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Cellular Respiration Lecture 8 Fall 2008 Overview of Cellular Respiration • All organisms need ATP to do cellular work • Cellular Respiration: – The conversion of chemical energy of carbon compounds into another form of chemical energy, ATP 1 2 Overview of Cellular Respiration Cellular respiration • Metabolic pathway: Catabolic – Series of multiple reactions – Specific enzymes catalyze each reaction • Function – To generate ATP for cellular work Three metabolic stages of cellular respiration • Glycolysis • The citric acid (Kreb’s) cycle • Oxidative phosphorylation Overview of Cellular Respiration Each process occurs in a specific area • Glycolysis – Cytosol of cell • Citric acid cycle – Matrix of mitochondria • Oxidative phosphorylation – Inner membrane of mitochondria Fig. 9.6 3 4 Mitochondria • Two membranes –outer & inner • Intermembrane space –bound within the inner and outer membranes • Matrix – bound within the inner membrane • Cristae – multiple infoldings of the inner membrane – Increases surface area Fig. 6.17 5 Overview of Cellular Respiration 1. Gycolysis • Glucose split into two molecules of pyruvate 2. The citric acid cycle • Acetate (derivative of pyruvate) broken down into CO2 3. Oxidative phosphorylation: electron transport & chemiosmosis • Electrons moved from NADH to oxygen • ATP produced Fig. 9.6 6 Overview of Cellular Respiration Purpose of Cellular Respiration •To produce ATP Fig. 9.6 – Some ATP generated at each step – Most ATP generated during electron transport (90%) •Oxidative phosphorylation – Production of ATP using energy derived from the redox reactions of an electron transport chain with oxygen as its final electron acceptor •Substrate-level phosphorylation – Production of ATP by an enzyme directly transferring a phosphate group from an intermediate substrate to ADP Fig. 9.7 Stage 1:Glycolysis Glycolysis (“splitting of sugar”) • Occurs in cytosol of cell • One glucose (6-carbon sugar) converted to 2 pyruvate molecules (3-carbon sugar) • Input – One glucose molecule – 2 ATP molecules • Output – 2 pyruvate molecules + 2 H2O – 2 ATP molecules (net: 4 made, but 2 spent) – 2 NADH (total of 4 electrons) + 2H+ • For electron transport chain 7 8 Stage 1:Glycolysis Two phases • Energy investment phase • Energy payoff phase Glycolysis: Energy Investment Phase Energy investment phase • 2 ATP molecules spent • Enzymes needed for each step • Hexokinase – Phosphorylates glucose – Charge prevents glucose from leaving cell – Makes glucose more chemically reactive • Phosphofructokinase – Phosphorylates intermediary (Fructose-6phosphate) – 6-carbon sugar with a phosphate group on each end • Aldolase – Splits 6-carbon sugar to 2 3-carbon sugars (one of which is glyceraldehyde-3-phosphate) Fig. 9.9 9 Glycolysis: Energy Investment Phase 10 Regulation : Phosphofructokinase • Inhibited by high concentrations of ATP • Stimulated by high concentrations of ADP (AMP: adenosine monophosphate) • Allosteric Fig. 9.21 Glycolysis: Energy Payoff Phase • Triose phosphate dehydrogenase – Oxidizes glyceraldehyde-3-phosphate • 2 electrons transferred to NAD+ (now NADH) – Exergonic reaction • Energy from redox reaction used to phosphorylate glyceraldehyde-3-phosphate • Phosphoglycerokinase – Transfers phosphate from intermediary (1,3 Bisphosphoglycerate) to ADP – Substrate-level phosphorylation • Intermediary is now an organic acid (no longer a sugar) Fig. 9.9 11 Glycolysis: Energy Payoff Phase • Enolase – Increases the potential energy of the intermediary by removing water, causing double bond to form (phosphoenolpyruvate -PEP) • Pyruvate kinase – Transfers phosphate group from PEP to ATP • Pyruvate is end product Fig. 9.9 12 Stage 2: The Citric Acid Cycle “Pre” citric acid cycle • Pyruvate transported into mitochondria • Carboxl group removed • 2-carbon compound oxidized to acetate – NAD+ reduced • Acetate bound to Coenzyme A = Acetyl CoA – Coenzyme A: carrier molecule that makes acetic acid more reactive (high potential energy) Fig. 9.10 13 14 Stage 2: The Citric Acid Cycle Regulation: Pyruvate dehydrogenase • Enzyme complex where pyruvate transformed to acetyl CoA • Requires vitamin B-complex as coenzymes • Inhibited by high concentration of ATP – Becomes phosphorylated/inactive • Stimulated by ADP (AMP) • Allosteric Fig. 9.10 15 Stage 2: The Citric Acid Cycle Citric Acid (Kreb’s) Cycle • Occurs in matrix of mitochondria • Acetyl CoA broken down into CO2 • Input (for every one glucose molecule) – 2 acetyl CoA molecules • Output – 6 CO2 (includes CO2 released during “pre” stage) – 2 ATP • (GTP – guanosine triphosphate) – 6 NADH & 2 FADH2 • To electron transport chain Fig. 9.11 Stage 2: The Citric Acid Cycle 16 • Acetyl CoA attached to oxaloacetate to form citrate • Oxaloacetate regenerated by intermediaries in cycle • ATP (GTP) produced by substrate level phosphorylation • Redox reactions – NAD+ reduced to NADH – FAD reduced to FADH2 Fig. 9.12 Stage 2: The Citric Acid Cycle 17 Regulation • Enzyme that converts acetyl CoA to citrate inhibited by ATP – Allosteric • Enzyme that converts isocitrate to a-ketogluterate inhibited by NADH – Competitive Fig. 9.12 18 Stage 3: The ETC & Chemiosmosis Majority of ATP produced in this stage Electron Transport Chain • Many protein complexes & nonprotein components embedded in the inner membrane of the mitochondria – High surface area (cristae), so many ETCs • Electrons provide by NADH & FADH2 – FADH2 adds electrons further down ETC – provides less energy • Electrons ”fall” down chain: Redox reactions – H2O formed • Produces energy at each step Fig. 9.13 19 Stage 3: The ETC & Chemiosmosis • Transfer of electrons activates pumping of H+ • H+ pumped from matrix, across the inner membrane, and into the intermembrane space of the mitochondria • Creates a concentration gradient (driving force) of H+ across the inner membrane Fig. 9.16 Stage 3: The ETC & Chemiosmosis Chemiosmosis • Concentration gradient of H+ harnessed to do work • Through ATP synthase – Enzymes generate ATP from ADP + P = phosphorylation – Oxidative phosphorylation Fig. 9.14 20 21 ATP Production ~ 38 ATP created per one glucose molecule • Glucose = 686 kcal/mol • ATP = 7.3 kcal/mol • 40% efficiency Fig. 9.17 22 Versatility of Cellular Respiration • Glucose is not the only molecule used in cellular respiration Fig. 9.20 23 Anabolic Pathways • Cellular respiration supplies precursor molecules for anabolic pathways Metabolic Diversity What about organisms that live in areas with no oxygen? – Anaerobic - lacking oxygen • They need to produce ATP, but do not have oxygen to act as an electron acceptor • Use anaerobic respiration or fermentation • Anaerobic respiration – Uses a different final electron acceptor (not oxygen) in ETC • E.g., sulfate ion • Fermentation – A process that makes a limited amount of ATP from glucose without using an ETC or oxygen – Utilizes glycolysis – Requires recycling of NADH to NAD+ (typically done in ETC) 24 25 Metabolic Diversity • Alcohol fermentation – Regeneration of NAD+ produces ethanol – Releases CO2 • Yeast: bread, beer • Lactic acid fermentation – Regeneration of NAD+ produces lactate – No release of CO2 • Cheese, yogurt Use of fermentation in humans? • Less efficient than cellular respiration – 2 ATP vs. ~38 ATP Fig. 9.18 Metabolic Diversity Obligate aerobes • Require O2 for cellular respiration Facultative anaerobes • Use O2 in cellular respiration if present • Use fermentation in an anaerobic environment Obligate anaerobes • Must live in anaerobic environment – poisoned by O2 • Use fermentation or anaerobic respiration 26 Evolution of PS & CR • Oldest prokaryote ~3.5 bya • No oxygen in atmosphere until ~2.7 bya • Glycolysis – Occurs in matrix – Does not require ETC or O2 – Enzymes from glycolysis observed in almost all bacteria, archaea & eukaryotes studied Fig. 26.10 27 Evolution of PS & CR • Evolution of electron transport systems > 3 bya – Evolution of photosystems – Evolution of anaerobic respiration • Development of atmospheric oxygen > 2.7 bya – Cyanobacteria – Evolution of aerobic respiration • Evolution of eukaryotes~ 2.1 bya 28 Endosymbiosis Endosymbiosis • One organism of a certain species lives inside another organism of a different species • Process where unicellular organisms engulf other unicellular organisms Endosymbiosis theory • The theory that mitochondria and chloroplasts evolved from prokaryotes that were engulfed by host cells and took up a symbiotic existence within those cells. • Over time, evolved into organelle of the cell 29 30 Endosymbiosis Mitochondria • Formed when early anaerobic eukaryotic cell engulfed an aerobic bacterium • Benefits? Plastids • Formed when early eukaryotic cell engulfed a photosynthetic cyanobacterium • Benefits? Mitochondria evolved before plastids – All eukaryotes have mitochondria or remnants of mitochondria – Not all eukaryotes have plastids Endosymbiosis Evidence Mitochondria & chloroplasts: • Similar size to bacteria • Have own ribosomes, similar to bacterial ribosomes • Inner membranes have enzymes and transport systems homologous to living prokaryotes • Reproduction - binary fission • Circular DNA with few or no proteins • Mitochondrial DNA sequencing and ribosomal RNA sequencing from chloroplasts support the structural and molecular evidence 31