What links Parkinson's disease, exercise intolerance, diabetes, and organ failure in sepsis? Anything common to such a disparate group would need to be quite fundamental, and there aren't many things more elementary than generating the energy needed to stay alive. This is the job of the mitochondrion—the dynamo of the cell—and recent research indicates that it contributes to a wide range of diseases.

Mitochondria are thought to have started off as free living prokaryotes that were engulfed by the ancestors of modern nucleated cells millions of years ago. One of the features hinting at their previous lives is that they have retained some of their own DNA.

Although the role of these tiny intracellular organelles is vital, their relevance to clinical practice has often seemed obscure. Cell biologists worked out how mitochondria make energy four decades ago. Since then medical students have had to trace out how, after a glucose molecule is broken down, electrons from its oxidised metabolites move along a series of mitochondrial membrane bound proteins, building up an electrochemical energy gradient that can be harnessed to make adenosine triphosphate (ATP), the main energy source for cellular reactions. But they have usually struggled to relate this to anything encountered on the wards.

Some clinicians may have encountered one or two of a handful of disorders attributed to mutations in mitochondrial DNA such as the maternally inherited Leber's hereditary optic neuropathy, which results in degeneration of the optic nerve. But diseases related to such mutations were regarded as rare, affecting perhaps one or two per million in the population, and the province of a few specialists.