Cytoplasm to Mitochondria substrate shuttle is:

Ans. (b) Malate shuttleRef: Harper's Biochemistry, 30 ed. pg. 134-135th* There are certain shuttle systems that operate for transport of reducing equivalents from cytosol into mitochondria. These shuttle systems operate because mitochondrial membrane is impermeable to cytosolic NADH produced during glycolysis. In order to gain entry of NADH into mitochondrial electron transport chain, these shuttle systems are helpful.* Malate shuttle (aka malate-aspartate shuttle): it is most common and universal shuttle system. Mitochondrial NADH which gains entry from cytosol is oxidized by mitochondrial ETC, leading to formation of 2.5 molecules of ATP.* Glycerophosphate shuttle: in this mechanism, cytosolic NADH is used to reduce dihydroxyacetone phosphate to glycerol-3 phosphate which diffuse through the outer mitochondrial membrane into the inter-membrane space of the mitochondria. In this space, glycerol 3 phosphate is reoxidized to dihydroxyacetone phosphate and the reducing equivalents are transferred to FAD to form is oxidized in ETC to form 1.5 molecules of ATP.FADH2 FADH2