Mitochondria is a site for an aerobic respiration to occur. The number of mitochondria in every organism cell is difference. The higher the energy required by cell, the more mitochondria number in the cell.
Mitochondria is covered by double membrane. The inner membrane is infold and called cristae. Cristae is necessary to extent mitochondria surface so that oxygen absorption process become more affective, to regulate enzyme transfer, and it is also responsible for energyzed molecule movement (ADP and ATP) trough membrane during cell respiration process.
Between membrane fold is a chamber called matrix. Matrix contains respiratory enzyme (cytochrome) and other compounds, such as nucleic acid and protein.
Source: Biology, 2004