The cell’s power plants
Mitochondria are the organelles that produce most of the cell’s usable energy, as ATP. Tissues with high energy demand — muscle, heart, brain — are especially dense with them, which is also why those tissues are among the first to show the effects of mitochondrial decline.
How cells make energy
Energy production runs by extracting electrons from food-derived fuel and passing them down a chain of proteins, using the released energy to manufacture ATP — a process that consumes most of the oxygen you breathe. It is far more efficient than the cell’s oxygen-free routes, which is why mitochondria are central to sustained effort.
Reactive oxygen species — and why a little is useful
Energy production generates reactive oxygen species (ROS) as a by-product. The simple “free-radical theory” treated these purely as damage, but the evidence now supports a more nuanced picture: low, transient ROS act as signals that trigger protective adaptations, while chronic excess is harmful. This idea — mitohormesis — reframes some stress as beneficial.
Mitochondria and aging
With age, mitochondrial function commonly declines — through reduced capacity, accumulated damage, and weaker quality control that fails to remove defective organelles. Reviews increasingly frame this not only as lost energy but as disrupted mitochondrial signalling that the rest of the cell reads as stress.
Exercise rebuilds mitochondria
Exercise is the most robust known stimulus for mitochondrial health. Endurance training in particular drives muscle to build new mitochondria and improve the quality of the existing pool, and it does so across the lifespan, including in older adults. That finding sits among the firmest in this field.
