Cell survival means that a biological cell remains alive over a specified interval and under specified conditions. It differs from cell proliferation: a neuron can survive for years without dividing, while an increasing cell population can conceal simultaneous cell division and cell death. A cell viability assay measures a chosen property, such as retention of an intact cell membrane or continuing metabolism, rather than every possible future function. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 17, Programmed Cell Death: survival signals and regulation of apoptosis
The balance is actively regulated. Survival signals can restrain apoptosis; loss of a required signal can permit a death programme. In a cell-free experiment, removing cytochrome c suppressed apoptotic activity and adding it back restored that activity in the presence of dATP. This linked a normally compartmentalized mitochondrial protein to activation of an apoptosis pathway, rather than identifying survival merely with a high metabolic readout. [ref]
Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c by X. Liu and others. 1996, abstract: immunodepletion, add-back and cytosolic release
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