The full name for acetyl CoA, the activated two-carbon unit that links the breakdown of food to energy production and biosynthesis. Coenzyme A itself is built from pantothenic acid, vitamin B5, which is why that vitamin is indispensable to metabolism: without it, the carrier molecule cannot be made and fuel processing stalls. In the mitochondria, acetyl coenzyme A feeds the citric acid cycle, generating the electron carriers that drive ATP synthesis, the energy currency of the cell. In the cytoplasm, the same molecule is the raw material for synthesizing fatty acids, cholesterol, and steroid compounds. Its dual role as both fuel and building block makes it one of the most studied molecules in biochemistry, and nutrition textbooks use it to show how a single compound unites the metabolism of carbohydrates, fats, and proteins.