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How Cells Release Stored Energy Chapter 7 ATP Is Universal Energy Source • Photosynthesizers get energy from the sun • Animals get energy second- or third-hand from plants or other organisms • Regardless, the energy is converted to the chemical bond energy of ATP Making ATP • Plants make ATP during photosynthesis • Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein Main Pathways Start with Glycolysis • Glycolysis occurs in cytoplasm • Reactions are catalyzed by enzymes Glucose (six carbons) 2 Pyruvate (three carbons) Overview of Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose carbon oxygen dioxide water Overview of Aerobic Respiration CYTOPLASM glucose ATP GLYCOLYSIS energy input to start reactions e- + H+ (2 ATP net) 2 pyruvate 2 NADH MITOCHONDRION 2 NADH 8 NADH 2 FADH2 e- e- + H+ 2 CO2 e- + H+ KREBS CYCLE e- + H+ ELECTRON TRANSPORT PHOSPHORYLATION H+ 4 CO2 2 32 ATP ATP water e- + oxygen TYPICAL ENERGY YIELD: 36 ATP The Role of Coenzymes • NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages • When reduced, they are NADH and FADH2 • In the third stage, these coenzymes deliver the electrons and hydrogen to the transport system Glucose • A simple sugar (C6H12O6) • Atoms held together by covalent bonds Glycolysis Occurs in Two Stages • Energy-requiring steps – ATP energy activates glucose and its six- carbon derivatives • Energy-releasing steps – The products of the first part are split into three-carbon pyruvate molecules – ATP and NADH form Energy-Requiring Steps glucose ATP ADP P glucose-6-phosphate P fructose-6-phosphate ATP ADP P fructose-1,6-bisphosphate 2 ATP invested EnergyReleasing Steps PGAL PGAL NAD+ NADH Pi P P NADH Pi P 1,3-bisphosphoglycerate ADP NAD+ ATP P 1,3-bisphosphoglycerate ADP ATP substrate-level phosphorylation 2 ATP invested P P 3-phosphoglycerate 3-phosphoglycerate P P 2-phosphoglycerate H2 O 2-phosphoglycerate H2 O PEP PEP P ADP ATP P ADP ATP substrate-level phosphorylation 2 ATP invested pyruvate pyruvate Net Energy Yield from Glycolysis • Energy requiring steps: 2 ATP invested • Energy releasing steps: 2 NADH formed 4 ATP formed • Net yield is 2 ATP and 2 NADH PREPARATORY STEPS pyruvate Second-Stage Reactions coenzyme A (CoA) NAD+ (CO2) NADH CoA Acetyl–CoA KREBS CYCLE • Occur in the mitochondria • Pyruvate is broken down to carbon dioxide • More ATP is formed • More coenzymes are reduced CoA oxaloacetate citrate H O 2 NADH H2O NAD+ malate NAD+ H2O isocitrate NADH fumarate FADH2 FAD a-ketogluterate CoA NAD+ NADH succinate CoA succinyl–CoA ATP ADP + phosphate group (from GTP) Two Parts of Second Stage • Preparatory reactions – Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide – NAD+ is reduced • Krebs cycle – The acetyl units are oxidized to carbon dioxide – NAD+ and FAD are reduced Preparatory Reactions pyruvate + coenzyme A + NAD+ acetyl-CoA + NADH + CO2 • One of the carbons from pyruvate is released in CO2 • Two carbons are attached to coenzyme A and continue on to the Krebs cycle What is Acetyl-CoA? • A two-carbon acetyl group linked to coenzyme A CH3 Acetyl group C=O Coenzyme A The Krebs Cycle (for each pyruvate) Overall Reactants Overall Products • • • • • • • • • Acetyl-CoA 3 NAD+ FAD ADP and Pi Coenzyme A 2 CO2 3 NADH FADH2 ATP Results of the Second Stage • All of the carbon molecules in pyruvate end up in carbon dioxide • Coenzymes are reduced (they pick up electrons and hydrogen) • One molecule of ATP is formed • Four-carbon oxaloacetate is regenerated Coenzyme Reductions During First Two Stages • Glycolysis • Preparatory reactions • Krebs cycle 2 NADH 2 NADH 2 FADH2 + 6 NADH • Total 2 FADH2 + 10 NADH For each glucose = 2 pyruvates Electron Transport Phosphorylation • Occurs in the mitochondria • Coenzymes deliver electrons to electron transport systems • Electron transport sets up H+ ion gradients • Flow of H+ down gradients powers ATP formation Electron Transport • Electron transport systems are embedded in inner mitochondrial compartment • NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system • Electrons are transported through the system • The final electron acceptor is oxygen Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT Making ATP: Chemiosmotic Model ATP INNER COMPARTMENT ADP + Pi Importance of Oxygen • Electron transport phosphorylation requires the presence of oxygen • Oxygen withdraws spent electrons from the electron transport system, then combines with H+ to form water Summary of Energy Harvest (per molecule of glucose) • Glycolysis – 2 ATP formed by substrate-level phosphorylation • Krebs cycle and preparatory reactions – 2 ATP formed by substrate-level phosphorylation • Electron transport phosphorylation – 32 ATP formed Energy Harvest from Coenzyme Reductions What are the sources of electrons used to generate the 32 ATP in the final stage? – 4 ATP - generated using electrons released during glycolysis and carried by NADH – 28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2 Efficiency of Aerobic Respiration • 686 kcal of energy are released • 7.5 kcal are conserved in each ATP • When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP • Efficiency is 270 / 686 X 100 = 39 percent • Most energy is lost as heat Overview of Aerobic Respiration CYTOPLASM glucose ATP GLYCOLYSIS energy input to start reactions e- + H+ (2 ATP net) 2 pyruvate 2 NADH MITOCHONDRION 2 NADH 8 NADH 2 FADH2 e- e- + H+ 2 CO2 e- + H+ KREBS CYCLE e- + H+ ELECTRON TRANSPORT PHOSPHORYLATION H+ 4 CO2 2 32 ATP ATP water e- + oxygen TYPICAL ENERGY YIELD: 36 ATP Anaerobic Pathways • Do not use oxygen • Produce less ATP than aerobic pathways • Two types – Fermentation pathways – Anaerobic electron transport Fermentation Pathways • Begin with glycolysis • Do not break glucose down completely to carbon dioxide and water • Yield only the 2 ATP from glycolysis • Steps that follow glycolysis serve only to regenerate NAD+ Lactate Fermentation GLYCOLYSIS C6H12O6 2 ATP energy input 2 NAD+ 2 ADP 2 4 NADH ATP energy output 2 pyruvate 2 ATP net LACTATE FORMATION electrons, hydrogen from NADH 2 lactate GLYCOLYSIS Alcoholic Fermentation C6H12O6 2 ATP energy input 2 NAD+ 2 ADP 2 4 NADH ATP 2 pyruvate energy output 2 ATP net ETHANOL FORMATION 2 H2O 2 CO2 2 acetaldehyde electrons, hydrogen from NADH 2 ethanol Carbohydrate Breakdown and Storage • Glucose is absorbed into blood • Pancreas releases insulin • Insulin stimulates glucose uptake by cells • Cells convert glucose to glucose-6-phosphate • This traps glucose in cytoplasm where it can be used for glycolysis Making Glycogen • If glucose intake is high, ATP-making machinery goes into high gear • When ATP levels rise high enough, glucose6-phosphate is diverted into glycogen synthesis (mainly in liver and muscle) • Glycogen is the main storage polysaccharide in animals Using Glycogen • When blood levels of glucose decline, pancreas releases glucagon • Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood • (Muscle cells do not release their stored glycogen) Energy Reserves • Glycogen makes up only about 1 percent of the body’s energy reserves • Proteins make up 21 percent of energy reserves • Fat makes up the bulk of reserves (78 percent) Energy from Proteins • Proteins are broken down to amino acids • Amino acids are broken apart • Amino group is removed, ammonia forms, is converted to urea and excreted • Carbon backbones can enter the Krebs cycle or its preparatory reactions Energy from Fats • Most stored fats are triglycerides • Triglycerides are broken down to glycerol and fatty acids • Glycerol is converted to PGAL, an intermediate of glycolysis • Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle GLYCOLYSIS outer mitochondrial compartment glucose inner mitochondrial compartment ATP 2NAD+ 2 PGAL ATP 2 NADH ATP a In glycolysis, 2 ATP used; 4 ATP form by substratelevel phosphorylation. So net yield is 2 ATP. 2 NADH for in the cytoplasm 2 2 pyruvate ATP b In Krebs cycle of second stage, 2 ATP form by substrate-level phosphorylation. cytoplasm 2 CO2 2 FADH2 2 acetyl-CoA 2 NADH ATP 2 KREBS CYCLE Electrons and hydrogen from cytoplasmic NADH are shuttled into inner compartment. Two coenzymes already inside transfer the electrons to a transport system. 4 c In third stage, NADH from glycolysis used to form 4 ATP by electron transport phosphorylation. Coenzymes (8 NAD+, 2 FAD total) transfer electrons and hydrogen from remnants of pyruvate to a transfer system. 6 NADH ATP 2 FADH2 ATP ATP ADP + Pi ATP 28 ATP Electrons flow through transport system Transport system pumps H+ to the outer compartment ELECTRON TRANSPORT PHOSPHORYLATION d In third stage, NADH and FADH2 from second stage used to make 28 ATP by electron transport Phosphorylation. 36 ATP At ATP synthases H+ flowing back in drives ATP formation Fig. 7.8, p. 139 FOOD fats fatty acids glycogen glycerol complex carbohydrates proteins simple sugars, e.g., glucose amino acids NH glucose-6-phosphate 3 urea carbon backbones PGAL 2 ATP 4 ATP GLYCOLYSIS pyruvate NADH acetyl-CoA NADH CO2 NADH FADH2 KREBS CYCLE e- 2 ATP CO2 ATP ELECTRON TRANSPORT PHOSPHORYLATION H+ e- oxygen ATP ATP many ATP water Fig. 7.12, p. 121