Citric acid cycle inputs outputs 2 (2c) acetyl groups 4 CO2 6 NAD+ 6 NADH 2 FADH2 2 FAD 2 ADP +2 P 2 ATP 34 Inside the Mitochondria • Electron Transport Chain (ETC) • Location: Eukaryotes: cristae of the mitochondria Aerobic prokaryotes: plasma membrane • Series of carrier molecules: Pass energy-rich electrons successively from one to another Complex arrays of protein and cytochrome • Cytochromes are proteins with heme groups with central iron atoms • The electron transport chain Receives electrons from NADH & FADH2 Produces ATP by oxidative phosphorylation • Oxygen serves as the final electron acceptor Oxygen combines with hydrogen ions to form water 35 Inside the Mitochondria • The fate of the hydrogens: Hydrogens from NADH deliver enough energy to make 3 ATPs Those from FADH2 have only enough for 2 ATPs “Spent” hydrogens combine with oxygen • Recycling of coenzymes increases efficiency Once NADH delivers hydrogens, it returns (as NAD+) to pick up more hydrogens However, hydrogens must be combined with oxygen to make water If O2 is not present, NADH cannot release H+ No longer recycled back to NAD+ 36 Inside the Mitochondria • The electron transport chain complexes pump H+ from the matrix into the intermembrane space of the mitochondrion • H+ therefore becomes more concentrated in the intermembrane space, creating an electrochemical gradient. • ATP synthase allows H+ to flow down its gradient. • Flow of H+ drives the synthesis of ATP from ADP and inorganic phosphate by ATP synthase. • This process is called chemiosmosis ATP production is linked to the establishment of the H+ gradient • ATP moves out of mitochondria and is used for cellular work It can be broken down to ADP and inorganic phosphate These molecules are returned to the mitochondria for more ATP production D:\ImageLibrary1-17\09CellularRespiration\09-15ElectronTransport.mov 37 Organization and Function of the Electron Transport Chain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. e- NADH NADH e- e- e- NADH and FADH2 ee- eGlycolysis glucose Electron transport chain and chemiosmosis Citric acid cycle Preparatory reaction pyruvate 2ATP 2ADP 4 ADP 4 ATP total 2 ATP net 2 H+ ATP Electron transport chain NADH-Q reductase H+ 32 or 34 32 ADP or 34 ADP H+ H+ H+ H+ cytochrome reductase H+ H+ H+ cytochrome c coenzyme Q H+ cytochrome oxidase H+ H+ e: H+ FADH2 high energy electron H+ NADH NAD+ FAD + 2 H+ low energy electron e: H+ H+ 2 H+ H+ H+ H+ ATP H2O ADP + P 1O 2 2 H+ H+ H+ Matrix H+ Intermembrane space H+ H+ H+ ATP synthase + complex H H+ H+ ATP channel protein ATP H+ Chemiosmosis H+ H+ H+ H+ Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis Figure 9.14 ATP synthase, a molecular mill Review: Cellular Respiration • Glycolysis: 2 ATP (substrate-level phosphorylation) • Kreb’s Cycle: 2 ATP (substrate-level phosphorylation) • Electron transport & oxidative phosphorylation: 2 NADH (glycolysis) = 6ATP 2 NADH (acetyl CoA) = 6ATP 6 NADH (Kreb’s) = 18 ATP 2 FADH2 (Kreb’s) = 4 ATP • 38 TOTAL ATP/glucose Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 42 Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 43