Bio Lecture 03/04: Cellular Respiration & Fermentation

Page 1: Overview of Redox Reactions

• Redox reactions are crucial for harvesting chemical energy from fuels.

• Cellular Respiration:

  • Reaction: C6H12O6 + 6O2 → 6CO2 + 6H2O

  • Free energy change (ΔG) = -686 kcal/mol

Page 2: Oxidation of Glucose

• Glucose and organic molecules are oxidized during cellular respiration.

  • Oxygen (O2) functions as an electron acceptor and gets reduced.

  • Reaction: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

• Oxidation and Reduction:

  • Glucose gets oxidized while oxygen gets reduced.

Page 3: Controlled Energy Release

• Oxygen is a powerful oxidizing agent.

• Energy needs to be released slowly and controlled for effective capture.

• Reaction: C6H12O6 + 6O2 → 6CO2 + 6H2O involves multiple enzyme-mediated steps in glycolysis and the citric acid cycle.

Page 4: Substrate-Level Phosphorylation

• Phosphorylation Process:

  • An enzyme transfers a phosphate group from a substrate to ADP, forming ATP.

  • Reaction: substrate-P + ADP → substrate + ATP

  • This is a slow reaction.

Page 5: Oxidative Phosphorylation

• Electron Carrier Molecules transfer H+ and e-:

  • Catalyzed by dehydrogenases.

  • Reactions for carriers:

    • NAD+ + H+ + 2e- → NADH (stores energy for later ATP production)

    • FAD + 2H+ + 2e- → FADH2.

Page 6: Electron Transport Chain

• Electrons carried by NADH and FADH2 enter the electron transport chain.

• Redox reactions occur, releasing energy in small amounts.

• Controlled energy release is essential for ATP production.

Page 7: Stages of Cellular Respiration

• Overview of Respiration:

  • Glycolysis: initial breakdown of glucose.

  • Citric Acid Cycle: completes the breakdown of glucose.

  • Oxidative Phosphorylation: final ATP synthesis stage.

Page 8: Glycolysis Phases

• Energy Investment Phase:

  • Uses 2 ATP.

• Energy Payoff Phase:

  • Produces 4 ATP through substrate-level phosphorylation and generates 2 NADH.

• Net Gain: 4 ATP produced - 2 ATP used = 2 ATP net gain.

Page 9: Glycolysis Stepwise Overview

• Key Enzymes and Intermediates in Glycolysis:

  • Hexokinase, Phosphofructokinase, Pyruvate Kinase, Glyceraldehyde 3-phosphate Dehydrogenase, etc.

  • Process includes: conversion of glucose to glucose-6-phosphate, fructose-6-phosphate, and ultimately pyruvate.

Page 10: Fate of Pyruvate

• If oxygen is present: Pyruvate enters mitochondria for further processing.

• If oxygen is absent: Pyruvate undergoes fermentation:

  • Reduced to lactic acid or ethanol.

  • This process regenerates NAD+.

Page 11: Alcohol Fermentation

• Process of Alcoholic Fermentation:

  • Releases CO2 from pyruvate, produces acetaldehyde.

  • Acetaldehyde gets reduced, moving electrons from NADH to form ethanol and regenerate NAD+.

  • No ATP is produced during fermentation, but NAD+ allows glycolysis to continue.

Page 12: Lactic Acid Fermentation

• Lactic Acid Fermentation:

  • Pyruvate is directly reduced from NADH.

  • Produces NAD+ and lactate (lactic acid).

  • Like alcoholic fermentation, no ATP is made, but NAD+ allows continued glycolysis.

Page 13: End Products of Glycolysis

• At the conclusion of glycolysis:

  • Yield: 2 ATP, 2 NADH, and 2 Pyruvate.

  • ATP can be utilized immediately; NADH and pyruvate are transported into the mitochondria for further processing.

Page 14: Pyruvate Oxidation

• Pyruvate must be converted into acetyl CoA to enter the citric acid cycle.

  • Catalyzed by the enzyme pyruvate dehydrogenase.

  • For each pyruvate, products are: 1 CO2, 1 NADH, and 1 acetyl CoA.

Page 15: Citric Acid Cycle Overview

• Key Steps in the Citric Acid Cycle:

  • Enzymes include citrate synthase, aconitase, isocitrate dehydrogenase, and others.

  • Cycle includes conversion of acetate to citrate and follows further reactions to produce CO2, NADH, and FADH2.

Page 16: Electron Transport Chain (ETC)

• Components of ETC:

  • 4 protein complexes in the inner mitochondrial membrane.

  • Electrons are transferred from one protein complex to another.

  • The released energy from these transfers is utilized to pump H+ ions into the intermembrane space.

Page 17: Cytochromes and Ubiquinone

• Carrier Molecules involved in ETC:

  • Cytochromes: proteins containing iron heme group.

  • Ubiquinone (Q): hydrophobic electron carrier, shuttles electrons between complexes in the electron transport chain.

Page 18: Alternate States of Electrons

• Each protein and carrier in ETC alternates between:

  • Reduced State: Accepts electrons.

  • Oxidized State: Donates electrons.

  • Energy is released in each redox reaction.

Page 19: Specifics of the Electron Transport Chain

• Detailed functions of complex interactions:

  • Complex I: receives 2 e- from NADH, pumps H+ into the intermembrane space.

  • Complex II: receives 2 e- from FADH2, does not pump.

  • Complex III: receives e- from Complexes I and II, pumps H+.

  • Complex IV: receives e- from Complex III, pumps H+.

Page 20: Final Electron Transfer

• Complex IV: Transfers e- to O2, producing water.

  • Reaction: 4H+ + 4e- + O2 → 2H2O.

Page 21: ATP Production Note

• ATP Generation: Not produced directly by the electron transport chain.

  • Instead, a H+ gradient is established (High [H+] vs Low [H+]).

Page 22: Chemiosmosis

• Mechanism of ATP synthesis:

  • Chemiosmosis: uses the flow of H+ ions to drive ATP synthesis.

  • ATP Synthase: enzyme that turns as H+ flows, activating catalytic sites to convert ADP + P into ATP.

Page 23: ATP Yield

• Ideally produces 32-34 ATP per glucose.

• Under normal cellular conditions, realistically produces about 26-28 ATP per glucose molecule.

Page 24: Overall Process Equation

• Overall Reaction: C6H12O6 + 6O2 → 6CO2 + 6H2O

  • Glucose is utilized in glycolysis and the citric acid cycle.

  • CO2 released during the citric acid cycle; O2 consumed and water produced in the electron transport chain.

  • In anaerobic conditions, the citric acid cycle stops since O2 is not available.

Page 25: Alternative Fuel Sources

• Cells can utilize various fuel sources:

  • Amino Acids: modified after removing the amino group to enter the citric acid cycle.

  • Glycerol: converted to G3P, can be broken down or synthesized into glucose.

  • Fatty Acids: undergo beta oxidation to be broken into acetyl CoA.

Page 26: Regulation of Cellular Respiration

• Feedback Inhibition is used to modulate the rate of cellular respiration:

  • Low levels of ATP lead to an increased rate of cellular respiration.

  • High levels of ATP result in a decreased rate of cellular respiration.

  • This mechanism prevents unnecessary energy expenditure when ATP is abundant.