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Cellular Respiration: Harvesting Chemical Energy Chapter 9 Cellular Respiration • Metabolic pathways that release stored energy by breaking down complex molecules are called catabolic pathways. • There are two types of catabolic processes- fermentation and cellular respiration. Catabolic Pathways •Catabolic pathways yield energy by oxidizing organic fuels •The breakdown of organic molecules is exergonic Catabolic Pathways Fermentation Is the partial degradation of sugars that occurs without the help of oxygen Cellular Respiration This is the most efficient and prevalent catabolic pathway, in which oxygen is consumed as a reactant along with organic fuel. Energy flow and Recycling • Energy – Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic + O2 CO2 + H2O Cellular molecules respiration in mitochondria ATP powers most cellular work Heat Energy Respiration harvests energy stored in organic molecules to generate ATP, Which powers most cellular work. The waste products are used by chloroplasts For photosynthesis. Thus chemicals essential to life are recycled. Redox Reaction • Example of Redox Reaction becomes oxidized (loses electron) Na + Cl Na+ + Cl– becomes reduced (gains electron) Some redox reactions Do not completely exchange electrons Change the degree of electron sharing in covalent bonds Redox Reactions • Redox reactions – Transfer electrons from one reactant to another by oxidation and reduction • In oxidation – A substance loses electrons, or is oxidized In reduction – A substance gains electrons, or is reduced • Cellular Respiration • During cellular respiration – Glucose is oxidized and oxygen is reduced becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Stepwise Harvest • • • Cellular respiration – Oxidizes glucose in a series of steps Electrons from organic compounds – Are usually first transferred to NAD+, a coenzyme NADH, the reduced form of NAD+ – Passes the electrons to the electron transport chain • If electron transfer is not stepwise – A large release of energy occurs – As in the reaction of hydrogen – and oxygen to form water Free energy, G H2 + 1/2 O2 Explosive release of heat and light energy H2O (a) Uncontrolled reaction Electron Transport Chain • 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 2H 1/ + 2 O2 (from food via NADH) 2 H+ + 2 e– Controlled release of energy for synthesis of ATP Free energy, G ATP ATP ATP 2 e– 1/ 2 H+ H2O (b) Cellular respiration 2 O2 Processes of Cellular Respiration • Respiration is a cumulative function of three metabolic stages – Glycolysis – The citric acid cycle – Oxidative phosphorylation • 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 Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolsis Pyruvate Glucose Cytosol Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation ATP Oxidative phosphorylation • Both glycolysis and the citric acid cycle – Can generate ATP by substrate-level phosphorylation Enzyme Enzyme ADP Substrate P + Product ATP Glycolysis • • • Glycolysis harvests energy by oxidizing glucose to pyruvate Glycolysis – Means “splitting of sugar” – Breaks down glucose into pyruvate – Occurs in the cytoplasm of the cell Glycolysis consists of two major phases – Energy investment phase – Energy payoff phase – Glycolysis – Can produce ATP with or without oxygen, in aerobic or anaerobic conditions – Couples with fermentation to produce ATP Glycolysis ATP •The energy input and output •of glycolysis Citric acid cycle Oxidative phosphorylation ATP ATP Energy investment phase Glucose 2 ATP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP 2 NADH 2 NAD+ + 4 e- + 4 H + formed + 2 H+ 2 Pyruvate + 2 H2O Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H + 2 Pyruvate + 2 H2O 2 ATP 2 NADH + 2 H+ CH2OH H H H HO HO OH H OH H Glycolysis Glucose ATP 1 Hexokinase ADP CH2OH P O H H H OH H HO H OH Glucose-6-phosphate 2 Phosphoglucoisomerase CH2O P O CH2OH H HO HO H HO H Fructose-6-phosphate 3 ATP Phosphofructokinase ADP P O CH2 O CH2 O HO H OH HO H Fructose1, 6-bisphosphate P 4 Aldolase 5 P O CH2 C O CH2OH Dihydroxyacetone phosphate H Isomerase C O CHOH CH2 O P Glyceraldehyde3-phosphate Citric acid cycle Oxidative phosphorylation •Glucose enters the cell and is phosphorylated by the enzyme hexokinase which transfers a phosphate group fro ATP to sugar •The charge of the phosphate group traps the sugar in the cell because the plasma membrane is impermeable to membranes. •Phosphorylation makes glucose more reactive chemically. 6 2 NAD+ 2 Triose phosphate dehydrogenase Pi 2 NADH + 2 H+ 2 P O C O CHOH P CH2 O 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP O– 2 C CHOH O CH2 3-Phosphoglycerate P 8 Phosphoglyceromutase O– 2 C H C O P O CH2OH 2-Phosphoglycerate 9 Enolase 2 H2 O 2 O– C O C O P CH2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 O– C O C O CH3 Pyruvate The Citric Acid Cycle • The citric acid cycle completes the energy-yielding oxidation of organic molecules • The citric acid cycle – Takes place in the matrix of the mitochondrion • Before the citric acid cycle can begin – Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylation 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 Oxidative Phosphorylation • • 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 ATP synthase – Is the enzyme that actually makes ATP At certain steps along the electron transport chain – 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 INTERMEMBRANE SPACE H+ H+ A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient. H+ H+ H+ H+ H+ 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. H+ ADP + Pi MITOCHONDRIAL MATRIX ATP Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP. Chemiosmosis • Chemiosmosis – Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane ATP ATP H+ H+ H+ Protein complex Intermembrane of electron space carners Q I Inner mitochondrial membrane Mitochondrial matrix H+ Cyt c IV III II FADH2 NADH+ NAD+ (Carrying electrons from, food) FAD+ 2 H+ + 1/2 O2 ATP synthase H2O ADP + ATP Pi H+ Chemiosmosis Electron transport chain + ATP synthesis powered by the flow Electron transport and pumping of protons (H ), + + Of H back across the membrane which create an H gradient across the membrane Oxidative phosphorylation ATP Production in Respiration • • During respiration, most energy flows in this sequence – Glucose to NADH to electron transport chain to proton-motive force to ATP About 40% of the energy in a glucose molecule – Is transferred to ATP during cellular respiration, making approximately 38 ATP Electron shuttles span membrane CYTOSOL MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH Glycolysis Glucose 2 Pyruvate 2 Acetyl CoA 6 NADH Citric acid cycle + 2 ATP + 2 ATP by substrate-level phosphorylation by substrate-level phosphorylation Maximum per glucose: 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 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 In alcohol fermentation – Pyruvate is converted to ethanol in two steps, one of which releases CO2 During lactic acid fermentation – Pyruvate is reduced directly to NADH to form lactate as a waste product – Both fermentation and cellular respiration – Use glycolysis to oxidize glucose and other organic fuels to pyruvate 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– Glycolysis 2 NAD+ O H 2 ATP C O C OH CH3 2 Lactate (b) Lactic acid fermentation 2 NADH C O C O CH3 Proteins Amino acids Fats Carbohydrates Sugars Glycolysis Glucose Glyceraldehyde-3- P Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation Glycerol Cellular respiration Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle Fatty acids