Power capacity, effective age, and life limit of the living cell.

Physical and chemical factors limit the maximum amount of energy that a living cell can expend per unit time. As the cell ages, its capacity to produce energy gradually decreases, ultimately vanishing when it reaches its natural life limit. The present analysis shows that the maximum energy-spending capacity of a cell, referred to as the cell's power capacity, depends on the overall solute concentration of the cytosol when the cell is in its rest state. An effective age is then introduced that is proportional to the cell's power capacity rather than time, as does chronological age. Being related to the cell's power, the effective age provides an indicator of the cell's vitality, irrespective of its chronological age. Old cells can be energetically young if their effective age is low. In contrast, a chronologically young cell with a high effective age is nearing the end of its life due to a lack of power capacity. This paper shows how the cell's effective age can be determined experimentally and how it can be used to know the state of health of the cells in an organ or to assess the efficacy of treatments aimed at rejuvenating or demising a cell. Finally, the experimental data available today to support the present study are discussed.