Here's why:

* Cellular Respiration: Mitochondria are the powerhouses of the cell. They are the sites of cellular respiration, a process that breaks down glucose (sugar) in the presence of oxygen to produce ATP (adenosine triphosphate). ATP is the primary energy currency of cells.

* ATP Production: Mitochondria have a complex internal structure with folds called cristae. These cristae increase surface area for the chemical reactions of cellular respiration to occur efficiently. The process generates a proton gradient across the mitochondrial membrane, which is used to generate ATP.

So, while other organelles contribute to the cell's overall function, it's the mitochondria that specifically break down fuel and release energy for the cell to use.