Plant respiration

• Aerobic Respiration

  1. Higher plants require the presence of molecular oxygen for normal metabolism

  2. They obtain energy and carbon by oxidizing photoassimilates
• Respiration: An Overview

  Takes place in 3 stages: glycolysis, Krebs cycle, and electron transport system (ETS)
• GLYCOLYSIS: Functions

  1. Partial oxidation of hexose, reduction of NAD to NADH, no O2 used, no CO2 released

  2. Produces ATP, net: 2 ATP
• Mitochondrial Structure and Function

  1. Inner membrane is impermeable to small molecules, outer membrane is permeable to some proteins

  2. Mitochondrial matrix contains circular DNA molecule, ribosomes, and enzymes for synthesis of RNA and proteins
• Respiration: An Overview

  Provides carbon skeleton for plant products such as amino acids, nucleotides, carbon precursors for fats, porphyrin pigments, and aromatic compounds (lignin)
• Respiration: An Overview

  1. Products are distributed and utilized for plant growth and maintenance

  2. In growth respiration, energy, reductants, or structural building blocks are used for new plant biomass synthesis

  3. In maintenance respiration, products are used for repair, maintenance of existing structural systems, and ion gradients
• Respiration: An Overview

  1. The direct oxidation of hexose by molecular oxygen, releasing all free energy as heat

  2. Breaking down the oxidation of hexose into small steps to control the release of energy for conservation in metabolically useful forms
• Glycolysis

  1. The first steps in oxidative metabolism, produces pyruvate, NADH, and 2 ATP

  2. Aerobic organisms extract more than 30 additional ATPs from pyruvate and NADH using O2
• Fate of Pyruvate

  When O2 is limiting, NADH and pyruvate
• GLYCOLYSIS


  1. Converts sugars to pyruvate in cytosol, synthesizes small amount of ATP through substrate level phosphorylation

  2. ATP synthesis involves transfer of phosphate group to ADP
• GLYCOLYSIS: Functions

  1. Leads to formation of molecules for synthesis of other compounds

  2. Pyruvate can be oxidized in mitochondrion for larger ATP amounts
• Overview of cellular respiration in eukaryotic cells

  Mitochondrial Structure and Function
• Mitochondrial Structure and Function

  1. Outer membrane serves as outer boundary, inner membrane is more than 75% protein

  2. Inner membrane contains cardiolipin but not cholesterol, outer membrane contains a large pore-forming protein called porin
• Tricarboxylic Acid Cycle/Krebs Cycle

  The cycle involves oxidation of substrate and energy conservation, starting with the formation of citrate from acetyl CoA and oxaloacetate
• The Electron Transport System involves oxidation of NADH and FADH2, producing ATP and involving O2 uptake
• Tricarboxylic Acid Cycle/Krebs Cycle


  Reaction intermediates in the cycle are common compounds generated in other catabolic reactions, making it the central metabolic pathway of the cell. ATP is formed from electrons donated by NADH and FADH2
• Lactic acid Fermentation

  Glucose converts to 2 Lactic acid and 2 ATP (catalyzed by lactic acid dehydrogenase)
• The TCA/Krebs Cycle functions include reduction of NAD and FAD to electron donors, direct synthesis of ATP, and formation of carbon skeletons for amino acid synthesis
• Aerobic organisms use O2 to extract more than 30 additional ATPs from pyruvate and NADH
• Glycolysis

  When O2 is limiting: NADH and pyruvate accumulate leading to fermentation
• Fermentation types

  • Ethanol (alcoholic fermentation)
  • Lactic acid (lactic acid fermentation)
  • Example: in roots growing under waterlogged conditions (paddy rice field condition)
• Fate of Pyruvate

  Pyruvate is transported across the inner membrane and decarboxylated to form acetyl CoA, which enters the next stage
• Tricarboxylic Acid Cycle/Krebs Cycle

  Four reactions in the cycle transfer electrons to NAD+ to form NADH or to FAD+ to form FADH2. One ATP is formed through substrate level phosphorylation
• Alcoholic Fermentation

  Glucose converts to 2 Ethanol + 2 CO2 and 2 ATP (catalyzed by pyruvic acid decarboxylase and alcohol dehydrogenase)
• The Role of Mitochondria in the Formation of ATP involves the Electron Transport System where electrons move through carriers to produce ATP
• ATP production in aerobic organisms

  Two molecules of ATP are produced initially
• The TCA cycle is also known as the Citric Acid Cycle or Krebs Cycle
• Electron Transport Complexes in mitochondria include Complex I, II, III, and IV which catalyze electron transfers and transport H+ ions
• Complex III (cytochrome bc1)

  Transfer of electrons from ubiquinone to cytochrome c and transports four H+ per pair
• Complex IV (cytochrome c oxidase)


  Transfer of electrons to O2 and transports H+ across the inner membrane
• Metabolic poisons that bind catalytic sites in Complex IV

  • CO
  • N3–
  • CN–
• Cytochrome oxidase

  Adds four electrons to O2 to form two molecules of H2O
• Electron-Transport System

  1. Electrons gradually lose energy as they pass along the chains of electron carriers

  2. Released energy pumps H+ into the intermembrane space, creating an H+ or pH gradient across the membrane
• ATP synthase

  Uses proton motive force to make ATP from ADP + Pi through chemiosmosis
• Proton motive force


  Free energy associated with the formation of the proton electrochemical gradient
• Chemiosmotic phosphorylation

  1. H+ ions diffuse back across the inner membrane through ATP synthase to make ATP

  2. Cell couples exergonic reactions of electron transport with endergonic synthesis of ATP

  3. Explained by the "Chemiosmotic Theory" by P. Mitchell in 1966
• Oxidative phosphorylation is the process when ATP formation is driven by energy released from electrons removed during substrate oxidation
• Unique Features of Plant Aerobic Respiration
• Unique Aspects of Plant Respiration
• Bioenergetics of Respiration: For every 1 molecule of hexose in glycolysis, 2 NADH and 2 ATP are produced