Here's why:
* Electron Transport Chain: NADH is a crucial electron carrier in the electron transport chain within the mitochondria. The energy from electrons flowing through the chain is used to pump protons across the mitochondrial membrane, creating a proton gradient. This gradient powers ATP synthesis.
* Shuttle Systems: NADH generated in the cytoplasm needs to be transported into the mitochondria. Two main shuttle systems exist:
* Glycerol-3-Phosphate Shuttle: This system produces 1.5 ATP per NADH.
* Malate-Aspartate Shuttle: This system produces 2.5 ATP per NADH.
Therefore, the exact number of ATP produced from one NADH molecule can range from 1.5 to 2.5.
Important Note: These are theoretical values. The actual ATP yield can vary due to factors such as:
* Proton Leak: Some protons can leak back across the mitochondrial membrane, reducing ATP production.
* Metabolic Conditions: The efficiency of the electron transport chain can be affected by factors like temperature and oxygen availability.
In summary, a simplified answer would be ~2.5 ATP per NADH, but the actual yield can vary.
