Glycolysis, Krebs cycle, and Electron Transport Chain (ETC)

1. Introduction to Cellular Respiration

1. Cellular respiration is the process by which cells break down glucose to produce energy in the form of ATP (Adenosine Triphosphate).
2. It occurs in three main stages:
   • Glycolysis (Occurs in the cytoplasm)
   • Krebs Cycle (Occurs in the mitochondrial matrix)
   • Electron Transport Chain (ETC) (Occurs in the inner mitochondrial membrane)

2. Glycolysis (First Step of Respiration)

1. Occurs in the cytoplasm of the cell.
2. It is the anaerobic phase of respiration and does not require oxygen.
3. One glucose (C₆H₁₂O₆) molecule is broken down into two molecules of pyruvate (C₃H₄O₃).
4. Net ATP Production: 2 ATP molecules per glucose.
5. Produces 2 NADH (Nicotinamide Adenine Dinucleotide) molecules, which store high-energy electrons.
6. Key steps in glycolysis:
   • Glucose phosphorylation: ATP is used to add phosphate groups.
   • Glucose splitting: Forms two three-carbon molecules.
   • ATP generation: High-energy electrons are transferred to NADH, and ATP is synthesized.
7. Pyruvate then enters the Krebs cycle in aerobic respiration or undergoes fermentation in anaerobic respiration.

3. Krebs Cycle (Citric Acid Cycle)

1. Occurs in the mitochondrial matrix.
2. It is an aerobic process (requires oxygen).
3. The two pyruvate molecules from glycolysis are converted into Acetyl-CoA, which enters the cycle.
4. Main products:
   • 2 ATP molecules
   • 6 NADH molecules (carry high-energy electrons)
   • 2 FADH₂ molecules (another electron carrier)
   • 4 CO₂ molecules (waste product)
5. Key steps of the Krebs cycle:
   • Acetyl-CoA combines with oxaloacetate to form citrate (citric acid).
   • Series of reactions: Citrate is broken down, releasing CO₂ and generating ATP, NADH, and FADH₂.
   • Regeneration of oxaloacetate: The cycle repeats for every Acetyl-CoA molecule.
6. Function: Provides high-energy electrons for the next stage: Electron Transport Chain (ETC).

4. Electron Transport Chain (ETC)

1. Occurs in the inner mitochondrial membrane.
2. It is the final and most efficient stage of aerobic respiration.
3. NADH and FADH₂ from glycolysis and the Krebs cycle donate electrons to the ETC.
4. Oxygen is the final electron acceptor, forming water (H₂O).
5. ATP Synthase: Uses a proton gradient to generate ATP.
6. Main Products:
   • 34 ATP molecules (highest ATP production)
   • Water (H₂O) as a byproduct

5. Summary of ATP Yield in Cellular Respiration

6. Importance of Cellular Respiration

1. Provides energy (ATP) for all cellular activities.
2. Releases CO₂, which is used in photosynthesis.
3. Maintains metabolic functions in plants, animals, and microorganisms.
4. ETC ensures maximum ATP production for energy efficiency.

7. Conclusion

1. Glycolysis is the first step and occurs in the cytoplasm.
2. Krebs cycle occurs in the mitochondria and provides high-energy carriers.
3. Electron Transport Chain produces the most ATP.
4. Overall, aerobic respiration yields 38 ATP per glucose molecule.