Cellular Respiration. Overview of Cellular Respiration. Lecture 8 Fall Overview of Cellular Respiration. Overview of Cellular Respiration

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1 Overview of Cellular Respiration 1 Cellular Respiration Lecture 8 Fall 2008 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 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 2 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 Mitochondria 4 Overview of Cellular Respiration 5 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 Gycolysis Glucose split into two molecules of pyruvate 2. The citric acid cycle Acetate (derivative of pyruvate) broken down into CO 2 3. Oxidative phosphorylation: electron transport & chemiosmosis Electrons moved from NADH to oxygen ATP produced Fig. 9.6

2 Overview of Cellular Respiration Purpose of Cellular Respiration To produce ATP 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. 9.6 Fig 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 H 2 O 2 ATP molecules (net: 4 made, but 2 spent) 2 NADH (total of 4 electrons) + 2H+ For electron transport chain 7 Stage 1:Glycolysis Two phases Energy investment phase Energy payoff phase 8 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-6- phosphate) 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 Glycolysis: Energy Investment Phase 10 Glycolysis: Energy Payoff Phase 11 Regulation : Phosphofructokinase Inhibited by high concentrations of ATP Stimulated by high concentrations of ADP (AMP: adenosine monophosphate) Allosteric 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 Fig. 9.9

3 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 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. 9.9 Fig 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 14 Citric Acid (Kreb s) Cycle Occurs in matrix of mitochondria Acetyl CoA broken down into CO 2 Input (for every one glucose molecule) 2 acetyl CoA molecules Output 6 CO 2 (includes CO 2 released during pre stage) 2 ATP (GTP guanosine triphosphate) 6 NADH & 2 FADH 2 To electron transport chain Fig Fig 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 Regulation Enzyme that converts acetyl CoA to citrate inhibited by ATP Allosteric Enzyme that converts isocitrate to a- ketogluterate inhibited by NADH Competitive Fig Fig. 9.12

4 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 & FADH 2 FADH 2 adds electrons further down ETC provides less energy Electrons fall down chain: Redox reactions H 2 O formed Produces energy at each step Fig 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 Stage 3: The ETC & Chemiosmosis 20 ATP Production 21 Chemiosmosis Concentration gradient of H+ harnessed to do work Through ATP synthase Enzymes generate ATP from ADP + P = phosphorylation Oxidative phosphorylation ~ 38 ATP created per one glucose molecule Glucose = 686 kcal/mol ATP = 7.3 kcal/mol 40% efficiency Fig Fig Versatility of Cellular Respiration 22 Anabolic Pathways 23 Glucose is not the only molecule used in cellular respiration Cellular respiration supplies precursor molecules for anabolic pathways Fig. 9.20

5 Metabolic Diversity 24 Metabolic Diversity 25 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) Alcohol fermentation Regeneration of NAD+ produces ethanol Releases CO 2 Yeast: bread, beer Lactic acid fermentation Regeneration of NAD+ produces lactate No release of CO 2 Cheese, yogurt Use of fermentation in humans? Less efficient than cellular respiration 2 ATP vs. ~38 ATP Fig Metabolic Diversity Obligate aerobes Require O 2 for cellular respiration Facultative anaerobes Use O 2 in cellular respiration if present Use fermentation in an anaerobic environment Obligate anaerobes Must live in anaerobic environment poisoned by O 2 Use fermentation or anaerobic respiration 26 Evolution of PS & CR Fig Oldest prokaryote ~3.5 bya No oxygen in atmosphere until ~2.7 bya Glycolysis Occurs in matrix Does not require ETC or O 2 Enzymes from glycolysis observed in almost all bacteria, archaea & eukaryotes studied 27 Evolution of PS & CR 28 Endosymbiosis 29 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 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

6 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 30 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