1. Glycolysis (in the cytoplasm):
- Glucose is broken down into two molecules of pyruvate.
- Each pyruvate undergoes a series of enzymatic reactions to form Acetyl CoA.
- During this process, 2 molecules of ATP (net gain) and 2 molecules of NADH (reduced form of nicotinamide adenine dinucleotide) are produced.
2. Krebs Cycle (in the mitochondrial matrix):
- Each Acetyl CoA enters the Krebs cycle, a series of chemical reactions that further oxidize it.
- For each Acetyl CoA, the Krebs cycle generates 3 molecules of NADH, 2 molecules of FADH2 (reduced form of flavin adenine dinucleotide), and 1 molecule of ATP (substrate-level phosphorylation).
3. Oxidative Phosphorylation (in the inner mitochondrial membrane):
- The high-energy electrons carried by NADH and FADH2 are passed along the electron transport chain, a series of protein complexes.
- This process generates a proton gradient across the inner mitochondrial membrane, which drives the synthesis of ATP through ATP synthase (also known as the chemiosmotic mechanism).
- For each pair of electrons transferred through the electron transport chain, 2-3 molecules of ATP are produced (estimates vary depending on the specific pathway and organism).
Considering the ATP generated at each stage:
- Glycolysis: 2 ATP (net gain)
- Krebs Cycle: 1 ATP + 3 NADH + 2 FADH2 (per Acetyl CoA)
- Oxidative Phosphorylation: Approximately 30-32 ATP (per pair of electrons transferred)
Assuming the complete oxidation of one molecule of glucose via glycolysis and the Krebs cycle, and taking into account the ATP generated through oxidative phosphorylation, the maximum theoretical yield is 36-38 ATP molecules for each glucose molecule. This represents the maximum energy that can be extracted and stored in the form of ATP during cellular respiration.
It's important to note that some ATP is used in the initial steps of glycolysis, and a small amount may be lost due to inefficiencies in the electron transport chain. However, the overall process is highly efficient in extracting energy from glucose and converting it into ATP, the "energy currency" of the cell.
