During gluconeogenesis, reducing equivalents from mitochondria to the cytosol are transported by:

Question

Image below highlights jugular foramen. Which of the following structures do not pass through the marked structure “A”?

Answer 1

Gluconeogenesis needs NADH out in the cytosol, but NADH itself is generated mainly inside mitochondria and cannot cross the inner mitochondrial membrane on its own. The cell solves this by disguising the reducing power as a small molecule that can cross. Let's look at each option.

Malate: Oxaloacetate inside the mitochondria is reduced to malate, using up mitochondrial NADH in the process. Malate then passes through a transporter in the inner membrane into the cytosol, where it is oxidized back to oxaloacetate, releasing NADH exactly where gluconeogenesis needs it. This is the carrier molecule the question is asking about.
Aspartate: Aspartate is part of the malate-aspartate shuttle too, but its usual job is to help move reducing equivalents into the mitochondria, and to carry the carbon skeleton back in the reverse direction of this specific export route, not to directly ferry NADH out for gluconeogenesis.
Glutamate: Glutamate donates or accepts an amino group in transamination reactions that support the aspartate-oxaloacetate interconversion, it acts as a helper in the shuttle chemistry but is not itself the molecule that crosses carrying the reducing equivalent.
Oxaloacetate: Oxaloacetate cannot cross the inner mitochondrial membrane through the transporter used in this pathway, this is precisely why the cell converts it to malate first.

Since malate is the molecule that physically crosses the membrane carrying the reducing equivalent, and is then oxidized in the cytosol to release NADH, it is the correct answer.

Let's summarize:

NADH cannot cross the mitochondrial membrane directly.
Oxaloacetate is reduced to malate inside the mitochondria, malate crosses over, and is oxidized back to oxaloacetate in the cytosol, releasing NADH there.

So malate is the carrier that exports reducing equivalents from mitochondria to the cytosol during gluconeogenesis.

Answer 2

Gluconeogenesis needs NADH out in the cytosol, but NADH itself is generated mainly inside mitochondria and cannot cross the inner mitochondrial membrane on its own. The cell solves this by disguising the reducing power as a small molecule that can cross. Let's look at each option.

Malate: Oxaloacetate inside the mitochondria is reduced to malate, using up mitochondrial NADH in the process. Malate then passes through a transporter in the inner membrane into the cytosol, where it is oxidized back to oxaloacetate, releasing NADH exactly where gluconeogenesis needs it. This is the carrier molecule the question is asking about.
Aspartate: Aspartate is part of the malate-aspartate shuttle too, but its usual job is to help move reducing equivalents into the mitochondria, and to carry the carbon skeleton back in the reverse direction of this specific export route, not to directly ferry NADH out for gluconeogenesis.
Glutamate: Glutamate donates or accepts an amino group in transamination reactions that support the aspartate-oxaloacetate interconversion, it acts as a helper in the shuttle chemistry but is not itself the molecule that crosses carrying the reducing equivalent.
Oxaloacetate: Oxaloacetate cannot cross the inner mitochondrial membrane through the transporter used in this pathway, this is precisely why the cell converts it to malate first.

Since malate is the molecule that physically crosses the membrane carrying the reducing equivalent, and is then oxidized in the cytosol to release NADH, it is the correct answer.

Let's summarize:

NADH cannot cross the mitochondrial membrane directly.
Oxaloacetate is reduced to malate inside the mitochondria, malate crosses over, and is oxidized back to oxaloacetate in the cytosol, releasing NADH there.

So malate is the carrier that exports reducing equivalents from mitochondria to the cytosol during gluconeogenesis.

During gluconeogenesis, reducing equivalents from mitochondria to the cytosol are transported by:

Show Hint

The Correct Option is A

Solution and Explanation

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