Updated November 07, 2013.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Definition:

Mitochondria are tiny structures within cells that provide the energy the cell needs to perform its functions (such as moving, dividing, secreting substances, or -- in the case of skeletal muscle cells -- contracting). Within the mitochondria, energy is produced by combining glucose and fatty acids with oxygen, a process called oxidation.