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Cellular Respiration A.P. Biology Energy – Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic CO2 + H2O + O2 Cellular molecules respiration in mitochondria ATP powers most cellular work Figure 9.2 Heat energy Catabolic pathways yield energy by oxidizing organic fuels • The breakdown of organic molecules is exergonic • Fermentation – Is a partial degradation of sugars that occurs without oxygen Catabolic pathways yield energy by oxidizing organic fuels • Cellular respiration – Is the most prevalent and efficient catabolic pathway – Consumes oxygen and organic molecules such as glucose – Yields ATP Redox Reactions: Oxidation and Reduction • Catabolic pathways yield energy – Due to the transfer of electrons • Redox reactions – Transfer electrons from one reactant to another by oxidation and reduction • Oxidation – A substance loses electrons, or is oxidized • Reduction – A substance gains electrons, or is reduced becomes oxidized (loses electron) becomes reduced (gains electron) Some redox reactions – Do not completely exchange electrons – Change the degree of electron sharing in covalent bonds Products Reactants becomes oxidized + CH4 CO 2O2 + Energy 2 H2O becomes reduced O O C O H O O H H H C + 2 H H Methane (reducing agent) Figure 9.3 Oxygen (oxidizing agent) Carbon dioxide Water 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 G = -686 kcal/mol Step by step catabolism of glucose • If electron transfer is not stepwise Figure 9.5 A Free energy, G – A large release of energy occurs – As in the reaction of hydrogen and oxygen to form water H2 + 1/2 O2 Explosive release of heat and light energy H2O (a) Uncontrolled reaction ETC 2H 1/ + 2 O2 1/ O2 (from food via NADH) • The electron transport chain Free energy, G – Passes electrons in a series of steps – Uses the energy from the electron transfer to form ATP 2 H+ + 2 e– Controlled release of energy for synthesis of ATP ATP ATP ATP 2 e– 2 H+ H2O Figure 9.5 B (b) Cellular respiration 2 NAD – Nicotinamide Adenine Dinucleotide (Electron Acceptor) • Electrons from organic compounds – Are usually 1st transferred to NAD+, a coenzyme 2 e– + 2 H+ NAD+ Dehydrogenase O NH2 H C CH2 O O– O O P O H O P O– HO O N+ Nicotinamide (oxidized form) H OH HO CH2 H HO N H OH Reduction of + 2[H] (from food) Oxidation of NADH N NADH H O C H N NH2 + Nicotinamide (reduced form) Dehydragenase – removes 2 hydrogen atoms N H O NAD+ NH2 N 2 e– + H+ H Figure 9.4 NADH • NADH, the reduced form of NAD+ – Passes the electrons to the electron transport chain – Electrons are ultimately passed to a molecule of oxygen (Final electron acceptor) G = -53 kcal/mol Electron path in respiration Food NADH ETC Oxygen Cellular Respiration • Respiration is a cumulative function of three metabolic stages – Glycolysis – The citric acid cycle (TCA or Krebbs) – Oxidative phosphorylation C6H12O6 + 6O2 <----> 6 CO2 + 6 H20 DG = -686 Kc/mole + e- ---> 36-38 ATP 263Kc = 38% Respiration • Glycolysis – Breaks down glucose into two molecules of pyruvate • The citric acid cycle – Completes the breakdown of glucose • Oxidative phosphorylation – Is driven by the electron transport chain – Generates ATP Respiration Overview Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolsis Pyruvate Glucose Cytosol ATP Figure 9.6 Substrate-level phosphorylation 2 ATP Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP Substrate-level phosphorylation 2 ATP ATP Oxidative phosphorylation 34 ATP Aerobic Respiration Substrates Substrate Level Phosphorylation • Both glycolysis and the citric acid cycle – Can generate ATP by substrate-level phosphorylation Enzyme Enzyme ADP P Substrate + Figure 9.7 Product ATP Glycolysis Glycolysis • Harvests energy by oxidizing glucose to pyruvate ATP Citric acid cycle Oxidative phosphorylation ATP ATP Energy investment phase Glucose • Glycolysis – Means “splitting of sugar” – Breaks down glucose into pyruvate – Occurs in the cytoplasm of the cell 2 ATP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 2 NAD+ + 4 e- + 4 H+ 4 ATP formed 2 + 2 H+ NADH 2 Pyruvate + 2 H 2O • Two major phases – Energy investment phase – Energy payoff phase Figure 9.8 Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O 2 ATP + 2 H+ 2 NADH Energy Investment Phase Energy Payoff Phase Glycolysis Summary The First Stage of Glycolysis •Glucose (6C) is broken down into 2 PGAL's (3C) •This requires two ATP's The Second Stage of Glycolysis •2 PGAL's (3C) are converted to 2 pyruvates •This creates 4 ATP's and 2 NADH's •The net ATP production of Glycolysis is 2 ATP's Citric Acid Cycle a.k.a. Krebs Cycle • Completes the energy-yielding oxidation of organic molecules • The citric acid cycle – Takes place in the matrix of the mitochondrion Krebs's Cycle (citric acid cycle, TCA cycle) •Goal: take pyruvate and put it into the Krebs's cycle, producing NADH and FADH2 •Where: the mitochondria •There are two steps •The Conversion of Pyruvate to Acetyl CoA •The Kreb's Cycle proper •In the Krebs's cycle, all of Carbons, Hydrogens, and Oxygeng in pyruvate end up as CO2 and H2O •The Krebs's cycle produces 2 ATP's, 8 NADH's, and 2FADH2's per glucose molecule Fate of Pyruvate Carboxyl (coo-) is cleaved CO2 is released Before the citric acid cycle can begin –Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA C O 2 C C O O 1 3 CH3 Pyruvate Transport protein Figure 9.10 CH3 Acetyle CoA CO2 Coenzyme A An overview of the citric acid cycle Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylatio n ATP CO2 CoA NADH + 3 H+ Acetyle CoA CoA CoA Citric acid cycle 2 CO2 3 NAD+ FADH2 FAD 3 NADH + 3 H+ ADP + P i ATP Figure 9.11 Glycolysis Citric Oxidative acid phosphorylation cycle S CoA C O CH3 Acetyl CoA CoA SH O NADH + H+ C COO– COO– 1 CH2 COO– NAD+ 8 Oxaloacetate HO C COO– COO– CH2 COO– HO CH H2O CH2 CH2 2 HC COO– COO– Malate HO Citrate CH2 COO– Isocitrate COO– H2O CO2 Citric acid cycle 7 COO– CH 3 NAD+ COO– Fumarate HC CH2 CoA SH 6 CoA SH COO– FAD CH2 CH2 COO– C O Succinate Pi S CoA GTP GDP Succinyl CoA ADP ATP 4 C O COO– CH2 5 CH2 FADH2 COO– NAD+ NADH + H+ NADH + H+ a-Ketoglutarate CH2 COO– Figure 9.12 CH CO2 The Kreb's Cycle •6 NADH's are generated •2 FADH2 is generated •2 ATP are generated •4 CO2's are released Net Engergy Production from Aerobic Respiration •Glycolysis: 2 ATP •Kreb's Cycle: 2 ATP •Electron Transport Phosphorylation: 32 ATP •Each NADH produced in Glycolysis is worth 2 ATP (2 x 2 = 4) - the NADH is worth 3 ATP, but it costs an ATP to transport the NADH into the mitochondria, so there is a net gain of 2 ATP for each NADH produced in gylcolysis •Each NADH produced in the conversion of pyruvate to acetyl COA and Kreb's Cycle is worth 3 ATP (8 x 3 = 24) •Each FADH2 is worth 2 ATP (2 x 2 = 4) •4 + 24 + 4 = 32 •Net Energy Production: 36 ATP Energy Yields: •Glucose: 686 kcal/mol •ATP: 7.5 kcal/mol •7.5 x 36 = 270 kcal/mol for all ATP's produced •270 / 686 = 39% energy recovered from aerobic respiration After the Krebs Cycle… •During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis •NADH and FADH2 –Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation The Pathway of Electron Transport •In the electron transport chain –Electrons from NADH and FADH2 lose energy in several steps •At the end of the chain –Electrons are passed to oxygen, forming water NADH 50 FADH2 Free energy (G) relative to O2 (kcl/mol) 40 I FMN Multiprotein complexes FAD Fe•S Fe•S II O III Cyt b 30 Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 20 10 0 Figure 9.13 2 H + + 12 O2 H2 O Chemiosmosis: The Energy-Coupling Mechanism •ATP synthase INTERMEMBRANE SPACE H+ H+ –Is the enzyme that actually makes ATP H+ H+ H+ H+ H+ A stator anchored in the membrane holds the knob stationary. H+ ADP + Pi Figure 9.14 A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient. MITOCHONDRIAL MATRIX ATP 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. ETC –Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space •The resulting H+ gradient –Stores energy –Drives chemiosmosis in ATP synthase –Is referred to as a proton-motive force Chemiosmosis –Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work H+ gradient = Proton motive force Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane ATP ATP H+ H+ H+ Intermembrane space H+ Cyt c Protein complex of electron carners Q I Inner mitochondrial membrane IV III ATP synthase II FADH2 FAD+ NADH+ 2 H+ + 1/2 O2 NAD+ H2O ADP + (Carrying electrons from, food) Mitochondrial matrix Figure 9.15 ATP Pi H+ Electron transport chain Electron transport and pumping of protons (H+), which create an H+ gradient across the membrane Chemiosmosis ATP synthesis powered by the flow Of H+ back across the membrane Oxidative phosphorylation 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 Electron shuttles span membrane CYTOSOL MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH Glycolysis Glucose 2 Acetyl CoA 2 Pyruvate Citric acid cycle + 2 ATP + 2 ATP by substrate-level phosphorylation by substrate-level phosphorylation Maximum per glucose: Figure 9.16 6 NADH About 36 or 38 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis + about 32 or 34 ATP by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol Anaerobic Respiration •Fermentation enables some cells to produce ATP without the use of oxygen •Glycolysis –Can produce ATP with or without oxygen, in aerobic or anaerobic conditions –Couples with fermentation to produce ATP Anaerobic Respiration •Fermentation consists of –Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis •Alcohol fermentation –Pyruvate is converted to ethanol in two steps, one of which releases CO2 •Lactic acid fermentation –Pyruvate is reduced directly to NADH to form lactate as a waste product 2 ADP + 2 Glucose P1 2 ATP Glycolysis O– C O C O CH3 2 Pyruvate 2 NADH 2 NAD+ H H 2 CO2 H C C OH CH3 O CH3 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation 2 ADP + 2 Glucose P1 O H C O OH CH3 2 Lactate Figure 9.17 O– Glycolysis 2 NAD+ C 2 ATP (b) Lactic acid fermentation 2 NADH C O C O CH3 Pyruvate is a key juncture in catabolism Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Figure 9.18 Acetyl CoA Citric acid cycle •Glycolysis –Occurs in nearly all organisms –Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Proteins Amino acids Carbohydrates Sugars Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Figure 9.19 Oxidative phosphorylation Fats Glycerol Fatty acids •Cellular respiration Glucose AMP Glycolysis –Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle Fructose-6-phosphate Stimulates + – Inhibits Phosphofructokinase – Fructose-1,6-bisphosphate Inhibits Pyruvate Citrate ATP Acetyl CoA Citric acid cycle Figure 9.20 Oxidative phosphorylation