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Cellular Respiration: Harvesting Chemical Energy • Aerobic respiration consumes organic molecules and O2 and yields ATP • Fermentation is a partial degradation of sugars that occurs without O2 • Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O 2 Cell respiration Overall C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy ΔG° = - 686 kcal/mole released But broken down into more controlled release Stepwise oxidation of carbon Each step releases significant energy Hallmarks of Oxidation (C loses H and/or C gains an O) releases energy NAD+, NADP+ Nicotinamide adenine dinucleotide (NAD+) Electron shuttling NAD+ + 2H+ + 2e- NADH + H+ Dehydrogenases - Enzymes perform redox reactions using NAD+/NADH as substrate The Stages of Cellular Respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolysis Pyruvate Glucose Citric acid cycle Pyruvate oxidation Oxidative phosphorylation: electron transport & chemiosmosis Mitochondrion Cytosol ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation I. Glycolysis Location: Cytosol •Simplest metabolic pathway •Most cells do it •Considered most primitive biochemical process •10 steps 10 enzymes Glycolysis: Phase I - Energy Investment Phase ATP is used to convert glucose into G3P Glycolysis: Phase II - Payoff Phase G3P is oxidized releasing energy captured by NAD+ Released energy also used for substrate level phosphorylation Investment Phase 4 ADP & 2 NAD+ 2 ATP Glucose 2 G3P 2 ADP Overall for glycolysis: Payoff phase 2Pyruvate 4 ATP 2 NADH + H+ (C3) 3-carbon molecule Glucose + 2ADP + 2Pi + 2NAD+ 2pyruvate + 2ATP + 2 NADH + 2H+ Transported into mitochondrial matrix Substrate level phosphorylation Transfer of a phosphate from substrate to ADP to generate ATP R-P + ADP R + ATP • A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation • Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration Fermentation • Result of anaerobic conditions (for organisms with a mitochondria) always for bacter • Removes buildup of NADH & resupplies NAD+, NAD+ is needed for glycolysis • Removes pyruvate • Prevents the overall glycolytic pathway from reaching equilibrium • Many other forms including production of methane and hydrogen gas Obligate Anaerobes • organisms that produce ATP using fermentation • sometimes killed by O2 (no SOD) • Some bacteria Facultative anaerobes • Fermentation in absence of O2 • Respiration in presence of O2 Obligate aerobes • can only do respiration (humans) Intermembrane space Outer membrane Free ribosomes Inner membrane Cristae Matrix II. Pyruvate decarboxylation & The Citric Acid Cycle • • • • Location: Mitochondrial Matrix Complete the oxidation of carbon to Generate more NADH & FADH2 More substrate-level phosphorylation Pyruvate decarboxylation • • • • Pyruvate pumped into matrix Generation / release of CO2 Oxidation step NADH + H+ Remaining 2-carbon molecule delivered to citric acid cycle by CoenzymeA Citric acid cycle • Carbons are further oxidized, until fully oxidized and released as CO2 • Each of the 2 pyruvates from glycolysis generate: 1 ATP 3 NADH + H+ 1 FADH2 (similar to NADH but less energetic) III. Oxidative Phosphorylation • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food (Oxidation products) • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation Electron Transport • Location: Mitochondria cristae • Most of the chain’s components are proteins, which exist in multiprotein complexes Fe3+ Fe2+ • The carriers alternate reduced and oxidized states as they accept and donate electrons • Electrons drop in free energy as they go down the chain and are finally passed to O 2, forming H2O • Release energy is used to power H+-pumps that are the protein complexes that the cytochromes are part of • This builds a high H+ gradient within the limited space of the mitochondrial intermembrane space. • Ultimately electrons are used to reduce O2, which combines with H+ to generate H2O. Remember this ultimately began with NADH + H+ NAD+ + 2e- + 2H+ • All steps are coupled together, meaning: if O2 is not available then none of these reactions can occur Chemiosmosis / ATP synthesis • Location: Intermembrane space, cristae and matrix • Use of the H+-gradient is chemiosmosis • This potential energy drives the enzyme ATP synthetase • Called proton-motive force • Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration • Glycolysis accepts a wide range of carbohydrates • Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle