The process of single-cell destruction or programmed cell death is known as apoptosis.
Apoptosis is a type of programmed cell death which occurs in multi-cellular organism in contrast to necrosis which is a kind of cell death occurring due to acute cellular injury. Apoptosis is a well-controlled and programmed process which produces apoptotic bodies or cell fragments that are engulfed by the phagocytic cells and removes them before the spilling of cell content which can damage the surrounding cells.
The initiation of apoptosis takes place by two pathways: intrinsic pathway when the cells kill themselves sensing cell stress and extrinsic pathway when the cells are killed due to the signals obtained from other cells.
An organ is the biggest out of all of these. Organs are formed from groups of tissues and tissues are groups of cells. A molecule is one of the small holes you see your hair grow out from.
