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Chapter 9 Cellular Respiration: Harvesting Chemical Energy PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Life Is Work • Living cells – Require transfusions of energy from outside sources to perform their many tasks Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chemical Cycling System • Energy – Flows into an ecosystem as sunlight and Light energy leaves as heat ECOSYSTEM Photosynthesis in chloroplasts Organic CO2 + H2O + O2 Cellular molecules respiration in mitochondria ATP powers most cellular work Figure 9.2 CD rom activity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heat energy • Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels • Catabolic –release energy stored in molecules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic • Exergonic – release of free energy, happens spontaneously Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 catabolic process during respiration • fermentation (no oxygen present) • Cellular respiration (oxygen present) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy • Cellular respiration – Is the most prevalent and efficient catabolic pathway – Consumes oxygen and organic molecules such as glucose – Yields ATP • ATP is regenerated to keep cells working Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Catabolic pathways yield energy – Due to the transfer of electrons • This transfer takes place by Redox Reactions: – Oxidation and Reduction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • If electron transfer is not stepwise – A large release of energy occurs – As in the reaction of hydrogen and oxygen to form water If cellular respiration took place in one step, all of the energy from glucose would be released at once, most of it would be in the form of heat and light. A living cell doesn’t want to just start a fire, it has to release the energy a little at a time. Figure 9.5 A Free energy, G H2 + 1/2 O2 Explosive release of heat and light energy H2O Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (a) Uncontrolled reaction We will see later that… • The electron transport chain – Passes electrons in a series of steps instead of in one explosive reaction – Uses the energy from the electron transfer to form ATP • What are redox reactions? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Principle of Redox • Redox reactions – Transfer electrons from one reactant to another by oxidation and reduction – Oxidations and reductions always go together since electrons are passed from one molecule to another. – Oxidation is the observation that all elements react with oxygen to form oxides. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In oxidation – A substance loses electrons, or is oxidized • Loss of Electrons is Oxidation LEO • In reduction – A substance gains electrons, or is reduced • Gain of Electrons is Reduction GER LEO says GER Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Examples of redox reactions becomes oxidized (loses electron) Na + Cl Na+ + becomes reduced (gains electron) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cl– Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration – Glucose is oxidized and oxygen is reduced becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Cellular Respiration: A Preview • Respiration is a cumulative function of three metabolic stages – Glycolysis – The citric acid cycle OR Krebs Cycle – Oxidative phosphorylation • The Electron Transport Chain – Preview biofilms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Major “Players” in oxidizing glucose • Enzymes called dehydrogenase and Coenzymes – Coenzymes are organic molecules that bind to an enzyme, helping with its catalytic function • Coenzyme NAD+ (nicotinamide adenine dinucleotide) – An organic molecule cells make from the vitamin niacin – Used to shuttle electrons . Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings What they do • Dehydrogenase strips 2 hydrogen atoms from glucose – (H carries 2 electrons) • NAD+ picks up the two electrons and is reduced to NADH – NADH delivers the electrons to the electron transport chain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cellular Respiration • An overview of cellular respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolsis Pyruvate Glucose Cytosol Electron Transport Train Krebs Mitochondrion ATP Figure 9.6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ATP ATP Glycolysis- Occurs In the Cytoplasm of the Cell • Glycolysis harvests energy by oxidizing glucose to pyruvate • Glycolysis – Means “splitting of sugar” • – Glyco “sweet” and lysis “split” Breaks down glucose into pyruvate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Glycolysis • Glycolysis occurs universally (all living organisms) • It does not require oxygen • It does not occur inside a membrane bound organelle • Thought to be a very ancient system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Glycolysis • Glycolysis consists of two major phases – Energy input Citric acid cycle Glycolysis – Energy output Oxidative phosphorylation ATP ATP ATP Energy investment phase Glucose 2 ATP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 2 NAD+ + 4 e- + 4 H + 4 ATP formed 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Glucose 4 ATP formed – 2 ATP used Figure 9.8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 NAD+ + 4 e– + 4 H + 2 Pyruvate + 2 H2O 2 ATP + 2 H+ 2 NADH Glycolysis 4 molecules of ATP are produced by glycolysis, but since the preparatory step used 2 ATP molecules to start the NET GAIN to the cell is 2 molecules of ATP (for each glucose) 2 NADH 2 pyruvate Complete first part of foldout Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Citric Acid Cycle • The citric acid cycle completes the energyyielding oxidation of organic molecules • The citric acid cycle – Takes place in the matrix of the mitochondrion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Citric Acid Cycle • Before the citric acid cycle can begin – Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis (this is a high energy fuel molecule) 1. A carbon atom is removed from pyruvate and released in CO2 2. The two-C compound remaining is oxidized while a molecule of NAD+ is reduced to NADH CYTOSOL NAD+ NADH O– MITOCHONDRION + H+ S C oA C O 2 C 3. A compound called coenzyme A (derived from a B vitamin), joins with the two-C group to form a molecule called acetyl coenzyme A Electrons are removed changing NAD+ to NADH C O O 1 3 CH3 Pyruvate Transport protein For each molecule of glucose that Entered glycolysis, two molecules of acetyl CoA are produced and enter the citric acid cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CO2 Coenzym eA CH3 Acetyle CoA Citric Acid Cycle: AKA – The Krebs Cycle • An overview of the Citric acid cycle Pyruvate (from glycolysis, 2 molecules per glucose) This will cycle twice because 2 pyruvate molecules are formed during glycolysis Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylatio n ATP CO2 CoA NADH + 3 H+ Acetyle CoA CoA Only the two-carbon acetyl part enters the cycle. Coenzyme A helps the acetyl group enter. It is then split off and recycled CoA Three NADH and one FADH2 are formed. These will be used to generate energy in the form of ATP later. Citric acid cycle 3 NAD+ FADH2 One ATP is formed by substratelevel phosphorylation. This can be used directly for cellular activities. FAD 3 NADH + 3 H+ ADP + P i ATP Figure 9.11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 CO2 Two carbons enter in the reduced form from acetyl CoA and two carbons exit, completely oxidized as CO2. This is the carbon dioxide you exhale. Substrate level phosphorylation • Directly adding a phosphate onto ADP to produce ATP • Takes place during glycolysis and Krebs cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Krebs Cycle • Complete Citric Acid Cycle foldout Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Pathway of Electron Transport – Electrons from NADH and FADH2 lose energy in several steps – Oxydative Phosphorylation is simply the transfer of electrons from NADH and FADH to O2 • The electron transport chain – takes place in the cristae (infoldings of the inner mitochondrial membrane) • It functions as a chemical machine – Uses energy released by the “fall” of electrons to pump hydrogen ions (H+) across the inner mitochondrial membrane – They store energy as they become more concentrated on one side of the membrane 2H 1/ + 2 O2 Free energy, G 2 H+ + 2 e– ATP ATP ATP 2 e– 2 H+ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1/ 2 O2 H2O Electron Transport Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane ATP 2. Electron transport chains use this energy to pump H+ across the inner membrane of the mitochondria ATP 4.H+ ions flow back through an ATP synthase. This spins a part of the synthase. H+ H+ H+ Intermembrane space Inner mitochondrial membrane H+ Protein complex of electron carners FADH2 FAD+ Figure 9.15 H2O 5.The ATP synthase NAD+ ATP uses the + Pi 1.NADH transfers electrons energy of From food to electron H+ the H+ Transport chains gradient Chemiosmosis Electron transport chain to Electron transport and pumping of protons (H+), ATP synthesis powered by the flowregenerate which create an H+ gradient across the membrane Of H+ back across the membrane ATP from ADP Oxidative phosphorylation NADH+ Mitochondrial matrix 2 H+ + 1/2 O2 ATP synthase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3. Oxygen functions to Pull electronsADP down the transport chain. Electron Transport • The hydrogen ions will eventually gush back to where they are less concentrated. • The membrane temporarily restrains the H+ ions – (like a dam holding back water) www2.nl.edu/jste/electron_transport_system.htm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chemiosmosis: The Energy-Coupling Mechanism • The energy of the H+ ions is generated like water in a dam. As it gushes through turbines that spin to create electricity , or in this case, energy. • ATP synthase is the enzyme that actually makes ATP INTERMEMBRANE SPACE It acts like the turbine in the mitochondria. H+ H+ H+ H+ Chemiosmosis H+ is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration or photosynthesis. H+ H+ H+ ADP + Pi Figure 9.14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings MITOCHONDRIAL MATRIX ATP A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient. A stator anchored in the membrane holds the knob stationary. A rod (for “stalk”) extending into the knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP. Electron Transport Chain • The resulting H+ gradient – Stores energy – Drives chemiosmosis in ATP synthase – Is referred to as a proton-motive force • Because some members of the chain that pass electrons also accept and release protons. • Protons are stored in the intermembrane space and are used to synthesize ATP. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An overview of cellular respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolsis Pyruvate Glucose Cytosol Enzyme transfers a phosphate group from a substrate to ATP ADP to make ATP Substrate-level phosphorylation re 9.6 Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP Substrate-level phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ATP Oxidative phosphorylation Redox reactions are used to synthsize ATP An Accounting of ATP Production by Cellular Respiration • During respiration, most energy flows in this sequence – Glucose to NADH to electron transport chain to proton-motive force to ATP – Each NADH is worth 3 ATP’s – Each FADH is worth 2 ATP’s – There are also the ATP’s produced directly Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Energy Production • About 40% of the energy in a glucose molecule – Is transferred to ATP during cellular respiration, making approximately 38 ATP Process Direct ATP NADH Glycolysis 2 2 Pyruvate to acetyl CoA Citric Acid 2 2 6 2 Total ATP 30 4 4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings FADH Fermentation • Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration – Relies on oxygen to produce ATP • In the absence of oxygen – Cells can still produce ATP through fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fermentation • Glycolysis – Can produce ATP with or without oxygen, in aerobic (with) or anaerobic (without) conditions – Couples with fermentation to produce 2 net ATP – The problem is recycling the electron acceptor NAD+ – During cellular respiration, the electron transport train will recycle NADH to NAD+ when it gives up its electrons. This will not occur when oxygen isn’t present Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fermentation • Fermentation allows recycling of NAD+ from NADH • Glycolysis can continue and small amounts of ATP can be generated Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fermentation in Human Muscle Cells • During lactic acid fermentation – Pyruvate is reduced directly to NADH to form lactate as a waste product – This will occur if your muscles must spend ATP at a rate that outpaces the delivery by the bloodstream of oxygen from your lungs to your muscles. – An example is when you are doing a “hard work out” Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lactic Acid Fermentation (the bottom picture) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; P1 2 ATP 2 ADP + 2 O– ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; C O ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; C O Glucose Glycolysis ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; CH3 2 Pyruvate ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; 2 NADH 2 NAD+ H H C OH CH3 2 Ethanol (a) Alcohol fermentation 2 ADP + 2 Glucose Figure 9.17 P1 2 CO2 H C O CH3 2 Acetaldehyde ‘ 2 ATP Glycolysis 2 NADH 2 NAD+ O C O H C OH CH3 2 Lactate (b) Lactic acid fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings O– C O C O CH3 NAD+ must be present as an electron acceptor during glycolysis. And with oxygen present the cell can regenerate its NAD+ when NADH drops its electron cargo down electron transport chains to oxygen. This cannot happen without oxygen present. Instead, NADH disposes of electrons by adding them to the pyruvic acid produced by glycolysis. This restores NAD+ and keeps glycolysis working as an ATP source. Reduction of pyruvic acid produces a waste product called lactic acid. Fermentation in Microorganisms • In alcohol fermentation – Occurs in yeast – Ethyl alcohol is produced as a waste product along with CO2 – Pyruvate is converted to ethanol in two steps, one of which releases CO2 • See next slide some bacteria and fungi produce lactic acid as their waste. This is used to flavor milk and cheese. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alcohol Fermentation . . Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fermentation and Cellular Respiration Compared • Both fermentation and cellular respiration – Use glycolysis to oxidize glucose and other organic fuels to pyruvate • NAD+ is the oxidizing agent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fermentation and cellular respiration – Differ in their final electron acceptor – Fermentation uses pyruvate or acetaldehyde as the final electron acceptor – Respiration uses oxygen, via electron trasport • Cellular respiration – Produces more ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Pyruvate is a key juncture in catabolism (release energy – break down) Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Figure 9.18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Acetyl CoA Citric acid cycle The Evolutionary Significance of Glycolysis • Glycolysis – Occurs in nearly all organisms – Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Versatility of Catabolism • Catabolic pathways – Funnel electrons from many kinds of organic molecules into cellular respiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The catabolism of various molecules from food. All are used to make ATP via cellular respiration Proteins are digested into amino acids, which can enter into respiration at several sites. Proteins Carbohydrates Amino acids Sugars Fats Glycerol Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Figure 9.19 Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fatty acids Digestion of fats yields glycerol, which is converted to an intermediate of glycolysis, and fatty acids, which are broken down to 2 carbon fragments that enter the citric acid cycle as acetyl CoA