Reducing equivalents in fatty acid synthesis are supplied by
**Core Concept**
Fatty acid synthesis involves the sequential addition of two-carbon units to elongate the fatty acid chain. This process requires reducing equivalents, which are supplied by the cell's energy-rich molecules.
**Why the Correct Answer is Right**
The correct answer involves the NADPH molecule, which is a high-energy electron carrier that donates reducing equivalents to the fatty acid synthase complex. This process is crucial for the reduction of acyl-CoA molecules to form the fatty acid chain. The NADPH is generated in the pentose phosphate pathway, which is a parallel pathway to glycolysis. The reducing equivalents from NADPH are then used to drive the reduction reactions in fatty acid synthesis.
**Why Each Wrong Option is Incorrect**
**Option A:** NADH is incorrect because it is primarily involved in the citric acid cycle and oxidative phosphorylation, and does not donate reducing equivalents to fatty acid synthesis.
**Option B:** ATP is incorrect because it is primarily involved in energy transfer reactions and does not directly participate in fatty acid synthesis.
**Option C:** GTP is incorrect because it is primarily involved in protein synthesis and signal transduction pathways, and does not donate reducing equivalents to fatty acid synthesis.
**Clinical Pearl / High-Yield Fact**
Fatty acid synthesis is a key process in lipid metabolism, and is regulated by hormones such as insulin, which promotes fatty acid synthesis by increasing NADPH production. This is an important clinical correlation to remember, as insulin resistance and type 2 diabetes are associated with impaired fatty acid synthesis.
**Correct Answer: D. NADPH**