In what way could two eukaryotic cells be different form each other

They differ from the number and types of organelles they contain

Think of plant cells verses animals cells. Both are eukaryotic but plant cells will have a cell wall and chloroplasts and animals cells lack both these things. 

Mitochondria is the cell organelle that produced the energy that is needed in the body system in form of atp, that is why it is called the power house of the cell. energy is needed by the living organisms to carry out the activities which sustain life.  if the mitochondria is destroyed atp will not be produced. if the mitochondria is destroyed, energy production will stop and the affected living organism will not have the energy to carry out the necessary life processes; this will lead to the death of the organism.