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Plants survive at night by respiration Sucrose is used for respiration, storage or construction. Developing and non-photosynthetic sink tissues depend on a supply of reduced carbon from the leaves. In this experiment 32P was used as a tracer to show which leaves were exporting metabolites via the phloem. What happens in the sink tissues? Invertase: enzyme that catalyzes the hydrolysis of sucrose Sucrose is a disaccharide composed of glucose and fructose The fate of sucrose in cells: Content of phloem and xylem exudates Sugars Phloem 100-300 mg ml-1 Amino acids 5-40 mg ml-1 Inorganics 1-5 mg ml-1 Total solutes 250-1200 mmol kg-1 Xylem 0 mg ml-1 0.1-2 mg ml-1 0.2-4 mg ml-1 10 -100 mmol kg-1 Ψs -0.6 to -3 MPa -0.02 to -0.2 MPa pH 7.3 - 8.0 5.0 - 6.3 Coke = 39 g of sugar/12 oz = 110 mg ml-1 and pH = 2.5 Respiration - the process of mobilizing organic compounds and oxidizing them. The released energy is transiently stored as ATP. The reduced carbon compounds that are substrates for respiration in plants include glucose, sucrose, fructose-containing polymers, lipids, and organic acids. C6H12O6 + 6O2 --> 6CO2 + 6H2O + 686 kcal energy Steps of respiration: Mobilization Glycolysis TCA (citric acid) cycle Electron transport and aerobic phosphorylation or Anaerobic fermentation The first step: mobilization. Production of glucose from starch. Glucose can be used to produce sucrose in the cytoplasm and that can be loaded into the phloem and transported to sink tissues such as roots and flowers. Glycolysis - the first step of respiration. Glucose -> 2 pyruvate + 2 ATP and 2 NADH Glycolysis occurs in the cytoplasm, no carbon is released, only about 20% of the energy available in glucose is obtained. ENERGY-REQUIRING STEPS OF GLYCOLYSIS: glucose 2 ATP invested glucose 6-phosphate fructose 6-phosphate fructose 1,6-bisphosphate 2 glyceraldehyde 3-phosphate 2 NAD+ dihydroxyacetone phosphate ENERGY-RELEASING STEPS OF GLYCOLYSIS: 2 NADH 2 Pi 2 1,3-bisphosphoglycerate 2 ADP 2 ATP phosphorylation, 2 ATP produced 2 3-phosphoglycerate 2 2-phosphoglycerate H 2O 2 PEP 2 ADP 2 ATP 2 pyruvate (to TCA cycle) phosphorylation, 2 ATP produced Net energy yield 2 ATP 2 NADH The fate of pyruvate depends on whether oxygen is available. If oxygen is not available then pyruvate goes through fermentation to recycle NAD+. The reason is because O2 is required to use NADH and NAD+ is required for glycolysis. Glycolysis C6H12O6 2 ATP energy input 2 ADP 2 NAD+ 2 4 NADH ATP 2 pyruvate energy output 2 ATP net Alcoholic fermentation 2H2O 2CO2 2 acetaldehyde electrons, hydrogen from NADH 2 ethanol In the absence of oxygen the energy stored as NADH is used to produce ethanol. In this process only 3% of the energy available in glucose is obtained in 2 moles of ATP formed per mole of glucose. 13% of the energy goes off as heat and 84% is present in the 2 moles of ethanol formed. When oxygen is present the pyruvate goes to the mitochondria where it goes through 1) the TCA cycle, also called the Krebs cycle or citric acid cycle and 2) the electron transport chain. Cytoplasm glucose energy input Glycolysis 2 ATP (net) 2 NADH 2 pyruvate 2 CO2 2 NADH TCA Cycle 6 NADH 2 FADH2 4 CO2 2 ATP water Electron transport phosphorylation Mitochondrion oxygen 32 ATP Pyruvate is decarboxylated to acetyl-CoA and NADH is produced. Acetyl CoA enters the TCA cycle. pyruvate from glycolysis coenzyme A NAD+ CO2 NADH CoA acetyl-CoA Function of the TCA cycle: Decarboxylation and production of reduced NADH and FADH2. These reactions occur in the mitochondrial matrix. Electron transport in chloroplasts and mitochondia generates ATP in the same way. H+ are transported out of the stroma/matrix by during electron transport and H+ flow back into the stroma/matrix through the ATP synthase and generates ATP in the stroma matrix. In chloroplasts the thylakoid lumen is acidified. In mitochondria, it is the intermembrane space. The mitochondrial matrix is equivalent to the chloroplast stroma. Most of the TCA cycle occurs in the matrix and ATP synthesis occurs in the matrix. To make ATP, the electron transport chain drives H+ out of the matrix into the intermembrane space. The electron donor is NADH and FADH2 and the terminal acceptor is O2. Oxidative phosphorylation NADH and FADH2 produced in the TCA cycle are the main electron donors, the ultimate electron acceptor is oxygen. The main product is ATP. The mitochondrial alternate oxidase (AOX) is used to generate heat. Skunk cabbage is a classic example of a plant that generates heat by thermogenic respiration. Exam on Tuesday Feb 21, 4:05 PM Multiple choice, 50 questions, bring a No. 2 pencil, no electronic devices allowed. Previous exams and a study guide are available on the course website.