What would happen if the mitochondria was missing from the cell?

they are unable to convert oxygen into energy that is found in the form of ATP.

Firstly not all cell have mitochondrion including multicellular and unicellular. example is the red blood cell it has no mitochondrion which make it not to covert the oxygen in them into energy they transport them to the body. If a cell have no mitochondrion it mean the body can not make energy for them self and need to depend on others for oxygen