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:
Glycolysis: The conversion of glucose into pyruvate.
Cellular Respiration (aerobic): The thorough oxidation of glucose utilizing oxygen.
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:
Energy-Investing Reactions: ATP is consumed to commence the breakdown of glucose.
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.