Noise-threshold model of apoptosis. (A) The cartoon illustrates the cellular processes that serve as a basis for the model. The metabolic state of the cell determines the intracellular ATP level. The ATP level controls negatively the release of cytochrome c (cyt c) from mitochondria into the cytosol, which promotes apoptosis. Antiapoptotic proteins such as Bcl-xL prevent the release of cytochrome c and can compensate for low ATP levels. (B) The model assumes that each individual cell has a given threshold for the ATP level that separates death from survival. Cells that survive, emphasized in blue, are those with thresholds below the ATP level (set here at ~1 fmol/cell for illustrative purposes).