Reducing equivalents produced in glycolysis are transpoed from cytosol to mitochondria by
## **Core Concept**
The question tests the understanding of cellular respiration, specifically the transport of reducing equivalents from glycolysis into mitochondria. Glycolysis occurs in the cytosol and produces NADH, which cannot directly cross the mitochondrial membrane. A mechanism is needed to transport these reducing equivalents into the mitochondria.
## **Why the Correct Answer is Right**
The correct answer, **B. Glycerol phosphate shuttle**, is a mechanism that facilitates the transfer of reducing equivalents from NADH in the cytosol into the mitochondria. In this shuttle, cytosolic NADH reduces dihydroxyacetone phosphate to glycerol-3-phosphate, which then donates electrons to the mitochondrial electron transport chain via glycerol-3-phosphate dehydrogenase, located on the outer mitochondrial membrane. This process effectively transports the reducing equivalents from NADH into the mitochondria, where they can contribute to ATP production.
## **Why Each Wrong Option is Incorrect**
- **Option A:** Malate-Aspartate shuttle is indeed another mechanism for transporting reducing equivalents into mitochondria, but it is not the only one and might not be as directly related to the question's context as the glycerol phosphate shuttle.
- **Option C:** Direct transport of NADH is not possible across the mitochondrial membrane, making this option incorrect.
- **Option D:** While the malate-Aspartate shuttle is a correct method for transferring reducing equivalents, the question seems to focus on another specific method.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the glycerol phosphate shuttle is particularly important in certain tissues and conditions. For example, it is significant in muscle cells and is used in some types of cells where the malate-aspartate shuttle is not active or is less efficient.
## **Correct Answer:** B. Glycerol phosphate shuttle