Lecture Notes: Respiratory Substrates

Introduction

• Respiratory Substrates: Organic molecules broken down to generate ATP.
• Common Substrates: Glucose (carbohydrate), lipids, and proteins.
• Focus: Determining which substrate is most efficient for ATP production.

Breakdown of Proteins

• Proteins: Composed of amino acids.
• Role of Amino Acids:
  • Used to build essential cellular components: antibodies, enzymes, hemoglobin, collagen.
  • Can be broken down for ATP, but this is not preferred due to their primary roles.

Carbohydrates vs. Lipids

• Examples:
  • Carbohydrate: Glucose (C6H12O6)
  • Lipid: Fatty acid (example: C18H34O2)
• ATP Yield:
  • Hydrogen Bonds: Fatty acids have more carbon-hydrogen bonds than glucose.
  • More hydrogen atoms released from fatty acids when broken down.
  • Result: Fatty acids produce more reduced NAD (NADH) and FAD (FADH2).
  • Oxidative Phosphorylation: Higher rates with fatty acids due to more hydrogen atoms, leading to more ATP production.

Glucose vs. Fatty Acids

• Why is Glucose Preferred?
  • Despite fatty acids producing more ATP, glucose is the main energy source.
  • Pathways:
    • Glucose: Undergoes glycolysis in the cytoplasm, then enters the mitochondrial matrix for further breakdown.
    • Fatty Acids: Enter the mitochondrial matrix directly, undergoing breakdown into two-carbon molecules.
  • Aerobic vs. Anaerobic Respiration:
    • Fatty Acids: Breakdown requires sufficient oxygen (aerobic conditions).
    • Glucose: Can undergo glycolysis without oxygen (anaerobic conditions), producing lactate in animals, or ethanol and CO2 in plants and yeast.
  • Efficiency: Glucose can be broken down when oxygen is limited, providing a consistent energy source.
  • Carbon Structure: Glucose is simpler (6-carbons) and easier to break down than complex fatty acids (e.g., 14-carbons).

Misconceptions

• Exercise: During high-intensity running, glucose is primarily broken down, not fats, due to the need for anaerobic respiration.

Conclusion

• Main Takeaway: Glucose is favored as a respiratory substrate because it can be metabolized in both aerobic and anaerobic conditions, making it a reliable and efficient energy source for cells.