Here's a breakdown of how ATP is generated in the different stages of cellular respiration:
1. Glycolysis: This occurs in the cytoplasm and doesn't require oxygen.
* Net production of 2 ATP molecules: Glycolysis breaks down glucose into pyruvate, producing a small amount of ATP through substrate-level phosphorylation. This means that an enzyme directly transfers a phosphate group from a substrate molecule to ADP to form ATP.
2. Krebs Cycle (Citric Acid Cycle): This takes place in the mitochondria and requires oxygen.
* Production of 2 ATP molecules: The Krebs cycle involves a series of reactions that oxidize pyruvate, generating a small amount of ATP through substrate-level phosphorylation. However, the primary role of the Krebs cycle is to produce electron carriers (NADH and FADH2) that will be used in the next stage.
3. Electron Transport Chain: This is the final stage and also occurs in the mitochondria.
* Production of approximately 28-34 ATP molecules: The electron transport chain utilizes the high-energy electrons from NADH and FADH2 to pump protons across the inner mitochondrial membrane, creating a proton gradient. This gradient drives the movement of protons back across the membrane through ATP synthase, which uses the energy to generate ATP through oxidative phosphorylation.
In summary:
* Glycolysis: 2 ATP
* Krebs Cycle: 2 ATP
* Electron Transport Chain: ~28-34 ATP
Therefore, the majority of ATP produced during cellular respiration comes from oxidative phosphorylation in the electron transport chain.
