Cellular stress is a demand-capacity mismatch: the demand on a cellular function exceeds the capacity currently available to perform it. The stressor is only one way to create that mismatch.
Two routes to the same mismatch
A mismatch can open from either side. Demand can climb while capacity holds steady, or capacity can fall while demand is unchanged. Both produce the same functional shortfall, and the cell's response is organized around closing the gap rather than around the original trigger.
Regulation versus stress
Ordinary regulation
- Deviation stays inside the existing regulatory reserve
- Already-active feedback corrects it
- No new gene expression program is required
- Example: a small, transient rise in cytosolic calcium is pumped back to resting level
Stress
- Deviation exceeds the regulatory reserve
- Existing feedback alone cannot restore the set-point
- Additional protective programs must be recruited
- Example: sustained calcium leak that overwhelms pump and buffer capacity
Because stress is defined against capacity, it is a property of the cell-stressor pair, not of the stressor alone. Two cells facing an identical environment can sit on opposite sides of the threshold.