About 32 ATP are produced from the oxidation of one molecule of glucose to CO₂. However, in many eukaryotes the inner mitochondrial membrane is impermeable to NADH, and a “toll” of one ATP must be paid for each NADH molecule produced in glycolysis that must be “shuttled” into the mitochondrial matrix. So in these organisms, the net yield of ATP is 30.

NADH shuttle systems transfer the electrons captured by glycolysis onto substrates that are capable of movement across the mitochondrial membranes. In muscle and liver tissues (and the brown fat in the opening story), an important shuttle involves glycerol 3-phosphate. In the cytoplasm,

NADH (from glycolysis) + dihydroxyacetone phosphate (DHAP) → NAD⁺ + glycerol 3-phosphate

Glycerol 3-phosphate is transferred to the outer surface of the inner mitochondrial membrane. At that surface,

FAD + glycerol 3-phosphate → FADH₂ + DHAP

The electrons flow from FADH₂ into the electron transport chain via ubiquinone (Q) (see Figure 9.8). DHAP is able to move back to the cytoplasm, where it is available to repeat the process. Note that the reducing electrons are transferred from NADH to FADH₂. As you know from Figure 9.8, the energy yield in terms of ATP from FADH₂ is lower than that from NADH. This lowers the overall energy yield.