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Plant Respiration Respiration overview • Retrieval of energy captured in photosynthesis; oxidation of carbohydrates C6H12O6 + 6O2 6CO2 + 6H2O + Energy Overall pathway • 1) Glycolysis – Glucose converted to pyruvate • 2) Citric Acid Cycle (aka Krebs or TCA cycle) – Pyruvate converted to CO2 and electrons • 3) Electron transport chain – Electrons reduce O2 to H20 and create ATP Energy storage • Plants store energy in the form of the carbohydrates sucrose and starch • In stroma of chloroplast, enzymes such as -amylase and -amylase break complex sugars down • Products exported from chloroplast to cytosol through transport proteins in plastid envelope Glycolysis • Conversion of hexoses to pyruvate • Anaerobic process that occurs in the cytosol Glycolysis Glucose + 2NAD+ + 2ADP + 2P 2 Pyruvate + 2NADH + 2H+ 2H2O Fate of pyruvate • If oxygen is present (i.e., conditions are aerobic), pyruvate will cross the mitochondrial membrane and enter the mitochondria • If oxygen is absent (i.e. conditions are anaerobic), pyruvate will undergo fermentation Fermentation • Under anaerobic conditions, pyruvate + NADH converted to lactate or primarily ethanol • Recycles NAD needed to oxidize Glyceraldehyde 3 Phosphate during glycolysis • Less efficient energy production than respiration (2 ATP per glucose) The citric acid cycle • • • AKA Krebs or Tricarboxylic Acid (TCA) cycle In mitochondrial matrix... Complete oxidation of pyruvate to CO2 and NADH (8 enzymatic steps) The citric acid cycle • Begins with oxidative decarboxylation of pyruvate (3C) to acetyl CoA (2C), catalyzed by pyruvate dehydrogenase Citric acid cycle • • Acetyl CoA (2C) condenses with oxaloacetate (4C) to form citrate (6C) catalyzed by citrate synthase Successive oxidation and decarboxylation of citrate to regenerate oxaloacetate (OAA) Citric acid cycle OAA + Acetyl CoA + 3 H2O + ADP + P +3NAD- + FAD OAA + 2CO2 + CoA + ATP + 3NADH + 3H+ + FADH2 • Two entire turns of the cycle to metabolize the equivalent of one hexose (glucose) yields Electron transport chain and oxidative phosphorylation • What happens to NADH? • Across mitochondrial inner membrane... • Electrons move through series of proteins in the mitochondrial membrane • Electrons move from higher energy level to lower energy level, releasing energy in the process • O2 is final electron acceptor Electron transport chain • Proton gradient used to produce ATP through ATP synthase Electron transport chain • Net 3 ATP per NADH; 2 ATP per FADH2 – Yield 22 ATP • 18 from 6 NADH • 4 from 2 FADH2 Unique features of plant respiration • Plants have alternate membrane proteins in mitochondria Unique plant respiration • External NAD(P)H dehydrogenase -alternate source of electrons used to reduce CoQ • Complex I not involved, thus only 2 ATP can be produced • Internal rotenone-insensitive dehydrogenase - oxidizes internal membrane NADH only Plant respiration • Complex IV inhibited by CN, but much plant respiration continues with CN • Cyanide (CN)-insensitive respiration - insensitive to many respiratory inhibitors, uses alternative oxidase • Energy converted to heat, rather than ATP Unique plant respiration • Role in heat generation (thermogenesis in skunk cabbage) • Energy overflow metabolism when cytochrome pathway is saturated