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Chapter 7 How Cells Release Chemical energy Overview of Carbohydrate Breakdown Pathway Plants and all other photoautotrophs get energy from the sun, heterotrophs get by eating plants and one another ATP is a common energy currency that drives metabolic reactions in cells Pathways of Carbohydrate Breakdown Starts with glycolysis in the cytoplasm – Convert glucose and other sugars to pyruvate Comparison of main pathways Fermentation pathway Aerobic Respiration Produces ATP under anaerobic conditions Ends in cytoplasm, do not use oxygen, yields 2 ATP per molecule of glucose Produces ATP by using oxygen Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule Oxygen acts as the final acceptor of electrons used during these reactions Overview of Aerobic Respiration Three main stages of aerobic respiration: 1. Glycolysis 2. Krebs cycle 3. Electron transfer phosphorylation Summary equation: C6H12O6 + 6O2 → 6CO2 + 6 H2O glucose Cytoplasm glucose 2 ATP ATP GLYCOLYSIS 4 ATP ATP (2net) 2 NADH 2 pyruvate The first stage, glycolysis, occurs in the cell’s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. During the reactions, 2 NAD+ pick up electrons and hydrogen atoms, so 2 NADH form. Mitochondrion Krebs Cycle 6CO2 2 ATP ATP 8 NADH, 2 FADH2 ATP oxygen Electron Transfer Phosphorylation 32 ATP The second stage, the Krebs cycle and a few steps before it, occurs inside mitochondria. The 2 pyruvates are broken down to CO2, which leaves the cell. During the reactions, 8 NAD+ and 2 FAD pick up electrons and hydrogen atoms, so 8 NADH and 2 FADH2 form. 2 ATP also form. The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 donate electrons and hydrogen ions at electron transfer chains. Electron flow through the chains sets up H+ gradients that drive ATP formation. Oxygen Overview of aerobic respiration Glycolysis – Glucose Breakdown Starts First step – Conversion of glucose to pyruvate Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate Reactions transfer electrons and hydrogen atoms to two NAD+ (reduces to NADH) 2 ATP is formed by substrate-level phosphorylation Products of Glycolysis Net yield of glycolysis: – 2 pyruvate, 2 ATP, and 2 NADH per glucose Pyruvate may: – Enter fermentation pathways in cytoplasm – Enter mitochondria and be broken down further in aerobic respiration ATP Requiring Steps Glycolysis glucose ATP ADP P glucose–6–phosphate ATP ADP p P fructose–1,6–bisphosphate ATP Generating steps 2 PGAL 2 NAD+ + 2 Pi NADH 2 reduced coenzymes 2 PGA 2 ADP ATP 2 ATP produced by substrate-level phosphorylation 2 PEP 2 ADP ATP 2 pyruvate to second stage 2 ATP produced by substrate-level phosphorylation Net 2 ATP + 2 NADH Second Stage of Aerobic Respiration Krebs Cycle Break down of pyruvate to Carbon dioxide The second stage of aerobic respiration takes place in the inner compartment of mitochondria It starts with acetyl-CoA formation and proceeds through the Krebs cycle outer membrane (next to cytoplasm) glucose (glycolysis) 2 pyruvate OUTER COMPARTMENT inner membrane inner mitochondrial compartment outer mitochondrial compartment (in between the two membranes) a An inner membrane divides a mitochondrion’s interior into two compartments. The second and third stages of aerobic respiration take place at this membrane. 2 acetyl–CoA Krebs Cycle CO2 ATP NADH FADH2 INNER COMPARTMENT Breakdown of 2 pyruvate to 6CO2 yields 2 ATP. Also, 10 coenzymes are reduced (8 NADH, 2 FADH2). The coenzymes carry hydrogen ions and electrons to sites of the third stage of aerobic respiration. b The second stage starts after membrane proteins transport pyruvate from the cytoplasm, across both mitochondrial membranes, to the inner compartment. Six carbon atoms enter these reactions (in two pyruvate), and six leave (in six CO2). Many coenzymes form. Acetyl-CoA Formation Two pyruvates from glycolysis are converted to two acetyl-CoA Two CO2 leave the cell Acetyl-CoA enters the Krebs cycle Krebs Cycle In each turn of the Krebs cycle, one acetylCoA is converted to two molecules of CO2 After two cycles – Two pyruvates are dismantled – Glucose molecule that entered glycolysis is fully broken down Energy Products Reactions transfer electrons and hydrogen atoms to NAD+ and FAD – Reduced to NADH and FADH2 ATP forms by substrate-level phosphorylation – Direct transfer of a phosphate group from a reaction intermediate to ADP Net Results Second stage of aerobic respiration results in Six CO2, two ATP, eight NADH, and two FADH2 for every two pyruvates Adding the yield from glycolysis, the total is – Twelve reduced coenzymes and four ATP for each glucose molecule Coenzymes deliver electrons and hydrogen to the third stage of reaction Acetyl–CoA Formation pyruvate NAD+ coenzyme A NADH CO2 acetyl–CoA coenzyme A Krebs Cycle oxaloacetate citrate CO2 NAD+ Krebs Cycle NADH NADH NAD+ NAD+ FADH2 CO2 NADH FAD ADP + Pi ATP Third Stage: Aerobic Respiration’s Big Energy Payoff Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane Energy released by electrons flowing through the transfer chains moves H+ from the inner to the outer compartment H+ gradient builds up across the inner membrane H+ ions flow by concentration gradient back to the inner compartment through ATP synthases The Aerobic Part of Aerobic Respiration At the end of electron transfer chain oxygen and H+ , forming water Overall, aerobic respiration yields up to 36 ATP for each glucose molecule glucose Glycolysis you are here Krebs Cycle Electron Transfer Phosphorylation INNER COMPARTMENT H+ NADH FADH2 H+ H+ H+ H2O INNER MITOCHONDRIAL MEMBRANE H+ OUTER COMPARTMENT H+ H+ H+ 1/2 O2 ATP ADP + Pi H+ H+ H+ H+ H+ glucose 2 ATP 2 NAD+ Glycolysis 2 NADH ATP (2 net) 2 pyruvate CYTOPLASM OUTER MITOCHONDRIAL COMPARTMENT 2 NADH 2 CO2 2 NADH 6 NADH 2 FADH2 INNER MITOCHONDRIAL COMPARTMENT 2 acetyl-CoA 4 CO2 Krebs Cycle 2 ATP ADP + Pi Electron Transfer Phosphorylation H+ water + H+ H H+ oxygen 32 ATP H+ Anaerobic Energy-Releasing Pathways Fermentation Pathway Begins with glycolysis and ends in the cytoplasm Do not use oxygen or electron transfer chains Two ATP is formed from glycolysis Final steps do not produce ATP; only regenerate oxidized NAD+ required for glycolysis to continue glucose Glycolysis you are here Fermentation Pathway Anaerobic Pathways Alcoholic fermentation End product: Ethyl alcohol (or ethanol) Pyruvate(3 carbon) is broken down Enzyme splits pyruvate into two - carbon acetaldehyde and carbon dioxide Alcoholic fermentation Role of Yeast in fermentation They are unicellular fungi Sacchromyces cerevisiae (Baker’s yeast) Cells release carbon dioxide in fementation and dough expands Other strains are used in the production of wine Alcoholic Fermentation Anaerobic Pathways Lactate fermentation End product: Lactate Conversion of pyruvate into three carbon lactate (lactic acid) Fermenters such as lactobacillus acidophillus can ferment milk, butter milk, cheese, yogurt Glycolysis glucose 2 NAD+ 2 ATP 2 NADH 4 ATP pyruvate Lactate Fermentation 2 NADH 2 NAD+ lactate The Twitchers Slow-twitch and fast-twitch skeletal muscle fibers can support different activity levels Aerobic respiration and lactate fermentation proceed in different fibers of muscles These pathways yeilds ATP for muscles Alternative Energy Sources Complex Carbohydrate break down They are broken down into simple sugars like glucose Glucose gets converted to glucose-6-phosphate. If the body doesn’t need glucose for energy,glucose-6-phosphate can be converted to glycogen for storage When blood sugar drops, glycogen is converted to glucose-6-phosphate and and enters the glycolysis pathway Alternative Energy Sources; Energy from Fats Most fat in the body are triglycerides Enzymes convert fat into glycerol and fatty acid Glycerol is converted into PGAL, an intermediate of glycolysis The carbon back bones of the fatty acid tail is broken apart, and fragments are converted into acetyl CoA, which can enter the Krebs cycle Energy from Proteins Enzymes split proteins into amino acid subunits The amino group is removed and becomes ammonia, then urea Urea is excreted Carbon Back bones can enter at several different points of Krebs cycle FOOD fats fatty acids COMPLEX CARBOHYDRATES glycerol glucose, other simple sugars PROTEINS amino acids acetyl-coA acetyl-coA PGAL Glycolysis NADH pyruvate oxaloacetate or another intermediate of the Krebs Krebs Cycle NADH, FADH2 Electron Transfer Phosphorylation Life’s Unity Photosynthesis and aerobic respiration are interconnected on a global scale In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels sunlight energy in Photosynthesis Driven by energy input from the sun, electrons and hydrogen are used to form ATP. ATP energy drives the synthesis of glucose from hydrogen, electrons (delivered by coenzymes), and carbon dioxide’s atoms. energy out (heat) glucose (stored chemical energy) carbon dioxide, water oxygen Aerobic Respiration Energy input from two ATP initiates three stages of reactions. Many ATP form during thecomplete breakdown of glucose to carbon dioxide and water. energy out (heat) chemical energy in many ATP available to drive nearly all cellular tasks