All the living animals and plants are made of millions of cells which have specific characters and functions.
Every cell undergoes stages like cell birth, multiplication and cell death (a means to end life).
The process namely apoptosis and necrosis are the two pathways by which cell undergoes death.
This article assumes that you already know what is cell and its organelles. If not take your time to go through once before reading further.
But why should a cell undergo death while the whole animal freaks the thought of death???
The answer is to keep up the repair and homeostasis. Repair is to replace damaged cells with healthy ones for efficient function and homeostasis is to keep cell count at a fixed level.
If not, the new cells born keep on adding while old cells still remain. Then the bodies structure would have lost and there would be huge mass of tissue growing every where. This is what happens in cancer. The cancer cells do not die but multiply adding to growth of tissue. The homeostasis is lost as the signals for cell death processes are lost.
So the cell death processes keeps cell count normal and this cell death is executed by Two processes namely apoptosis, necrosis.
Apoptosis and necrosis are the two pathways by which a cell undergoes death and eventual destruction. Both lead to cell death but are quite different in terms of process, the action time and the intention for cell death.
Necrosis is an acute (sudden) unintended death process seen after most injuries or assaults. It is a fast process.
The cell destruction in this mode is quite drastic and is seen by cell swelling, rupture or bursting (example you can notice this type of cell death in ulcers in the mouth) and initiates inflammation around.
Apoptosis is a normal (slow), intentional and predetermined cell death process which ends by series of activities inside the cell before its dies and also gets destroyed.
Here the cell DNA and other important material is damaged, cell shrinks and gets detached from surrounding cells and by changes in its cell membrane structure (membrane blebbing) it gets consumed by macro-phages without troubling surrounding cells unlike that of necrosis.
So apoptosis seems to be a slow yet precise mechanism of cell death. The aim is to remove cells unfit for normal function and hoemostasis.
Apoptosis cancer: Cancer is a condition with loss of homeostasis of cell formation and cell death. The signals to induce apoptosis mechanism are destroyed in cancer cells and hence they continue to stay alive indefinitely while other new cancer cells are formed adding to huge mass of tumor tissue.
This loss of mechanism to induce apoptosis leading to cell death is the key focus in cancer treatment. The drugs used in cancer treatment are focused to cause toxicity to cancer cells by substituting the metabolites or causing other problems. But in doing so the other normal cells also die.
Hence there is focus to develop drugs which can induce apoptosis by caspase activation (an apoptotic protein) in cancer cells to enable programmed cell death.
Autophagy: This is a repair mechanism, where in the cell accumulates all the waste or worn out organelles into a vesicle namely lysosome and destroys them. Thus it derives energy and proteins from that degradation. It is a type of cell survival phenomenon but it some times gets initiated before and helps the apoptosis pathway by providing energy for complete cell destruction. Thus sometimes autophagy appears to be involved in cell death.
Thus it can be said apoptosis is a spontaneous cell death process and necrosis is cell death process mostly initiated by trauma. Yet recent research shows necrosis to be partly programmed with non apoptosis inducers.