Here's why:

* Mitochondria are the "powerhouses" of the cell: They are responsible for producing ATP (adenosine triphosphate), the main energy currency of the cell.

* Respiration involves breaking down glucose: This process happens in the mitochondria, where glucose is converted into ATP.

* The process is complex: Respiration involves several steps, including glycolysis (which occurs in the cytoplasm), the Krebs cycle (which occurs in the mitochondrial matrix), and the electron transport chain (which occurs on the inner mitochondrial membrane).

While respiration begins in the cytoplasm with glycolysis, the majority of the energy-producing steps occur within the mitochondria.