Cells that have no mitochondria are unable to convert oxygen into energy, found in the form of adenosine triphosphate (ATP). All multicellular eukaryotic organisms, including plants and animals, have mitochondria in some cells, but prokaryotes and some single-cell eukaryotes do not have mitochondria.
Although all multicellular eukaryotes have mitochondria, mitochondria do not exist in all cells. For instance, human red blood cells don't contain mitochondria, which prevents them from using the oxygen they carry. If these cells had mitochondria, they would use the oxygen instead of transporting it to other cells. Most unicellular eukaryotes that do not have mitochondria are parasitic, as they are unable to make energy for themselves and therefore must live off a host organism.
In addition to producing ATP, mitochondria serve a number of other functions. Mitochondria play a role in the construction of blood and certain hormones, while also helping to regulate the concentration of calcium ions within the cell. They also play a determining role in programmed cell death by releasing a certain chemical that signals the cell to die. Mitochondrial disorders can affect this function, causing the premature death of a large number of cells, which could damage organs. Most of these disorders are related to mutations within the mitochondrial DNA.