Main Fuel Molecule: GlucoseFuels: Molecules whose stored energy can be released for use.The most common fuel in organisms is glucose. Other molecules are first converted into glucose or other intermediate compounds.
How is Glucose Used to Make EnergyBurning or metabolism of glucose:C6 H12O6 + 6O2 → 6CO2 + 6 H 2O + free energyGlucose metabolism pathway traps the free energy in ATP: ADP + Pi + free energy → ATP
How is Glucose Used to Make EnergyΔG is the change in free energyΔG from complete combustion of glucose= –686 kcal/molHighly exergonic; drives endergonic formation of many ATP molecules.
Three metabolic pathways are involved in harvesting the energy of glucose:Glycolysis: glucose is converted to pyruvateCellular respiration: aerobic and converts pyruvate into H2O, CO2, and ATPFermentation: anaerobic and converts pyruvate into lactic acid or ethanol, CO2, and ATP
The Big Picture If O2 is present (aerobic): Glycolysis is followed by three pathways of cellular respiration: • Pyruvate oxidation • Citric acid cycle • Electron transport chain If O2 is not present (anaerobic): Pyruvate from glycolysis is metabolized by fermentation.
Redox ReactionsRedox reactions: One substance transfers electrons to another substanceReduction: Gain of one or more electrons by an atom, ion, or moleculeOxidation: Loss of one or more electronsAlso occurs if hydrogen atoms are gained or lost (H = H+ + e-)
Redox Reactions and GlucoseIn glucose combustion,glucose is the reducingagent, O2 is the oxidizingagent.Energy is transferred in aredox reaction.Energy in the reducingagent (glucose) istransferred to the reducedproduct.
Electron CarriersExamples: NAD, FADH2, NADPHCoenzyme NAD+ is a key electron carrier in redox reactions.Two forms: NAD+ (oxidized) NADH (reduced)
Complete vs. Incomplete Oxidation of Glucose Incomplete Oxidation Complete Oxidation
The five metabolic pathways occur in different parts of the cell.
GlycolysisTakes place in the cytosolConverts glucose into pyruvateProduces a small amount of energyGenerates no CO2
GlycolysisInvolves ten enzyme-catalyzed reactions.Energy-investing reactions: Require ATPEnergy-harvesting reactions: Yield NADH and ATP.Results (net per glucose): 2 molecules of pyruvate 2 molecules of ATP 2 molecules of NADH
“Energy Investing” Steps of GlycolysisSo what is a kinase?
Substrate Level PhosphorylationEnzyme-catalyzed transfer of a phosphate group from a donor to ADP to form ATP is called substrate-level phosphorylation.Phosphorylation: addition of a phosphate group.
Anaerobic ConditionsWithout O2, ATP can be produced by glycolysis and fermentation.Fermentation occurs in the cytosol, to regenerate NAD+.Pyruvate from glycolysis is reduced by NADH + H+.
Lactic Acid Fermentation• Occurs in microorganisms, some muscle cells• Pyruvate is the final electron acceptor• Lactate is the product and can build up
Alcohol Fermentation• Requires two enzymes to metabolize pyruvate to ethanol• Acetaldehyde is reduced by NADH + H+, producing NAD+ and glycolysis continues
Summary of Anaerobic RespirationCellular respiration yields more energy than fermentation per glucose molecule.• Glycolysis plus fermentation = 2 ATP• Glycolysis plus cellular respiration = 32 ATP• So why do it?
Aerobic Respiration: Pyruvate OxidationLinks glycolysis and the citric acid cycle; occurs in the mitochondrial matrixPyruvate is oxidized to acetate and CO 2 is releasedNAD+ is reduced to NADH, capturing energySome energy is stored by combining acetate and Coenzyme A (CoA) to form acetyl CoA
Pyruvate Oxidation Per Glucose: 2 NADH 2CO2 2 Acetyl-CoA
Citric Acid CycleInputs: acetyl CoA, water and electron carriers NAD+, FAD, and GDPEnergy released is captured by ADP and electron carriers NAD+, FAD, and GDPOutputs: CO2, reduced electron carriers, and ATP (really GTP)
Citric Acid CycleThe citric acid cycle is in steady state: The concentrations of the intermediates don’t change.The cycle continues when starting materials are available:• Acetyl CoA• Reoxidized electron carriers
Citric Acid Cycle Per Glucose: 6 NADH 2 FADH2 4 CO2 2 ATP
Recycle Electron CarriersThe electron carriers that are reduced during the citric acid cycle must be reoxidized to take part in the cycle again.Fermentation—if no O2 is presentOxidative phosphorylation—O2 is present
Oxidative PhosphorylationOxidative phosphorylation: ATP is synthesized by reoxidation of electron carriers in the presence of O2.Two stages:• Electron transport• Chemiosmosis
Electron Transport Chain (ETC)Electrons from NADH and FADH2 pass through the respiratory chain of membrane-associated carriers. Electron flow results in a proton concentration (Membrane Potential) gradient in mitochondria.
ETCThe respiratory chain is located in the inner mitochondrial membrane (cristae).Energy is released as electrons are passed between carriers.Examples: protein complexes I, II, III, IV; Cytochrome c, ubiquinone (Q)
Proton Motive Force “Membrane Potential”During electron transport protons are also actively transported.Protons accumulate in the intermembrane space and create a concentration gradient and charge difference— potential energy!This proton-motive force drives protons back across the membrane.
Proton Motive Force I, III, and IV Pump Protons
ChemiosmosisProtons diffuse back into the mitochondria through ATP synthase, a channel protein.Diffusion is coupled to ATP synthesis.Oxidative Phosphorylation is one example of Chemiosmosis
ATP SynthaseF0 subunit: transmembraneF1 subunit: projects into the mitochondrial matrix, rotates to expose active sites for ATP synthesis
Summary of Aerobic RespirationGlycolysis ETC4 ATP – 2ATP used =2 ATP NET NADH (TOTAL=10 x 2.5=25 ATP)2 NADH FADH2 (2 x 1.5= 3 ATP)(IF FERMENTATION use 2 NADH to Total ATP = 28reduce pyruvate)Pyruvate oxidation Total for Cell Respiration2 NADH 4 ATP by substrate level2 CO2 respirationTCA Cycle 28 ATP by oxidative2 ATP Phosphorylation6 NADH = 32 ATP4 CO22 FADH2