Mitochondria convert the energy stored in glucose into a form that the cell can use, which is ATP (adenosine triphosphate).

Here's a breakdown of the process:

* Glucose: This is a sugar that is the primary source of energy for most cells.

* Mitochondria: These are organelles within cells that are responsible for cellular respiration.

* Cellular Respiration: This is a series of chemical reactions that break down glucose and release energy in the form of ATP.

* ATP: This is a molecule that acts as the cell's energy currency.

So, the answer is glucose.