Just as you can work out a power station's efficiency by seeing how much electricity it produces for each unit of oxygen and fuel it burns, you can assess the efficiency of mitochondria by monitoring the amount of ATP produced for every unit of oxygen used.
Researchers from various departments of the University of Washington, Seattle, compared resting muscle cells from young (7-month) and old (30-month) mice. They found that old muscle used around half as much energy as young muscle, but that the mitochondria used just as much oxygen at both ages. This represents a 50% loss in efficiency.
"The best explanation for this loss of efficiency is that the mitochondria become leaky as they get old. Protons leak back into the mitochondria without making ATP, and so reduce the coupling between oxygen use and ATP production," says lead author David Marcinek, who works in the department of radiology.
This inefficiency means that elderly people's muscle produce less useable energy (ATP) for every unit of oxygen consumed, making normal activity seem more challenging and limiting their range of activities.