What is the first step in fatty acid synthesis?

The first step in fatty acid synthesis involves the transport of acetyl CoA to the cytoplasm. Acetyl CoA is the key building block for fatty acid synthesis and is generated in the mitochondria during the breakdown of carbohydrates, fats, and proteins. However, fatty acid synthesis primarily occurs in the cytoplasm. Thus, for synthesis to begin, acetyl CoA must be transported from the mitochondrial matrix, where it is produced, to the cytoplasm.

Once in the cytoplasm, acetyl CoA can then participate in the series of reactions that lead to the elongation of fatty acid chains. This transport process is essential because the mitochondrial and cytoplasmic compartments of the cell have distinct biochemical environments and functions, and the movement of acetyl CoA across the mitochondrial membrane is a critical regulatory step in the metabolic pathways that synthesize fatty acids.

The other options do not accurately represent the initial step in this process. Converting glucose into fatty acids is a broader metabolic pathway encompassing several steps rather than a first step. Starting with free fatty acids implies that synthesis is already underway, while combining glycerol with fatty acids implies the formation of triglycerides rather than the initiation of fatty acid synthesis itself.