* Krebs Cycle (Citric Acid Cycle): This cycle is a central part of cellular respiration, taking place in the mitochondria. It breaks down pyruvate (derived from glucose) to generate energy in the form of ATP.

* FADH2: This is a reduced form of flavin adenine dinucleotide, a coenzyme involved in cellular respiration. It is produced during the Krebs cycle as electrons are transferred from certain molecules to FAD. These electrons will later be used in the electron transport chain to generate ATP.

* Pyruvic Acid: This is the starting molecule for the Krebs cycle. It is produced during glycolysis (the breakdown of glucose). Pyruvic acid is not formed during the Krebs cycle but is consumed by it.

In summary: FADH2 is formed during the Krebs cycle as a result of the breakdown of pyruvate. Pyruvic acid is the starting molecule for the Krebs cycle, not a product.