9/16/13 1 Chapter 14 Energy generation in mitochondria and chloroplasts Be Able To • Predict the outcome of oxidative phosphorylation if the mitochondrial inner membrane is disrupted. Explain why. • Compare/contrast electron transport chain in mitochondria to the electron transport photosystem in chloroplasts. • Compare/contrast the citric acid cycle with the Calvin cycle. • Identify the electron donor and terminal electron acceptor in the mitochondria and chloroplasts. • Explain the role of NADH dehydrogenase, cytochrome oxidase and rubisco. • Identify the sub-cellular locations of various processes and enzymes critical to cellular respiration and photosynthesis. • Explain the relationship between photosynthesis and respiration in plants.9/16/13 2 Be Able To • Identify the products of Calvin Cycle. • How does the light harvesting complex convert light energy into useful molecule for cells? • Compare/contrast the role of O2 and CO2 in photosynthesis and cellular respiration. • Describe the role for the two stages of photosynthesis. mitochondrial matrix9/16/13 3 Energy released during passage of electrons through the electron-transport chain is harnessed to pump protons across the mitochondrial inner membrane - How does this work? NAD+ is regenerated O2 is the terminal electron acceptor Reductions often involve the transfer of a proton (charge neutralization) The orientation of electron carrier complexes in the membrane allows release of the proton to the opposite face of the membrane when the electron is released (oxidation)9/16/13 4 The energy released by electron transfer can be quantified Redox Potential See Panel 14-1 Oxidation-reduction (redox) reactions proceed depending on ΔG just like any other chemical reaction. Standard redox potentials (E0’) are also defined in a way similar to ΔGo Reduced and Oxidized molecules are equimolar, pH is 7.0 Compared to a reference (2H+ + 2e- ⇔ H2) The voltage difference (compared to the reference) is measured Compounds with negative redox potentials have low affinity for electrons and are likely to donate them (NADH, E0’= -320 mV) Electrons flow to more positive redox potentials, releasing energy ΔGo= -nF ΔE0’ Each step of the electron-transport chain has a more positive redox potential so the direction of electron flow is constant and energy is harnessed at each step until the terminal electron acceptor is reached: O2 + 4H+ + 4e- ⇔ H2O E0’= +820 mV9/16/13 5 Electrons are transferred to tightly bound metal atoms within the transmembrane enzyme complexes of the electron-transport chain Iron-sulfur centers - NADH dehydrogenase Heme (iron atoms) - cytochrome b-c1, cytochrome oxidase Copper atoms - cytochrome oxidase Electrons are carried between the different complexes by mobile electron carriers: Ubiquinone (hydrophobic, not part of a protein) Cytochrome c (heme, part of small membrane-associated protein) The electron carriers can diffuse along the membrane to accept and donate electrons to the enzyme complexes How are electrons transferred? 14_22_Iron_sulfur.jpg Iron-sulfur centers are used to transfer electrons within the NADH dehydrogenase complex9/16/13 6 14_20_Quinones.jpg Membrane-diffusible ubiquinone carries electrons between electron-transport chain enzyme complexes 14_24_Cytochrome_ox.jpg Cytochrome oxidase catalyzes the reduction of O2 This is the only step in respiration where O2 plays a direct role9/16/13 7 14_12_deltaV_deltapH.jpg ΔG = RT ln [H+]inside/[H+]outside + FEM Proton pumping produces a large electrochemical force The proton electrochemical gradient drives ATP synthesis - chemiosmotic coupling9/16/13 8 The proton electrochemical gradient also drives coupled transport Net yield of ATP from respiration is variable - dependent on concentration of reactants and products - account for energy spent for coupled transport Estimates (from 1 molecule of glucose): Glycolysis 2 ATP 2 ATP 2 NADH 3 ATP Pyruvate dehydrogenase 2 NADH 5 ATP Citric acid cycle 6 NADH 15 ATP 2 FADH2 3 ATP 2 GTP 2 ATP Net yield ~30 ATP9/16/13 9 Electrons are transferred from FADH2to ubiquinone See question 14-19 What happens if the proton gradient is dissipated? Uncouplers See question 14-19/16/13 10 mitochondrial matrix Chloroplasts have three membranes9/16/13 11 Compare and contrast structural features Two stages of photosynthesis 1. Light energy captured and used to transfer electrons from H2O to an electron-transport chain generating O2, NADPH and a proton gradient that ultimately yields ATP. 2. Chemical energy of ATP and NADPH used to convert CO2 to carbohydrate.9/16/13 12 Chlorophyll is the primary light-absorbing pigment When chlorophyll molecules absorb photons, an electron is raised to an orbital with higher energy When this energy is released, the chlorophyll returns to its unexcited, ground state. Energy could be released in several ways, but only electron transfer can be converted to chemical work: 2H20 + 4chl+ → 4H+ + O2 + 4chl9/16/13 13 14_34_photosystem.jpg Light-absorbing chlorophyll is located in a photosystem Transmembrane complex of proteins and organic pigments Plants use two photosystems for electron transport Charge separation Electron transfer Water splitting NADP+ is the terminal electron acceptor, yielding NADPH H+ gradient established9/16/13 14 Very electropositive Second stage of photosynthesis Carbon Fixation The ATP and NADPH generated from the first stage of photosynthesis is used to synthesize carbohydrate from CO2 and water (Calvin cycle). Reactions take place in the chloroplast stroma and can continue in the dark until all ATP and NADPH is exhausted A cycle but the net product is glyceraldehyde 3-phosphate Converted to sucrose in the cytosol Feeds into glycolysis in the cytosol9/16/13 15 Ribulose bisphosphate carboxylase (Rubisco) May be the most abundant protein on Earth 50% of chloroplast protein The carbon of CO2 ends up in the carboxyl group of glyceraldehyde 3-phosphate Carbon fixation is energetically expensive9/16/13 16 Compare the flow of protons and the orientation of the ATP synthase in mitochondria and chloroplasts9/16/13 17 Chloroplasts and mitochondria work together to supply plant cells with metabolites and ATP Origin of organelles and eukaryotic cells9/16/13 18 Endosymbiotic Theory (aerobic) Endosymbiotic Theory (cyanobacterium)9/16/13 19 What is the evidence supporting the