All living cells need to perform a variety of activities, and these activities require energy. Reproduction, in particular, takes a significant amount of energy. Cells that are starved of energy sources eventually die.
Even though cells in complex organisms serve specific purposes, they are alive by all accepted definitions of life. Because of this, they have to absorb energy, release waste and most importantly, reproduce. Each of these activities requires energy.
Cellular reproduction requires multiple steps. Before it can reproduce, a cell needs to split its DNA into two parts. After doing so, cells pull apart at their centers and eventually become two cells. Each of the resulting cells needs to have sufficient energy to survive and thrive.
Oxygen is an important source of energy for animal cells. When cells are deprived of oxygen, they eventually die. Different human cells have different energy requirements, and those that need the most energy tend to die faster than those that need less. Because of this, cells in the human brain, which require a significant amount of energy, begin to die within minutes. There are, however, ways to slow this death. Lowering the temperature of cells stops certain processes that damage them, and they can often recover once the flow of oxygen through blood has been restored.