The NADH dehydrogenase of the inner mitochondrial membrane of animal cells can accept electrons only from NADH in the matrix. The inner membrane is not permeable to NADH. This has to be reoxidised to NAD⁺ by O₂ via the respiratory chain. This can be overcome by the special shuttle systems carry reducing equivalents from cytosolic NADH into mitochondria by an indirect route.

1. Malate-aspartate shuttle system
2. Glycerol 3-phosphate shuttle system