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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]

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]

The mitochondrial intermembrane space is the compartment between the outer and inner mitochondrial lipid bilayers, including the spaces within cristae. It is separated from the mitochondrial matrix by the inner lipid bilayer. During oxidative phosphorylation, respiratory proton pumps transfer hydrogen ions from the mitochondrial matrix toward this compartment. Return through ATP synthase couples the resulting proton-motive force to ATP synthesis. The relevant stored energy includes both an electrical potential difference and a chemical concentration difference. ^[ref]

The compartment also contains cytochrome c, an electron carrier associated with the outer face of the inner lipid bilayer. Its release into the cytosol can contribute to apoptosis. In a cell-free study, depletion of cytochrome c reduced apoptotic activity and add-back restored it, connecting loss of compartmentalization to a new biochemical function. ^[ref]

X. Liu (apoptosis researcher) is the first author of the 1996 paper reconstructing aspects of apoptosis in cell-free extracts. Fractionation identified a requirement for cytochrome c together with deoxyadenosine triphosphate; removal of cytochrome c diminished the apoptotic response, and adding it back restored activity.^[ref]

This experiment connected a protein associated with mitochondria to activation of a cellular death program. Removing and restoring one component is especially informative because it tests a biochemical requirement within the extract. The paper's indexed byline is X. Liu; this entry does not infer a full identity from those initials.