Final answer:
FAD is reduced to FADH2 during step 6 of the citric acid cycle, where succinate is dehydrated into fumarate, leading to the reduction of FAD inside the mitochondria's matrix.
Step-by-step explanation:
In the citric acid cycle, Flavin Adenine Dinucleotide (FAD) is reduced to FADH2 during step 6. This process takes place in the matrix of the mitochondria and is a crucial part of cellular respiration. Specifically, in the sixth step, the conversion of succinate into fumarate occurs via a dehydration process that simultaneously transfers two hydrogen atoms to FAD, thereby reducing it to FADH2. It is important to note that the enzyme responsible for this step is located within the inner membrane of the mitochondrion, and unlike NADH, FADH2 remains attached to the enzyme, which then transfers the electrons directly to the electron transport chain. This distinction is vital because the energy held in the electrons from FADH2 is insufficient to reduce NAD+ but it can reduce FAD.