Cellular Respiration: Pyruvate Oxidation & The Krebs Cycle

Cellular Respiration Overview

  • Cellular respiration is a process that enables cells to release and transfer energy from food molecules.
  • Key stages include:
    • Glycolysis: Breakdown of glucose to pyruvate.
    • Pyruvate Oxidation: Conversion of pyruvate to Acetyl-CoA, producing NADH and CO2.
    • Krebs Cycle (Citric Acid Cycle): Further breakdown of Acetyl-CoA to produce NADH, FADH2, GTP, and CO2.
    • Electron Transport Chain: Utilizes NADH and FADH2 to synthesize ATP.

Pyruvate Oxidation

  • Pyruvate (from glycolysis) is oxidized to form Acetyl-CoA.
  • Key steps in Pyruvate Oxidation:
    • Pyruvate is converted into a 2-C acetyl group.
    • The acetyl group combines with 4-C oxaloacetate to form 6-C citrate.
    • Through oxidation and decarboxylation reactions, NADH and CO2 are produced.
    • The cycle regenerates oxaloacetate, completing the cycle.
  • Key Outputs:
    • 1 Acetyl-CoA per pyruvate
    • 1 NADH
    • 1 CO2 (as waste)

Krebs Cycle (Citric Acid Cycle)

  • The Krebs Cycle occurs in the mitochondrial matrix, consisting of eight enzymatic reactions that oxidize Acetyl-CoA.
  • Reactions:
    • Inputs: 1 Acetyl-CoA, 3 NAD+, FAD, GDP
    • Outputs: 3 NADH, 1 FADH2, 1 GTP, 2 CO2 (as waste)
  • The cycle regenerates oxaloacetate and produces energy-rich molecules that are essential for ATP production.
  • Each NADH molecule can yield approximately 2.5-3 ATP molecules, while each FADH2 yields about 1.5-2 ATP molecules.

Role of Electrons in Energy Transfer

  • Redox Reactions: Cells utilize oxidation (loss of electrons) and reduction (gain of electrons) to transfer energy.
    • Oxidation: Loss of electrons or hydrogen.
    • Reduction: Gain of electrons or hydrogen.
  • These reactions synthesize molecules like ADP to ATP, GDP to GTP, NAD+ to NADH, and FAD to FADH2.

Energy Synthesis in Mitochondria

  • ATP Synthesis Location:
    • Most ATP is synthesized in mitochondria during cellular respiration:
    • Pyruvate oxidation and the Krebs cycle occur in the matrix.
    • The Electron Transport Chain occurs in the inner mitochondrial membrane.
  • These processes require oxygen, highlighting the importance of aerobic respiration in energy production.

Vitamin Cofactors in Enzymatic Reactions

  • Enzymes in pyruvate oxidation and Krebs cycle require vitamin B cofactors (e.g., thiamine, niacin).
  • These vitamins assist in the conversion of substrates and production of energy-rich molecules.

Waste Management

  • CO2 produced from both pyruvate oxidation and the Krebs cycle exits the body via exhalation, preventing toxic accumulation.

Summary of Products and Energy Flow

  • Pyruvate oxidation produces:

    • 1 Acetyl-CoA
    • 1 NADH
    • 1 CO2 (waste)

  • Krebs cycle generates:

    • 3 NADH
    • 1 FADH2
    • 1 GTP (convertible to ATP)
    • 2 CO2 (waste)

Important Study Questions

  • How do redox reactions facilitate energy transfer in cells?
  • What are the outputs of pyruvate oxidation and the Krebs cycle?
  • Why do mitochondria play a vital role in cellular respiration?
  • Why are B vitamins important as cofactors in these metabolic processes?