So the inner mitochondrial membrane is actually impermeable to NAD plus or NADH molecules. And that means these NADH molecules, once formed in glycolysis, cannot simply move across the membranes of the mitochondria. And their movement basically depends on a specialized membrane transport system that we call glycerol three phosphate shuttle. So let's begin in glycolysis. So in glycolysis, we oxidize glucose into pyruvate molecules. In the process, we also generate these NADH molecules.