Kreb's cycle, also known as the citric acid cycle or TCA cycle, occurs in the mitochondria. It is a series of chemical reactions that occur in the presence of oxygen and play a critical role in the production of energy in the form of ATP during cellular respiration. The cycle starts with the entry of acetyl-CoA into the cycle, which then undergoes a series of reactions to produce NADH and FADH2, which are important electron carriers. These electron carriers then go on to participate in the electron transport chain, leading to the production of ATP.