In a mitochondria chemical energy becomes available to cells through a process called

Oxidative phosphorylation.

Explanation:

Mitochondria use oxygen, which is available within the cell to convert chemical energy from food in the cell to energy in the usable form to the host cell. This process is called oxidative phosphorylation. It occurs inside of the mitochondria. In the matrix of mitochondria, the production of NADH happens by the citric acid or Krebs cycle. After the creation of NADH, it is used by enzymes that are fixed in the mitochondrial inner membrane to produce ATP. Energy is stored in the ATP as a form of chemical bonds.

These bonds can be opened, and the energy can be delivered.