Mitochondria are small organelles, found within the cells of plants and animals. Mitochondria supply energy to the cell in which they reside. They produce a molecule called ATP, which stores energy in its chemical bonds. ATP is important in aerobic respiration and provides the primary site for the respiration to occur. Cells that have high energy demands typically have greater numbers of mitochondria than those with low energy demands.
Mitochondria are roughly oval in shape and possess a double membrane. The outer layer of the membrane forms the physical boundary that separates the external and internal environments. The inner layer of the membrane is highly convoluted and possesses extensions called cristae. The interior of the mitochondria is called the matrix. Mitochondria are small organelles, and a given cell may have thousands of the structures. The average mitochondria in an animal cell is about 0.5 to 1 micron in size.
Mitochondria have their own DNA that is different from that of the larger organism. The presence of this mitochondrial DNA, or mDNA, has led some scientists to propose that mitochondria are actually symbiotic bacteria that live within the cells of organisms. Nevertheless, the instructions for constructing some of the components within the mitochondria lie in the nuclear DNA of the organism, demonstrating the complex relationships between the organelles and their host cells.