Cellular Respiration Overview

Introduction

• Cellular respiration includes pathways that do not directly use oxygen.
• Focus on glycolysis and the citric acid cycle.

Glycolysis

• Definition: Breakdown of glucose into two pyruvate molecules.
  • Glucose (6 carbons) -> 2 pyruvate (3 carbons each).
• Location: Cytosol.
• ATP Generation: Inefficient, substrate-level phosphorylation.
  • Produces a net of 2 ATP.
• Key Outputs: 2 ATP, 2 NADH, 2 Pyruvate.
• Phases:
  1. Investment Phase: 2 ATP used to phosphorylate glucose.
     • Enzymes: Hexokinase creates glucose 6-phosphate.
     • Ensures continuous glucose intake by altering solute gradient.
  2. Cleavage Phase: Split 6-carbon sugar into two 3-carbon molecules.
     • Important molecules: Dihydroxyacetone phosphate, Glyceraldehyde 3-phosphate (G3P).
  3. Payout Phase: ATP formation and NAD+ reduction to NADH.
     • Produces 2 NADH and 4 ATP (net gain of 2 ATP).

Pyruvate Metabolism

• Transitional stage between glycolysis and citric acid cycle.
• Conversion: Pyruvate to Acetyl-CoA.
  • Generates 1 NADH per pyruvate (2 per glucose).
  • Produces CO2 as a waste product.

Citric Acid Cycle (Krebs Cycle)

• Location: Mitochondrial matrix.
• Process:
  • Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons).
  • Cycle runs twice per glucose molecule.
• Key Outputs:
  • 3 NADH and 1 FADH2 per Acetyl-CoA (total: 6 NADH, 2 FADH2 per glucose).
  • 2 ATP total through substrate-level phosphorylation.
  • Produces 4 CO2 as waste.

Oxidative Phosphorylation

• Location: Occurs after citric acid cycle.
• Steps:
  1. Electron Transport Chain: Repurposes energy from electrons.
  2. Chemiosmosis: ATP generation via oxidative phosphorylation.
• Dependence on Oxygen:
  • Oxygen recycles co-enzymes to continue ATP production.
  • Lack of oxygen leads to fermentation, but it is not sustainable long-term.

Fermentation

• Occurs in anaerobic conditions.
• Converts NADH back to NAD+ to allow glycolysis to continue.
• Produces lactic acid, which can lead to pH imbalances.

Conclusion

• Efficient ATP production requires oxygen for oxidative phosphorylation.
• Oxygen is crucial for recycling co-enzymes and maintaining cellular respiration pathways.