Introduction

• Life Processes and Energy: All living organisms require energy for essential life processes.
• Energy Sources: Energy is obtained through the oxidation of macromolecules known as “food.”

Photosynthesis and Food Production

• Green Plants and Cyanobacteria: Capable of photosynthesis, converting light energy into chemical energy stored in carbohydrates (glucose, sucrose, starch).
• Food Translocation: Not all cells in green plants photosynthesize; food must be translocated to non-green parts.

Heterotrophic Organisms

• Animal and Fungal Nutrition: Animals (herbivores, carnivores) obtain food directly or indirectly from plants. Fungi are saprophytes, relying on dead and decaying matter.
• Ultimate Source: Regardless of the organism’s nutritional strategy, the ultimate source of food for respiration is photosynthesis.

Cellular Respiration Process

• Cellular Locations: Photosynthesis occurs in chloroplasts, while cellular respiration takes place in the cytoplasm and mitochondria (eukaryotes).
• C-C Bond Breakdown: Respiration involves the breakdown of C-C bonds through oxidation, releasing energy.
• Respiratory Substrates: Carbohydrates are commonly oxidized, but proteins, fats, and organic acids may also serve as respiratory substrates in specific conditions.

ATP Synthesis and Utilization

• Series of Reactions: Energy release in respiration occurs in a series of controlled, enzymatic reactions.
• ATP Synthesis: Energy released is trapped as chemical energy in the form of ATP.
• Energy Currency: ATP serves as the energy currency of the cell, utilized in various energy-requiring processes.
• Carbon Skeleton Utilization: Carbon skeletons produced during respiration serve as precursors for the biosynthesis of other molecules in the cell.

• Photosynthesis Adaptations: During photosynthesis, O₂ release within cells addresses O₂ availability.

Gaseous Exchange Mechanisms

• Cell Proximity to Surface: Living cells in plants are close to the surface, facilitating efficient gas exchange.
• Parenchyma Cells: Loose packing of parenchyma cells in leaves, stems, and roots creates interconnected air spaces.

Energy Production and Utilization

• Complete Glucose Combustion: Glucose combustion produces CO₂, H₂O, and energy (mostly as heat).
• Energy Utilization Strategy: Cells catabolize glucose in multiple small steps, coupling energy release to ATP synthesis.
• Story of Respiration: Respiration involves oxygen utilization, releasing CO₂, water, and energy.

Anaerobic Adaptations

• Oxygen Availability: Some cells live in environments with varying oxygen availability.
• Adaptation to Anaerobic Conditions: Early cells likely existed in oxygen-lacking atmospheres; present-day organisms adapted to anaerobic conditions.

Glycolysis: Anaerobic Glucose Breakdown

• Enzymatic Machinery: Living organisms retain enzymatic machinery for partial glucose oxidation without oxygen.
• Glycolysis: Breakdown of glucose to pyruvic acid in the absence of oxygen.

Glycolysis: Breaking Down Glucose for Energy

Introduction

• Origin of the Term: “Glycolysis” from Greek words glycos (sugar) and lysis (splitting).
• Pioneers: Gustav Embden, Otto Meyerhof, and J. Parnas developed the glycolysis scheme.
• Anaerobic Process: Predominant in anaerobic organisms; occurs in the cytoplasm.

Glycolytic Pathway in Plants

• Glucose Source: Derived from sucrose (photosynthesis end product) or storage carbohydrates.
• Sucrose Conversion: Invertase converts sucrose into glucose and fructose.
• Initial Steps: Glucose and fructose phosphorylation to form glucose-6-phosphate.

Ten Steps of Glycolysis

1. Hexokinase Activity: Glucose to glucose-6-phosphate.
2. Isomerization: Glucose-6-phosphate to fructose-6-phosphate.
3. Common Metabolism: Subsequent metabolism steps for glucose and fructose are the same.
4. ATP Utilization: Twice during glucose conversion and fructose-6-phosphate conversion.
5. Fructose 1,6-Bisphosphate Formation: ATP consumption.
6. Bisphosphate Split: Fructose 1,6-bisphosphate to dihydroxyacetone phosphate and 3-phosphoglyceraldehyde.

NADH + H+ Formation

• Redox Reaction: 3-phosphoglyceraldehyde to 1, 3-bisphosphoglycerate.
• Hydrogen Transfer: NADH + H+ formation.

ATP Synthesis

• Energy Yielding Steps: Conversion of 1, 3-bisphosphoglycerate to 3-phosphoglyceric acid.
• ATP Formation: During the conversion of PEP to pyruvic acid.

Overall ATP Calculation

• Direct ATP Synthesis: Calculate the total ATP molecules directly synthesized from one glucose molecule.

Pyruvic Acid as a Product

• Metabolic Fate: Determined by cellular needs.
• Possibilities: Lactic acid fermentation, alcoholic fermentation, or aerobic respiration.

Fermentation: Anaerobic Energy Production

Overview

• Definition: Incomplete oxidation of glucose under anaerobic conditions.
• Byproducts: CO2 and ethanol (yeast), or lactic acid (bacteria and muscles).
• Enzymes: Pyruvic acid decarboxylase and alcohol dehydrogenase (for alcohol fermentation).

Yeast Fermentation

1. Pyruvic Acid Conversion: Catalyzed by pyruvic acid decarboxylase.
2. Ethanol Formation: Through alcohol dehydrogenase.
3. Energy Release: Limited energy release (less than 7% of glucose energy).
4. ATP Synthesis: Not highly efficient; some ATP formed.

Lactic Acid Fermentation

• Bacterial Process: Some bacteria convert pyruvic acid to lactic acid.
• Animal Cells: Muscles during anaerobic conditions convert pyruvic acid to lactic acid.
• NADH+H+ Reoxidation: NAD+ regeneration via reduction of pyruvic acid.

Energy Yield and Hazards

• Limited ATP Production: Not all energy is trapped as high-energy ATP bonds.
• Hazardous Byproducts: Production of acid (lactic acid) or alcohol.
• Yeast Poisoning: Yeasts are affected when alcohol concentration reaches about 13%.

Net ATP Synthesis

• Calculation: Deduct ATP used during glycolysis from ATP synthesized during fermentation.

Maximum Alcohol Concentration

• Yeast Limitation: Yeasts poison themselves at high alcohol concentrations.
• Natural Fermentation: Maximum alcohol content limited by yeast tolerance.

Complete Oxidation in Aerobic Respiration

• Mitochondrial Steps: Complete oxidation occurs within mitochondria.
• Oxygen Dependency: Requires oxygen (O₂).
• Products: CO₂, water, and substantial energy release for ATP synthesis.
• Prevalence: Common in higher organisms.

Aerobic Respiration: Mitochondrial Processes

Overview

• Location: Mitochondria (matrix and inner membrane).
• Pyruvate Transport: From cytoplasm to mitochondria.
• Crucial Events:
  1. Oxidation of Pyruvate:
     • Removal of all hydrogen atoms.
     • Production of three molecules of CO₂.
     • Occurs in the matrix of mitochondria.
  2. Electron Transfer to O₂:
     • Electrons passed to molecular O₂.
     • Simultaneous ATP synthesis.
     • Located on the inner membrane of mitochondria.

Pyruvate Oxidation

1. Pyruvate Formation: Glycolytic catabolism in cytosol produces pyruvate.
2. Transport to Mitochondria: Pyruvate enters mitochondrial matrix.
3. Oxidative Decarboxylation: Catalyzed by pyruvic dehydrogenase.
4. Coenzymes: Involves NAD+ and Coenzyme A.
5. Reaction:

NADH Production

• NADH Synthesis: Two molecules of NADH from the metabolism of two pyruvic acid molecules (from one glucose during glycolysis).

Tricarboxylic Acid Cycle (TCA Cycle)

Overview

• Start: Condensation of acetyl group with oxaloacetic acid (OAA) and water.
• Enzyme: Catalyzed by citrate synthase.
• Product: Citric acid and release of CoA.
• Isomerization: Citrate is isomerized to isocitrate.
• Decarboxylation Steps:
  1. Formation of α-ketoglutaric acid.
  2. Formation of succinyl-CoA.
• Oxidation Steps:
  • Succinyl-CoA to succinic acid.
  • GTP synthesis in substrate-level phosphorylation.
  • GTP converted to GDP, producing ATP.
• Reduction Steps:
  • Three points where NAD+ is reduced to NADH + H+.
  • One point where FAD+ is reduced to FADH₂.
• Continued Cycle: Requires replenishment of oxaloacetic acid and regeneration of NAD+ and FAD+.

Substrate-Level Phosphorylation

• Process: Conversion of succinyl-CoA to succinic acid.
• Result: Synthesis of one molecule of GTP.
• Coupled Reaction: GTP converted to GDP, simultaneously synthesizing ATP from ADP.

NADH and FADH2 Production

• NADH Production: Three points in the cycle.
• FADH2 Production: One point in the cycle.

Cycle Significance

• Purpose: Continued oxidation of acetyl CoA.
• Requirements:
  1. Replenishment of oxaloacetic acid.
  2. Regeneration of NAD+ and FAD+ from NADH and FADH₂.
• Products So Far: CO₂, eight NADH + H+, two FADH₂, and two ATP.

Electron Transport System (ETS) and Oxidative Phosphorylation

Overview

• Objective: Release and utilize the energy stored in NADH+H+ and FADH₂.
• Location: Inner mitochondrial membrane.
• Process: Oxidation of NADH and FADH₂ through the electron transport system (ETS).
• Result: Formation of H₂O, coupled with ATP synthesis.

Electron Transport System (ETS) Components

1. NADH Dehydrogenase (Complex I):
   • Oxidizes electrons from NADH produced in the mitochondrial matrix.
   • Transfers electrons to ubiquinone.
2. Ubiquinone:
   • Receives electrons from NADH dehydrogenase (complex I) and FADH₂ (complex II).
   • Transfers electrons to cytochrome c.
3. Cytochrome c and Cytochrome bc1 Complex (Complex III):
   • Cytochrome c acts as a mobile carrier.
   • Receives electrons from reduced ubiquinone.
   • Transfers electrons to cytochrome c oxidase complex (complex IV).
4. Cytochrome c Oxidase Complex (Complex IV):
   • Contains cytochromes a and a3, and two copper centers.
   • Accepts electrons from cytochrome c.
   • Transfers electrons to oxygen, forming H₂O.

ATP Synthesis

• Coupled with ETS: ATP synthase (complex V) couples with the electron transport chain (ETS).
• Components:
  • F1 Headpiece: Peripheral membrane protein complex with ATP synthesis site.
  • F0 Integral Membrane Protein: Forms a channel for proton movement.
• Proton Movement: Protons move through F₀ from the intermembrane space to the matrix down the electrochemical proton gradient.
• Proton Passage Coupling: Coupled to the catalytic site of the F₁ component for ATP production.
• ATP Synthesis: For each ATP produced, 4H+ passes through F₀.

Role of Oxygen

• Terminal Stage: Oxygen acts as the final hydrogen acceptor.
• Importance: Drives the entire process by removing hydrogen from the system.
• Role: Oxygen is the final electron acceptor, forming water.

Oxidative Phosphorylation

• Definition: The process is termed oxidative phosphorylation due to the utilization of energy from oxidation-reduction for ATP synthesis.
• Comparison to Photophosphorylation: Unlike photophosphorylation, where light energy is utilized, oxidative phosphorylation uses the energy from oxidation-reduction.

ATP Synthase (Complex V)

• Components:
  • F1 Headpiece: Peripheral membrane protein complex.
  • F0 Integral Membrane Protein: Forms the proton channel.
• Energy Source: Energy released during the electron transport system.
• Function: Synthesizes ATP from ADP and inorganic phosphate.
• Hypothesis: Mechanism explained by the chemiosmotic hypothesis.

ATP Production

• NADH Oxidation: Produces 3 molecules of ATP.
• FADH2 Oxidation: Produces 2 molecules of ATP.

Importance of Oxygen in Respiration

• Limited Role: Oxygen’s role is primarily in the terminal stage of respiration.
• Vital Presence: Vital for driving the entire process by acting as the final hydrogen acceptor.

THE RESPIRATORY BALANCE SHEET

Overview

• Objective: Calculate the theoretical net gain of ATP for every glucose molecule oxidized.
• Assumptions:
  1. Sequential, orderly pathway: Glycolysis, TCA cycle, and ETS pathway in succession.
  2. Transfer of NADH to mitochondria for oxidative phosphorylation.
  3. No utilization of intermediates for synthesizing other compounds.
  4. Sole respiration of glucose; no alternative substrates entering the pathway.
• Limitations: Real-life pathways are simultaneous, substrate entry/withdrawal dynamic, and enzymatic rates are controlled by multiple factors.

Theoretical Net Gain

• Aerobic Respiration:
  • Assumptions-Based Net Gain: 38 ATP molecules for one glucose molecule.
  • Considerations: Sequential functioning, controlled substrate flow, and no diversion of intermediates.

Comparison with Fermentation

• Fermentation vs. Aerobic Respiration:
  • Partial vs. Complete Breakdown:
    • Fermentation: Partial breakdown of glucose.
    • Aerobic Respiration: Complete degradation to CO₂ and H₂O.
  • ATP Production:
    • Fermentation: Net gain of only two ATP molecules per glucose degraded to pyruvic acid.
    • Aerobic Respiration: Generates many more ATP molecules under aerobic conditions.
  • NADH Oxidation:
    • Fermentation: Slow oxidation of NADH to NAD+.
    • Aerobic Respiration: Vigorous oxidation of NADH to NAD+.

Appreciating the Living System

• Dynamic Pathways:
  • Pathways work simultaneously, not sequentially.
  • Substrates enter/exit pathways dynamically.
• Utilization of ATP:
  • ATP is used as needed, and enzymatic rates are controlled by multiple factors.
• Exercise Purpose:
  • Appreciate the efficiency of the living system in energy extraction and storage.

AMPHIBOLIC PATHWAY

Overview

• Glucose as the Primary Substrate:
  • Glucose is the preferred substrate for respiration.
  • All carbohydrates are usually converted into glucose before entering the respiratory pathway.

Entry Points for Different Substrates

• Points of Entry:
  • Glucose: Primary substrate entering the respiratory pathway.
  • Fats: Break down into glycerol and fatty acids, with entry after degradation to acetyl CoA.
  • Proteins: Degraded by proteases, and amino acids enter the pathway at various stages (as pyruvate, acetyl CoA, or within Krebs’ cycle).

Catabolism vs. Anabolism

• Traditional Understanding:
  • Respiration seen as a catabolic process, and the respiratory pathway as catabolic.
  • Catabolism involves the breakdown of substrates.

Amphibolic Nature of the Respiratory Pathway

• Dual Role in Breakdown and Synthesis:
  • Fatty Acids:
    • Breakdown to acetyl CoA during respiration.
    • Acetyl CoA is withdrawn for fatty acid synthesis.
  • Proteins:
    • Involved in both breakdown and synthesis.
  • Recognition of Amphibolic Nature:
    • The respiratory pathway plays a role in both catabolism and anabolism.
    • Recognized as an amphibolic pathway.

RESPIRATORY QUOTIENT (RQ)

Overview

• Definition:
  • The respiratory quotient (RQ) or respiratory ratio is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during aerobic respiration.

Calculation of RQ

• Formula:
  • RQ=Volume of CO₂ evolved Volume of O₂ consumed RQ =Volume of O₂​ consumed volume of CO₂​ evolved​

Dependence on Respiratory Substrate

• Carbohydrates:
  • RQ is 1 when carbohydrates are completely oxidized.
  • Example: C₆H₁₂O₆+6O₂→6CO₂+6H₂O+Energy
  • RQ=6CO₂/6O₂=1
• Fats:
  • RQ is less than 1 when fats are used.
  • Example (Tripalmitin): C₅₁​H₉₈​O₆​+145O₂​→102CO₂​+98H₂​O+Energy
  • RQ=102CO₂​​/145O₂≈0.7
• Proteins:
  • RQ for proteins is about 0.9.

Importance

• Multiple Substrates:
  • Living organisms often use a combination of respiratory substrates.
  • Pure proteins or fats are rarely used as sole respiratory substrates.