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Chapter 9 Cellular respiration: Harvesting Chemical energy Figure 9.1 Energy flow and chemical recycling in ecosystems Figure 9.3 Methane combustion as an energy-yielding redox reaction Figure 9.4 Nicotinamide Adenine Dinucleotide + as an electron shuttle • • • • NAD+ is reduced to form NADH by gaining a hydrogen atom..and an electron to balance the charge The electron come from water therefore a proton is formed Therefore… NAD+ > NADH + H+ FAD +2H FADH2 Figure 6.8 The structure and hydrolysis of ATP Cell and Organismal Biology 2009 Figure 6.10 The ATP cycle Cell and Organismal Biology 2009 8.11 A review of how ATP drives cellular work Roles for ATP ATP also provides phosphates and energy to phosphorylate and therefore activate or deactivate proteins 2 ways to make ATP • Substrate level phosphorylation – – • Transfer of a phosphate from an organic molecule to ADP to form ATP Phosphoenolpyruvate (PEP) Oxidative phosphorylation/electron transport system – Use of NADH + H+ (reduced form) to provide energy to make ATP – Requires presence of oxygen (O2) Mitochondria Figure 9.6 An overview of cellular respiration (Layer 3) Glucose 6 5 1 4 3 2 C6H12O6 + 6O2 ---> 6CO2 + 6H2O Figure 9.9 A closer look at glycolysis: energy investment phase (Layer 2) Figure 9.9 A closer look at glycolysis: energy payoff phase (Layer 4) Figure 9.8 The energy input and output of glycolysis Metabolic Energy Production Summary C6H12O6 + 6O2 (36 ATP) 6H2O + 6CO2 + energy 1. Glycolysis: (Outside mitochondria in the cytoplasm) •Glucose (6-carbon) is phosphorylated to Fructose 1,6-bisphosphate (2 ATP used up) •F 1,6 BP split to two PGAL(G3P)(3-carbon each) •Two PGAL phosphorylated to two DPGA (BPG)(2 NADH +2H+ produced) •Two DPGA dephosphorylated to two pyruvate molecules (3-carbon) (4 ATP produced) Net Gain: 2 NADH + H+; 2ATP. (These two ATPs may be used up transporting the NADH + H+ to the mitochondria) Figure 9.6 An overview of cellular respiration (Layer 3) Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle (Citric Acid Cycle) 2. Preparation for Krebs cycle: (In mitochondria) CO2 removed from pyruvate as Acetyl-CoA formed (2 NADH + 2H+ produced per 2 pyruvate) Net Gain: 2 NADH + H+ Figure 9.11 A summary of the Krebs cycle Figure 9.11 A closer look at the Krebs cycle (Layer 4) 3. Citric Acid Cycle (Krebs Cycle, CAC) (In mitochondria) Acetyl-CoA combines with oxaloacetic acid (4-carbon) to give 6-carbon molecule. Molecule gradually rearranged and broken down. Co-A is re-used. Oxaloacetic acid regenerated for re-use. 2 x 3 NADH + H+ produced (2 x 3 because two Acetyl-CoA enter cycle per glucose) 2 x 1 FADH2 produced 2 x 1GTP produced (later converted to ATP) 2 x 2 CO2 released So, glucose completely broken down to CO2 and high energy compounds produced Net Gain 10 NADH + H+ 2 FADH2 2 ATP (via GTP) (This is used directly in the cell) Figure 9.6 An overview of cellular respiration (Layer 3) Figure 9.5 An introduction to electron transport chains Mitochondria Figure 9.14 ATP synthase, a molecular mill Figure 9.13 Free-energy change during electron transport • FMN • FeS • Q • Cyt Flavin Mononucleotide Iron Sulfur protein Ubiquinone Cytochromes Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis 4. Chemiosmotic ATP Synthesis: (In inner mitochondrial membrane) NADH + H+ releases electrons (2e-) plus hydrogen ions (2H+) to electron transport chain. NAD+ goes back to earlier stages and is reused. Electrons pass across the membrane three times, carrying 2H+ across each time, and leaving them between the inner and outer mitochondrial membranes. So each NADH +H+ carries six hydrogen ions across. Electrons from FADH2 carry four hydrogen ions across. FAD is also re-used in CAC. So, for each glucose molecule, 68 hydrogen ions are moved across the inner mitochondrial membrane. It takes 2 hydrogen ions moving through the ATP synthase enzyme to convert ADP + Pi to ATP (34 total.) The electron pairs must now combine with half of an O2 molecule, and two H+ to give H2O. This is the source of the water in the respiration equation. Net Gain 10 NADH + H+ 2 FADH2 2 ATP (via GTP) (This is used directly in the cell) Figure 9.16 Review: how each molecule of glucose yields many ATP molecules during cellular respiration 5. Conclusion So, 34 ATP are formed by chemisosmotic ATP synthesis. Add these to the net gain of 2 ATP from the Krebs Cycle to give a grand total of 36 ATPs formed from one molecule of glucose as it is broken down to six CO2 and 6H2O molecules. The ATP is then used elsewhere in the cell. Depending on how the cytosolic electrons are transferred to the mitochondria, yield may include the ATPs from Glycolysis (so, yield may be 38 ATP) Note that this is the theoretical yield. Actual yield is lower Figure 9.18 Pyruvate as a key juncture in catabolism Figure 9.17a Fermentation Figure 9.17b Fermentation Figure 9.20 The control of cellular respiration Figure 9.19 The catabolism of various food molecules Cell and Organismal Biology 2009 -oxidation • Invest ATP to prime system (-1ATP) • Harvest FADH2 (2ATP) • Harvest NADH (3ATP) • Net gain 4ATP • Plus 12 ATP from Krebs cycle