Cellular Respiration I

Glycolysis & Krebs Cycle Detailed Overview

Biological Redox Reactions

• A comprehensive examination of glucose oxidation and the consequent release of chemical energy.

Glycolysis

• Pyruvate Oxidation: The biochemical transformation of pyruvate into acetyl-CoA, a vital intermediate that feeds into the Krebs cycle.

Citric Acid Cycle

• Commonly referred to as the Krebs cycle, this crucial cycle is responsible for the complete oxidation of glucose, resulting in the generation of energy.

Biological Redox Reactions

Concept 1: Utilization of Redox Reactions

• Cells harness free energy through redox processes involving reduced chemical compounds, predominantly glucose.

Concept 2: Glucose as a Primary Fuel

• Among various metabolic substrates, glucose stands out as the most common fuel, undergoing various pathways that lead to its oxidation for ATP synthesis.

Redox Reactions

• Definition: A class of reactions characterized by the transfer of electrons, which enable cells to extract energy from nutrients.

• Key Processes:

  • Reduction: The process of gaining electrons or hydrogen atoms, leading to a state that is more reduced.

  • Oxidation: The loss of electrons or hydrogen atoms, resulting in a more oxidized state.

  • Coupling: In biological systems, oxidation and reduction reactions occur simultaneously.

Roles of Agents in Redox Reactions

• Oxidizing Agent: A substance that accepts electrons or hydrogen atoms (e.g., oxygen).

• Reducing Agent: A substance that donates electrons or hydrogen atoms (e.g., glucose).

  • In the context of glucose metabolism, glucose undergoes oxidation (serving as the reducing agent), while oxygen is reduced (acting as the oxidizing agent).

• The overall reaction in this context is exergonic, evidenced by a negative change in free energy (∆G).

Metabolic Significance of Glucose

• As a fundamental biochemical entity, glucose plays a pivotal role in synthesizing:

  • Fats: Critical for maintaining cell membrane structure and serving as energy reserves.

  • Carbohydrates: Essential for structural purposes and energy storage within cells.

  • Additional Compounds: Including amino acids and nucleic acids, vital for various cellular processes and functions.

• The complete oxidation of glucose yields approximately 686 kcal, underscoring its importance as a vital energy source.

ATP: The Energy Currency of Cells

• Electrons provide a significant source of chemical potential energy, which is integral to the synthesis of ATP:

  • The energy derived from the oxidation of carbohydrates contributes to the formation of ATP, making it the primary energy carrier in cellular activities.

• The high potential energy associated with electrons in ATP is critical for facilitating a wide range of cellular processes.

Overview of Glucose Oxidation

Concept 3: Major Metabolic Pathways

• The extraction of energy from glucose occurs through three principal pathways:

  1. Glycolysis: The conversion of glucose into pyruvate.

  2. Cellular Respiration (aerobic): The thorough oxidation of glucose utilizing oxygen.

  3. Fermentation: An anaerobic alternative yielding lower energy and producing by-products such as ethanol or lactic acid.

Aerobic Respiration Versus Fermentation

• Concept 4: Aerobic respiration ensures the full oxidation of glucose into carbon dioxide (CO2), with oxygen being reduced to water (H2O).

• Concept 5: In contrast, fermentation leads to the partial oxidation of glucose, resulting in lower energy yields and by-products such as ethanol or lactic acid.

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

Cellular Respiration

• Complete Glucose Oxidation:

  • Reaction: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + free energy.

  • The standard free energy change (∆G) for this reaction is -686 kcal/mole, highlighting its nature as a highly exergonic reaction that drives ATP formation.

Electron Carriers in Redox Reactions

Concept 6: Function of Electron Carriers

• Electron carriers, including NAD, are essential for transferring electrons from reduced compounds to the ultimate electron acceptor in metabolic pathways.

NAD as an Electron Carrier

• NAD (Nicotinamide Adenine Dinucleotide): A vital coenzyme in redox processes, it exists in two forms:

  • Oxidized Form: NAD+

  • Reduced Form: NADH + H+

• Key Reactions:

  • Reduction Reaction (energy-intensive): NAD+ + 2H → NADH + H+

  • Oxidation Reaction (exothermic): NADH + H+ + ½ O2 → NAD+ + H2O

Glycolysis: Transitioning from Glucose to Pyruvate

Concept 7: Glycolysis Stages

• Glycolysis occurs within the cytoplasm and comprises two major stages:

  1. Energy-Investing Reactions: ATP is consumed to commence the breakdown of glucose.

  2. Energy-Harvesting Reactions: ATP and NADH are produced as G3P is oxidized to form pyruvate.

Energy-Investing Reactions in Glycolysis

Concept 8: Glucose Splitting

• In the glycolytic process, glucose is cleaved into two 3-carbon molecules known as G3P.

  • Reaction: Glucose + 2 ATP → 2 G3P + 2 ADP + 2 Pi

  • ATP Requirement: Critical for activating glucose for subsequent reactions.

Energy-Harvesting Reactions in Glycolysis

Concept 9: Pyruvate Formation

• Through the oxidation of G3P, pyruvate is produced, along with the generation of ATP.

  • Reaction: 2 G3P + 2 NAD+ + 4 ADP + 4 Pi → 2 pyruvate + 2 NADH + 2 H+ + 4 ATP

  • Substrate-Level Phosphorylation: ATP is generated by transferring a phosphate group from an activated intermediate to ADP.

Free Energy Changes Throughout Glycolysis

• The glycolytic pathway highlights the contrast between energy-investing and energy-harvesting phases, indicating significant free energy transformations.

Pyruvate Oxidation

Concept 11: Location and Process of Pyruvate Oxidation

• This essential biochemical process takes place at the plasma membrane in prokaryotic cells and within the mitochondrial matrix in eukaryotic cells.

Conversion of Pyruvate to Acetyl-CoA

Concept 12: Decarboxylation Process

• In eukaryotic cells, pyruvate is transformed into acetyl-CoA, a key intermediary compound.

  • Reaction: 2 pyruvate + 2 NAD+ + 2 CoA → 2 acetyl-CoA + 2 NADH + 2 H+ + 2 CO2

Citric Acid Cycle (Krebs Cycle)

Concept 13: Site for Krebs Cycle

• The Krebs cycle occurs in the cytoplasm of prokaryotic cells and the mitochondrial matrix of eukaryotic cells, highlighting its universality in metabolism.

Concept 14: Cycle Initiation

• The cycle begins with the combination of acetyl-CoA and oxaloacetate, giving rise to citric acid (citrate).

Concept 15: Series of Reactions

• Comprising eight enzyme-catalyzed reactions, the oxidation of citrate involves the reduction of NAD and FAD as electron carriers and the regeneration of oxaloacetate to perpetuate the cycle.

Concept 16: Completing the Oxidation

• This cycle ultimately serves to finalize the oxidation of glucose into carbon dioxide (CO2), contributing significantly to the energy yield via cellular respiration.