Mitochondria generate energy for cells by converting oxygen and nutrients into adenosine triphosphate (ATP). ATP is a molecule that is able to store energy for cells to use. The production of ATP using oxygen in mitochondria is about fifteen times greater than the production of ATP without oxygen outside of the mitochondria. Without the mitochondria, cells would probably not be able to generate enough ATP to function.
The number of mitochondria in a cell depends on how much energy the cell needs to function. A cell may contain one large mitochondria or thousands of mitochondria.
Mitochondria contain their own DNA. This is thought to have originated when smaller prokaryotes were engulfed by larger prokaryotes. Over time, a symbiotic relationship developed where the smaller prokaryote provided energy for the larger prokaryote. Eventually, the larger prokaryotes evolved into a eukaryotic cell, while the smaller prokaryotes evolved into mitochondria.