Which shuttle is more efficient at transferring cytosolic NADH equivalents into the mitochondrial matrix?

Which shuttle is more efficient at transferring cytosolic NADH equivalents into the mitochondrial matrix?

• A. Glycerol phosphate shuttle
• B. Malate-aspartate shuttle
• C. Both equally efficient
• D. Neither shuttles transfers NADH equivalents

Answer: (B)

The main idea is how cytosolic NADH becomes usable inside the mitochondrial matrix. Cytosolic NADH can’t cross the inner membrane directly, so cells use shuttles to move its reducing power into the matrix. In the malate-aspartate shuttle, oxaloacetate in the cytosol is reduced to malate by NADH, carrying the reducing equivalents across into the matrix where malate is oxidized back to oxaloacetate, generating NADH inside the matrix. This means the electron donors entering the matrix are true NADH, providing the same reducing power as cytosolic NADH and feeding the electron transport chain at its standard entry point. The glycerol phosphate shuttle takes a different route. It transfers electrons to FAD in the inner membrane, forming FADH2, which donates electrons to the chain at a later point (lower energy yield). Because the reducing equivalents don’t arrive as NADH in the matrix, the overall ATP yield per cytosolic NADH is smaller. So, the malate-aspartate shuttle is more efficient at delivering NADH equivalents into the mitochondrial matrix.

The main idea is how cytosolic NADH becomes usable inside the mitochondrial matrix. Cytosolic NADH can’t cross the inner membrane directly, so cells use shuttles to move its reducing power into the matrix.

In the malate-aspartate shuttle, oxaloacetate in the cytosol is reduced to malate by NADH, carrying the reducing equivalents across into the matrix where malate is oxidized back to oxaloacetate, generating NADH inside the matrix. This means the electron donors entering the matrix are true NADH, providing the same reducing power as cytosolic NADH and feeding the electron transport chain at its standard entry point.

The glycerol phosphate shuttle takes a different route. It transfers electrons to FAD in the inner membrane, forming FADH2, which donates electrons to the chain at a later point (lower energy yield). Because the reducing equivalents don’t arrive as NADH in the matrix, the overall ATP yield per cytosolic NADH is smaller.

So, the malate-aspartate shuttle is more efficient at delivering NADH equivalents into the mitochondrial matrix.