In the electron transport chain (ETC), which is a critical component of cellular respiration in mitochondria, the movement of electrons is coupled with the pumping of protons (H+) across the inner mitochondrial membrane. This creates a proton gradient that is used to generate ATP through ATP synthase.
For ATP synthase to produce ATP, it typically requires the movement of protons back into the mitochondrial matrix down their concentration gradient. The exact number of electrons that must be transferred through the electron transport chain does not directly correspond to a specific number of ATP molecules produced because the yield of ATP can vary based on several factors, including the type of electron donor and overall cellular conditions.
However, generally speaking, as a rule of thumb in human mitochondria, the complete oxidation of one molecule of glucose through glycolysis, the Krebs cycle, and oxidative phosphorylation can theoretically produce about 30 to 32 ATP molecules. The electrons that move down the chain results in the generation of a proton motive force, and typically, the movement of about 4 protons back into the matrix will generate 1 ATP via ATP synthase.
In practical terms, for one pair of electrons (such as those from NADH), approximately 10 protons are pumped out during their passage through the ETC, theoretically resulting in the synthesis of about 2.5 ATPs. For FADH2, about 6 protons are pumped out, yielding about 1.5 ATPs.
Thus, the number of electrons that need to move for ATP to be made is not fixed and depends on the starting substrates and conditions, but movement of pairs of electrons (from NADH or FADH2) is what initiates the synthesis of ATP through the resultant proton gradient.
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