Which statement best matches shuttle systems with their electron donors in mitochondria?

• A. Malate-aspartate uses FADH2; glycerol-3-phosphate uses NADH
• B. Malate-aspartate uses NADH; glycerol-3-phosphate uses NADH
• C. Malate-aspartate uses NADH; glycerol-3-phosphate uses FADH2
• D. Malate-aspartate uses FADH2; glycerol-3-phosphate uses NADH

Answer: (C)

The main idea is how cytosolic NADH energy is conveyed into the mitochondrial electron transport chain via two shuttles, each using a different mitochondrial electron donor. In the malate-aspartate shuttle, electrons from NADH in the cytosol are carried as malate into the mitochondria and then oxidized back to oxaloacetate by mitochondrial malate dehydrogenase, yielding NADH inside the matrix. So, the mitochondrial donor in this route is NADH. In the glycerol-3-phosphate shuttle, dihydroxyacetone phosphate is reduced to glycerol-3-phosphate by cytosolic NADH, and then mitochondrial glycerol-3-phosphate dehydrogenase reoxidizes glycerol-3-phosphate using FAD, transferring electrons to ubiquinone as FADH2. Thus, the mitochondrial donor here is FADH2. This is why the pairing “malate-aspartate uses NADH” and “glycerol-3-phosphate uses FADH2” correctly matches the two shuttles.