Mitochondria are energy factories in each cell that burn oxygen to make energy. Unless neutralized by antioxidants, byproduct (free radicals) damage can accumulate in cells, slow energy production, accelerate the process of aging, and kill brain cells. PET scans: a reduction in the efficiency of energy production in the mitochondria of brain cells occurs with age. (Carper, Jean. Your Miracle Brain. p 19-20. NY:HarperCollins Publishers, Inc., 2000.)
Mitochondria are sometimes described as cellular power plants because they generate most of the cell's supply of adenosine triphosphate or ATP, used as a source of energy for chemical processes. The number of mitochondria in a cell varies widely by tissue type and organism. Many cells have only a single mitochondrion, whereas others can contain several thousand mitochondria. (http://en.wikipedia.org/wiki/Mitochondrion.)
Disturbances in mitochondrial metabolism are now known to play a role not only in rare childhood diseases, but have also been implicated in many common diseases of aging. Mitochondrial diseases include Alzheimer’s, Diabetes Mellitus and deafness, Non-Insulin Dependent Diabetes Mellitus, and Sudden Infant Death Syndrome. (Source)
Refer to Energy and the Brain for additional information.