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Lecture11 - MCDB321 Introductory Plant Physiology MCDB321...

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Unformatted text preview: MCDB321 Introductory Plant Physiology MCDB321 Lecture 10, Feb 15, 2011 Lecture Plant Respiration Plant The chemical equation for respiration is: C6H12O6 + 6O2------> 6CO2 + 6H2O Glucose + Oxygen ------> Carbon Dioxide + Water I. OVERVIEW OF PLANT RESPIRATION II. STARCH AND SUCROSE BREAKDOWN [In cytosol or stroma of chloroplast] A. hydrolytic breakdown of starch B. phosphorylytic breakdown of starch C. sucrose is hydrolyzed to hexoses III. GLYCOLYSIS AND FERMENTATION conversion of hexoses to pyruvate [In cytosol…] IV. THE CITRIC ACID CYCLE (Krebs or tricarboxylic acid cycle) [Inside of mitochondria] V. RESPIRATORY ELECTRON TRANSPORT and OXIDATIVE PHOSPHORYLATION [on the inner membrane] Aerobic Respiration is a redox process by which electrons are transferred from glucose (oxidized) to oxygen (which is reduced) Three Stages of Respiration: 1) glycolysis, 2) the citric acid cycle, 3) electron transport chain/ chemiosmosis. 1. Preparation Phase Preparation Conversion of sucrose/starch to glyceraldehyde-3-phosphate Conversion Requires at least two ATP molecule/glucose or Requires 1 ATP for a 3C-compound ATP In plants carbohydrate is stored as starch, a homopolymer of glucose. It is composed of branched macromolecules (amylopectin) and a linear polymer (amylose.) The linear amylose forms a helix and these helices stack to form impenetrable sheets. A common test for starch is a blue complex formed with iodine, and this reaction is based on the ability of iodine to fit inside the helix structure. It can be digested by amylases, which are found in saliva and secreted from the pancreas. Amylase cleaves the alpha 1-> 4 bonds at random, releasing di and tri maltose molecules and dextrin at the branch points. 1. The release of soluble glucans requires phosphorylation of starch 1. 2. Debraching enzymes converted amylopectin into amylose 3 . β-amylase catalyzes hydrolysis while phosphohydrolase catalyzes phosphorolysis amylose[glucose]n + H2O ----> amylose [glucose]n-2 + maltose Amylose[glucose]n + Pi -------> amylose [glucose]n-1 + glucose-1-P Maltotriose 2[glucose]3 ----> maltopentaose [glucose]5 + glucose • Matose [glucose] + branched glucan [glucose] ---- glucose + branched glucan [glucose] • in the cytosol. glyceraldehyde-3-phosphate (GAP) is converted to pyruvate pyruvate generating 1 NADH and 2 ATP molecules Net energy production of glycolysis: 1 NADH + 1 ATP (1 ATP is needed in the preparation phase Three routes of pyruvate pyruvate 1. TCA cycle-aerobic oxidation 2. Lactate fermentation anaerobic 3. Ethanol fermentation to regenerate NAD+ needed for glycolysis The efficiency of energy conversion for anaerobic fermentation is <4% (4 X 50 kJ/mol(ATP) / 5760 kJ/mol(sucrose) With the fermentation operating, only 1 ATP is produced for every 1 molecule of GAP oxidized by glycolysis. Alternative pathway for mobilizing Alternative phosphoenopyruvate (PEP) (in the cytosol) costing 1 NADH molecule? catalyzed by two enzymes PEP Carboxylase Malate Dehydrogenase Malate producing two 4C-acid producing Oxaloacetate and Malate pentose phosphate pathway erates NADPH and toses (5-carbon sugars). luble enzymes in cytosol & chloroplasts) o phases: Two consecutive oxidative reactions, losing 1 CO2, and producing 2 NADPH (no ATP production) Conversion of ribulose-5-phosphate to glycolytic intermediates: F6P and GA3P cose-6-P + 2 NADP+ + H2O----> ulose-5-P + CO2 + 2 NADPH + 2H+ jor Functions: Production of NADPH [60% NADPH production in human] Supply of biosynthetic subbstrates Ribose-5-P: RNA/DNA Erythrose-4-P aromatic AA Generation of Calvin Cycle Intermediates C5 + C5 ---> C3 + C7 ---> C6 + C4 C5 C5 C6 C5 + C4 ---> C6 + C3 C6 C3 Mitochondria are semi-autonomous organelles Mitochondria Outer membrane Inner membrane: cristae Pyruvate entering Mt requires a specific transporter Pyruvate-/OH- antiporter on the inner membrane 1 OH- moving out is equivalent of 1 H + entering the Mt matrix 1 A large enzyme complex Is a 3-step process: decarboxylation, oxidation, & conjugation to CoA Mitochondria Matrix Mitochondria 2 3 Two successive Two oxidative decarboxylation 6C---> 4C + 2CO2 2 NADH At this point, 3 CO2 are produced from 1 mol of pyruvate (12 CO2 from 1 Sucrose) Regeneration of 5 Regeneration (OAA) Oxaloacetate Oxaloacetate is a membrane-bound protein complex 4 Substrate-level phosphorylation Substrate-level ATP instead of GTP(animal) 1 Pyruvate ---> 3CO2 Pyruvate 4 NADH NADH 1 FADH2 FADH 1 ATP The Citric Acid Cycle of Plants The Has Unique Features Has 1. Substrate level phosphorylation leads to the 1. formation of ATP instead of GTP formation 2. The matrix of plant mitocondria contains 2. NAD+-malic enzyme NAD two implications: two A. provides an alternative pathway for metabolism of PEP derived from glycolysis of B. makes it possible for complete net oxidation citric acid cycle intermediates. citric Mitochondria Matrix cytosol cytosol C. The malate produced from PEP can replenish intermediates of the TCA cycle. intermediates Anaplerosis Anaplerosis The plant Mt innermembrane carries The a malate2- /Pi2- antiporter that moves malate into the Mt matrix; It also has a malate2-/citrate2- antiporter that It moves citrate2- into the Mt matrix while moving malate2- out Electron Transport and ATP Synthesis at the Inner Mitochondria Membrane at Two additional important transporters are needed for ATP synthesis Two Pi-/OH- antiporter & ADP3-/ATP4- antipoter Some Electron Transport Enzymes Are Unique to Plant Mitochondria Both animals and plants have UCPs 1. External (rotenone-insensitive) NAD(P)H dehydrogenases 1. can accept electrons direct from NAD(P)H produced in the cytosol can [FAD+ --> FADH2] [FAD 2. Rotenone-insensitive 2. NAD(P)H dehydrogenases NAD(P)H exist on the matrix side of the inner membrane the The structure and regulatory features of the alternative oxidase in plant mitochondria 1. 2. Its transcription is induced by stresses Its activity is regulated by oxidation/reduction of a disulfide bridge, the reduction level of Ub , & disulfide the [pyruvate] Major functions of the alternative oxidase: 1. Adjust the rates of ATP synthesis and synthesis of carbon skeletons 2. Prevent a potential overreduction of the Ub pool 3. Heat production 4. Acclimation of plant growth at low temperature Heat Production Arum (skunk cabbage) Nelumbo (sacred lotus) Temperatures of the receptacle (Tr) and ambient air (Ta), and rates of oxygen consumption and heat production, throughout a complete sequence of flowering. Col-0: wild-type Col-0: XX-2, E-9 AOX-overexpression AS-12: antisense AOX line ...
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