Scientific Art

From Power, Sex, Suicide: Mitochondria and the Meaning of Life


Mighty Mitochondria!

The cover illustration is based on this original artwork by Odra Noel

To view more of Odra Noel's scientific art, visit her web site


A mitochondrion—one of many tiny power-houses within cells that control our lives in surprising ways

Mitochondria are tiny organelles inside cells that generate almost all our energy in the form of ATP. On average there are 300–400 in every cell, giving ten million billion in the human body. Essentially all complex cells contain mitochondria. They look like bacteria, and appearances are not deceptive: they were once free-living bacteria, which adapted to life inside larger cells some two billion years ago. They retain a fragment of a genome as a badge of former independence. Their tortuous relations with their host cells have shaped the whole fabric of life, from energy, sex, and fertility, to cell suicide, ageing, and death.


The first eukaryote—one cell engulfed another to form an extraordinary chimera two billion years ago

All true multicellular life on earth is made up of eukaryotic cells—cells with a nucleus. The evolution of these complex cells is shrouded in mystery, and may have been one of the most unlikely events in the entire history of life. The critical moment was not the formation of a nucleus, but rather the union of two cells, in which one cell physically engulfed another, giving rise to a chimeric cell containing mitochondria. Yet one cell engulfing another is commonplace; what was so special about the eukaryotic merger that it happened only once?


The elementary particles of life—energy generating proteins in the mitochondrial membranes 

The way in which mitochondria generate energy is one of the most bizarre mechanisms in biology. Its discovery has been compared with those of Darwin and Einstein. Mitochondria pump protons across a membrane to generate an electric charge with the power, over a few nanometres, of a bolt of lightning. This proton power is harnessed by the elementary particles of life—mushroom-shaped proteins in the membranes—to generate energy in the form of ATP. This radical mechanism is as fundamental to life as DNA itself, and gives an insight into the origin of life on Earth


A large cell with things inside—in eukaryotes, energy generation is internalized in mitochondria 

Bacteria ruled supreme on Earth for two billion years. They evolved almost unlimited biochemical versatility but never discovered the secrets of greater size or morphological complexity. Life on other planets may get stuck in the same rut. On Earth, large size and complexity only became possible once energy generation had been internalized in mitochondria. But why did bacteria never internalize their own energy generation? The answer lies in the tenacious survival of mitochondrial DNA, a two-billion-year-old paradox.


The more the merrier—mitochondrial numbers dictate the evolution of size and complexity

Does life inherently become more complex? There may be nothing in the genes to push life up a ramp of ascending complexity, but one force lies outside the genes. Size and complexity are usually linked, for larger size requires greater genetic and anatomical complexity. But there is an immediate advantage to being bigger: more mitochondria means more power and greater metabolic efficiency. It seems that two revolutions were powered by mitochondria—the accumulation of DNA and genes in eukaryotic cells, giving an impetus to complexity, and the evolution of warm-blooded animals, which inherited the earth


Death by apoptosis—mitochondria determine whether a cell lives or dies by enforced suicide

When cells in the body become worn out or damaged, they die by enforced suicide, or apoptosis. The cell blebs, is packaged up, and reabsorbed. If the mechanisms controlling apoptosis fail, the result is cancer, a conflict of interest between cells and the body as a whole. Apoptosis seems to be necessary for the integrity and cohesion of multicellular individuals, but how did once-independent cells come to accept death for the greater good? Today apoptosis is policed by mitochondria, and the machinery of death was inherited from their bacterial ancestors, suggesting a history of murder. So was the cohesion of the individual forged in deadly conflict.


Mitochondrial DNA—a tiny circular genome in the mitochondria, inherited from the mother

Males have sperm and females have eggs. Both pass on the genes in their nucleus, but under normal circumstances only the egg passes on mitochondria to the next generation—along with their tiny but critical genomes. The maternal inheritance of mitochondrial DNA has been used to trace the ancestry of all human races back to ‘Mitochondrial Eve’, in Africa 170 000 years ago. Recent data challenge this paradigm, but give a fresh insight into why it is normally the mother who passes on mitochondria. The new findings help explain why it was ever necessary for two sexes to evolve at all.


Ageing and death—mitochondria divide or die, depending on their interactions with the nucleus

Animals with a fast metabolic rate tend to age quickly and succumb to degenerative diseases such as cancer. Birds are an exception because they combine a fast metabolic rate with a long lifespan, and a low risk of disease. They achieve this by leaking fewer free radicals from their mitochondria. But why does free-radical leakage affect our vulnerability to degenerative diseases that on the face of it have little to do with mitochondria? A dynamic new picture is emerging, in which signalling between damaged mitochondria and the nucleus plays a pivotal role in the cell’s fate, and our own.

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