Here's why:

* Energy Production: Mitochondria are responsible for producing ATP (adenosine triphosphate), which is the main energy currency of cells. They do this through cellular respiration, a process that breaks down glucose (sugar) and converts it into energy.

* Cellular Respiration: This process involves a series of chemical reactions that take place within the mitochondria, using oxygen and releasing carbon dioxide as a byproduct.

* Food Source: The food that cells use for energy comes from sources like glucose (from carbohydrates) and fatty acids (from fats).

So, while cells need other organelles for various functions, the mitochondria are the ones that actually "manufacture" the energy the cell needs to operate.