Mitochondria and ageing: why quality beats quantity

Two people of the same age give a small muscle sample. Their mitochondria, the tiny structures that make a cell’s energy, are counted. The counts come back almost the same. Yet one person climbs four flights of stairs without noticing, and the other stops on the second landing.

The difference is not how many mitochondria they have. It is how good those mitochondria are. This article explains how a cell keeps its mitochondria in working order, and why that system, not the count, is what ageing wears down.

Are mitochondria fixed parts, or a changing population?

Textbook drawings show mitochondria as tidy ovals, fixed in number, sitting in the cell like batteries in a drawer. A living cell looks nothing like that. Its mitochondria form a restless network that keeps merging and splitting, over minutes and hours.

That movement is the cell’s quality control system. When two mitochondria merge, they pool their contents: proteins, membrane, and copies of their own DNA. Scientists call this merging fusion. A damaged unit gets diluted by a healthier neighbour, whose working parts cover for the faulty ones. Mitochondria are a population in constant turnover, and that turnover is what keeps their quality up.

Splitting does the opposite job. The network pinches apart, a process called fission. A segment too damaged to rescue is cut off and tagged for removal. The cell then breaks it down through a disposal route reserved for mitochondria, called mitophagy. Fusion lets a damaged unit borrow from healthy neighbours. Fission isolates what cannot be rescued, so it can be removed.

Both halves of this system protect one special weakness. Mitochondria carry something no other part of the cell does.

Why do mitochondria have their own DNA?

Almost every gene you have sits in the cell’s nucleus. Those genes come in two copies, are wrapped in protective proteins, and are served by a full repair toolkit. Mitochondria keep a small genome of their own. It is a circle of DNA about 16,500 letters long, carrying 37 genes. Thirteen of those genes are parts of the energy machinery itself.

Three features make this DNA different. First, each cell holds hundreds to thousands of copies, so no single copy is decisive. Second, it comes only from your mother, because the egg supplies the embryo’s mitochondria. Third, it sits close to where energy production runs. It lacks the protective packaging of nuclear DNA, and it has fewer repair tools. Mitochondrial DNA is a second, smaller genome: many copies, inherited down the maternal line, and less protected than the DNA in the nucleus.

If copies pick up faults over decades, why does the cell not fail much earlier?

Why can a cell carry faulty copies for decades?

Because faulty and healthy copies live side by side in the same cell. Scientists call that mixture heteroplasmy. It is the most useful single idea in this field.

Picture the faulty share of copies climbing from one in five to one in two. For a long time, nothing measurable happens. Healthy copies make enough working machinery for the whole population. Fusion spreads that machinery through the network, and the faulty copies are carried. Then the faulty share crosses a line, roughly 60 to 80 per cent depending on the fault and the tissue. At that point the cover runs out. Function stays flat until the faulty share crosses a threshold. That is why decline can be silent for years and then look sudden.

You can see this in ageing muscle. Single fibres carry short segments where the energy machinery has gone dark. In each dark segment, one faulty version of the genome has copied itself past the threshold. The rest of the fibre carries on. So why was the faulty version not cleared out?

What does ageing actually change?

Mostly not the count. The turnover.

With age, the cycle of fusion and fission slows down. The clearance of tagged units also becomes less efficient. A mitochondrion that is neither rescued nor removed stays in the network. While it stays, it keeps copying its DNA. Faulty copies are not just kept, they are multiplied.

This is where the popular version goes wrong. The goal is almost always stated as more mitochondria, as if number were the variable. A tissue can hold or even raise its count while producing less energy, because the units being counted are poorer. Ageing slows turnover more than it changes the count. Average quality falls while the number stays respectable, which is why more mitochondria is not the goal.

If quality is the real variable, what actually improves it?

What can you do about mitochondrial quality?

The biology itself is settled. Fusion, fission, mitophagy and heteroplasmy thresholds are established laboratory science. The open questions are about measuring and improving them in a living person, and that research is moving quickly.

Combined endurance and resistance training has the strongest human evidence for improving mitochondrial function in older adults. The reason fits the biology above. Repeated demand pushes the population to turn over, and turnover holds quality up. Nutrient support is a frontier researchers are actively mapping, and it sits best alongside training rather than instead of it.

Quality control is the thread running through this whole field. The knowledge check will show you how firmly that idea has settled.