1. Glycolysis (1 net ATP):
- Glycolysis occurs in the cytoplasm and is the initial stage of cellular respiration.
- Two molecules of ATP are consumed during the initial steps of glycolysis.
- Four molecules of ATP are produced later in glycolysis through the substrate-level phosphorylation of ADP.
- The net gain from glycolysis is 4 ATP minus the 2 ATP used, resulting in 1 net ATP molecule.
2. Krebs Cycle (2 net ATP):
- The Krebs cycle takes place in the mitochondrial matrix.
- Acetyl CoA, derived from the breakdown of glucose or fatty acids, enters the cycle.
- During the cycle, high-energy molecules are generated, which are then used to produce ATP.
- For each acetyl CoA that enters the cycle, 3 molecules of NADH, 2 molecules of FADH2, and 1 molecule of GTP (which can be exchanged for ATP) are produced.
- This yields a total of 12 potential ATP molecules (3 NADH x 3 ATP + 2 FADH2 x 2 ATP + 1 GTP x 1 ATP).
- However, 2 ATP molecules are used in the cycle for the activation of acetyl CoA, so the net gain from the Krebs cycle is 2 ATP molecules.
3. Oxidative Phosphorylation (up to 34 net ATP):
- Oxidative phosphorylation occurs in the inner mitochondrial membrane.
- During this process, the NADH and FADH2 produced in glycolysis and the Krebs cycle pass their electrons through the electron transport chain, creating an electrochemical proton gradient.
- ATP synthase uses the energy from this gradient to convert ADP into ATP through oxidative phosphorylation.
- Approximately 3 molecules of ATP are produced for each NADH molecule, and 2 molecules of ATP for each FADH2 molecule.
- Glycolysis and the Krebs cycle produce a total of 10 NADH and 2 FADH2 molecules per glucose molecule.
- Therefore, the potential ATP yield from oxidative phosphorylation is 34 ATP (10 NADH x 3 ATP + 2 FADH2 x 2 ATP).
In summary:
- Glycolysis: 1 net ATP
- Krebs Cycle: 2 net ATP
- Oxidative Phosphorylation: up to 34 net ATP
When all the stages are combined, the complete breakdown of one molecule of glucose via cellular respiration can potentially yield up to 37 net ATP molecules (1 net ATP from glycolysis, 2 net ATP from the Krebs cycle, and 34 ATP from oxidative phosphorylation). This process allows the cell to generate ATP, which is the primary energy currency for cellular functions.
