The reduced coenzyme NADH produced in glycolysis in the cytoplasm reaches the electron transport chain by

  • A. diffusing freely through both mitochondrial membranes
  • B. being carried in by shuttle systems that transfer its electrons
  • C. being converted into FADH2 first
  • D. being broken down and never entering the mitochondrion

Explanation

The inner mitochondrial membrane is impermeable to NADH itself, so shuttle mechanisms transfer its electrons inside, sometimes at the cost of some ATP. This is why estimates of the total ATP yield vary between 36 and 38 per glucose.

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