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Respiration In open systems, cells require E to perform work (chemical, transport, mechanical) E flows into ecosystem as sunlight à autotrophs transform it into chemical E, O2 released as a byproduct à cells use some chemical E in organic molecules to make ATP à E leaves as heat Respiration: exergonic (releases E)C6H12O6 + 6O2 à 6H2O + 6CO2 + ATP (and heat) Photosynthesis: endergonic (requires E)6H2O + 6CO2 + ATP + Light à C6H12O6 + 6O2 Redox Reactions (oxidation-reduction)Xe- + Y à X + Ye-Where X is the electron donor. It loses an electron in oxidationWhere Y is the electron acceptor. It gains an electron in reduction Oxidation = lose an electronReduction = gain an electron Here in the reaction for respiration…C6H12O6 + 6O2 à 6H2O + 6CO2 + ATP (and heat)C6H12O6 is oxidized and O2 is reduced à 6H2O + 6CO2 + ATP (and heat) C6H12O6 is oxidized to CO2O2 is reduced to H2OThrough the process ATP is formed Energy Harvest-E is released as electrons ‘fall’ from organic molecules to O2-Broken down into steps: food (glucose) à NADH à ETC à O2 -coenzyme NAD+ = electron acceptor -NAD+ picks up 2 electrons and 2H+ à NADH (stores E) -NADH carries electrons to the electron transport chain (ETC) -ETC transfers electrons to O2 to make H2O-so NAD+ acts as an electron shuttle to help control the release of energy for ATP synthesis (otherwise it would be like an uncontrolled explosion) STAGES OF CELLULAR RESPIRATION1. Glycolysis – happens in cytosol2. Pyruvate oxidation + citric acid cycle (krebs cycle) – happens in mitochondria3. Oxidative phosphorylation (electron transport chain (ETC) snd chemiosmosis) – happens in mitochondria STAGE 1: GLYCOLYSIS (sugar splitting)-believed to be ancient (as early prokaryotes had no O2 available)-occurs in cytosol-partially oxidizes glucose (6C) to 2 pyruvates (3C)-net gain: 2 ATP + 2NADH-also makes 2 H2O-no O2 required Part 1 of Glycolysis: energy investment stage -cell uses ATP to phosphorylate compounds of glucosePart 2 of Glycolysis: energy payoff stage -two 3-C compounds are oxidized -for each glucose molecule:--2 net ATP produced by substrate-level phosphorylation--2 molecules of NAD+ à NADHSubstrate-level phosphorylation-generates small amount of ATP-phosphorylation: enzyme transfers a phosphate to other compounds-ADP + Pi à ATP Glycolysis summary: glucose à 2 pyruvate, 2 ATP, 2 NADH STAGE 2: PYRUVATE OXIDATION 2AND CITRIC ACID CYCLE-pyruvate oxidation happens across the inner mitochondrial membrane-citric acid cycle happens in the mitochondrial matrix-ETC is located on the inner mitochondrial membrane Pyruvate oxidation:As pyruvate crosses from the cytosol into the mitochondria…Pyruvate à Acetyl CoA (use to make citrate)Pyruvate is oxidized to Acetyl CoA, CO2 and NADH are produced in the process Citric Acid Cycle:-occurs in the mitochondrial matrix-Acetly CoA à citrate, CO2 released through a series of 8 steps1. Acetly CoA enters cycle, joined by oxaloacetate, forms citrate, CoA-SH released 2. citrate converted to isocitrate3. isocitrate converted to alpha-ketogluterate, NAD+ to NADH and H+, CO2 released4. alpha-ketogluterate converted to succinyl CoA, NAD+ to NADH and H+, CO2 released5. succinyl CoA converted to succinate, CoA-SH released, GDP + P to GTP coupled to ADP to ATP6. succinate converted to fumerate, FAD to FADH27. fumerate converted to malate8. malate converted to oxaloacetate, NAD+ to NADH and H+ -net gain: 2 ATP, 6 NADH, 2 FADH2 (electron carrier)-ATP produced by substrate-level phosphorylation-summary – inputs are 2 pyruvate à Acetly CoA, also 2 oxaloacetate outputs are 2 ATP, 8 NADH, 6 CO2, 2 FADH2 STAGE 3: OXIDATIVE PHOSPHORYLATION (ETC + CHEMIOS)-Electron transport chain: occurs in inner membrane of mitochondria, produces 26-28 ATP by oxidative phosphorylation via chemiosmosis-Chemiosmosis: H+ ions pumped across inner mitochondrial membrane, H+ diffuse through ATP synthase, ADPàATP Electron Transport Chain-collection of molecules embedded in inner membrane of mitochondria-tightly bound protein and non-protein components-alternate between reduced/oxidized states as accept/donate electrons-does not make ATP directly-as electrons move through the ETC, proton pumps move H+ across inner mitochondrial membrane into the inter-membrane space creating a proton gradient Chemiosmosis is the energy-coupling mechanism-chemiosmosis = H+ gradient across the membrane is used to drive cellular work-proton-motive force: use the proton gradient to perform work-ATP synthase = enzyme that makes ATP – uses E from proton gradient – the flow of H+ back across the membrane-as the H+ moves back across the membrane, E is used to phosphorylate ADP to ATP Per molecule of glucose:2 ATP are made in glycolysis2 ATP are made in the citric acid cycle26 or 28 ATP are made in oxidative phosphorylationtotal yield = 30 or 32 ATP per molecule of glucose Anaerobic Respiration – generates ATP using other electron acceptors besides O2-final electron acceptors: sulfate (SO4), nitrate, sulfur (produces H2S)-like obligate anaerobes – can’t survive in the presence of oxygen-facultative anaerobes – make ATP by aerobic respiration (with O2 present) or switch to fermentation (when no O2 available)-fermentation = glycolysis + regeneration of NAD+ With oxygen present-respiration: release E from breakdown of food w O2, occurs in mito, O2 required as the final electron acceptor, produces CO2, H2O, and up to 32 ATP Without oxygen present-fermentation: keeps glycolysis going by regenerating NAD+, occurs in the cytosol, no O2 needed, creates ethanol (+CO2) or lactate Types of fermentation:-Alcohol fermentation: pyruvate à ethanol + CO2 ex: bacteria, yeast used in brewing, winemaking, baking-Lactic acid fermentation: pyruvate à lactate ex: fungi, bacteria, human muscle cells used to make cheese, yogurt, acetone, methanol fyi: lactate build-up does not cause muscle fatigue and pain Various sources of fuel-carbs, fats and proteins can all be used as fuel for cellular respiration-monomers enter glycolysis or citric acid cycle at different points Phosphofructokinase:-allosteric enzyme that controls rate of glycolysis and citric acid cycle-inhibited by ATP and citrate-stimulated by AMPAMP + P + P à ATP
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