Power, Sex, Suicide: Mitochondria and the Meaning of Life (13 page)

When we think of a rise in oxygen levels, we tend to think of more fresh air, but the effects can actually be startlingly counterintuitive. As I discussed in an earlier book,
Oxygen: The Molecule that Made the World
, what actually happens is this. The foul sulphurous fumes emanating from volcanoes contain sulphur in forms such as elemental sulphur and hydrogen sulphide. When this sulphur reacts with oxygen, it is oxidized to produce sulphates. This is the same problem we face today with acid rain—the sulphur compounds released into the atmosphere from factories become oxidized by oxygen to form sulphuric acid, H
₂
SO
₄
. The ‘SO
₄
’ is the sulphate group, and it is this group that the sulphate-reducing bacteria need to oxidize hydrogen—which in chemical terms is exactly the same thing as reducing the sulphate, hence the name of the bacteria. Here is the rub. When oxygen levels rise, sulphur is oxidized to form sulphates, which accumulate in the oceans—the more oxygen, the more sulphate. This is the raw material needed by the sulphate-reducing bacteria, which convert sulphate into hydrogen sulphide. Although a gas, hydrogen sulphide is actually heavier than water, and so it sinks down towards the bottom of the
oceans. What happens next depends on the dynamic balance in the concentrations of sulphate, oxygen, and so on. However, if hydrogen sulphide is formed more rapidly than oxygen in the deep oceans (where photosynthesis is less active because sunlight does not permeate down) then the outcome is a ‘stratified’ ocean. The best example today is the Black Sea. In general, in stratified oceans the depths become stagnant, reeking of hydrogen sulphide (or technically, ‘euxinic’), whereas the sunlit surface waters fill up with oxygen. Geological evidence shows that this is exactly what happened in the oceans throughout the world two billion years ago, and the stagnant conditions apparently persisted for at least a billion years, and probably longer.

Now to my point. When the oxygen levels rose, so too did the population of sulphate-reducing bacteria. If, like today, the methanogens couldn’t compete with these voracious bacteria, then they would have faced a pressing shortage of hydrogen. This would have given the methanogens a good reason to enter into an intimate partnership with a hydrogen-producing bacterium, such as
Rhodobacter
. So far, so good. But what forced the prototype eukaryote up into the oxygenated surface waters before it lost its genes for oxygen respiration? Again, it may have been the sulphate-reducing bacteria. This time, the competition could have been for nutrients like nitrates, phosphates, and some metals, which are more plentiful in the sunlit surface waters. If the prototype eukaryote were no longer tied to its waterhole, then it would benefit from moving up in the world. If so, competition may have pressed the first eukaryotic cells up into the oxygenated surface waters long before they lost their genes for oxygen respiration, where they would have found good use for them. What an ironic turn of events! It seems the majestic rise of the eukaryotes was contingent on unequal competition between incompatible tribes of bacteria, the glories of nature upon the flight of the weak. The Bible was right: the meek really did inherit the Earth.

Is this truly what happened? It’s too early to say for sure. I’m reminded of that amiably cynical Italian turn of phrase, which translates roughly as ‘It may not be true, but it is well contrived’. In my view, the hydrogen hypothesis is a radical hypothesis, which makes better use of the known evidence than any other theory; and it has about the right combination of probability and improbability to explain the fact that the eukaryotes arose only once.

Beyond that there is another consideration, which makes me believe the hydrogen hypothesis, or something like it, is basically correct—and this relates to a more profound advantage provided by mitochondria. It explains why
all
known eukaryotes either have, or once had (then lost) mitochondria. As we noted earlier, the eukaryotic lifestyle is energetically profligate. Changing shape and engulfing food is highly energetic. The only eukaryotes that can do it without mitochondria are parasites that live in the lap of luxury, and they
barely need to do anything
but
change their shape. In the next few chapters, we’ll see that virtually every aspect of the eukaryotic lifestyle—changing shape with a dynamic cytoskeleton, becoming large, building a nucleus, hoarding reams of DNA, sex, multicellularity—all these depend on the existence of mitochondria, and so can’t, or are at least highly unlikely to, happen in bacteria.

The reason relates to the precise mechanism of energy production across a membrane. Energy is generated in essentially the same way in both bacteria and mitochondria, but the mitochondria are internalized within cells, whereas bacteria use their cell membrane. Such internalization not only explains the success of the eukaryotes, but it even throws light on the origin of life itself. In
Part 2
, we’ll consider how the mechanism of energy-generation in bacteria and mitochondria shows how life might have originated on earth, and why it gave the eukaryotes, and only the eukaryotes, the opportunity to inherit the world.

Proton Power and the Origin of Life

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.

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

Energy and life go hand in hand. If you stop breathing, you will not be able to generate the energy you need for staying alive and you’ll be dead in a few minutes. Keep breathing. Now the oxygen in your breath is being transported to virtually every one of the 15 trillion cells in your body, where it is used to burn glucose in cellular respiration. You are a fantastically energetic machine. Gram per gram, even when sitting comfortably, you are converting 10 000 times more energy than
the sun
every second.

This sounds improbable, to put it mildly, so let’s consider the numbers. The sun’s luminosity is about 4 × 10
²⁶
watts and its total mass is 2 × 10
³⁰
kg. Over its projected lifetime, about 10 billion years, each gram of solar material will produce about 60 million kilojoules of energy. The generation of this energy is not explosive, however, but slow and steady, providing a uniform and long-lived rate of energy production. At any one moment, only a small proportion of the sun’s vast mass is involved in nuclear fusions, and these reactions take place only in the dense core. This is why the sun can burn for so long. If you divide the luminosity of the sun by its mass, each gram of solar mass yields about 0.0002 milliwatts of energy, which is 0.0000002 joules of energy per gram per second (0.2 αJ/g/sec). Now let’s assume that you weigh 70 kg, and if you are anything like me you will eat about 12 600 kilojoules (about 3000 calories) per day. Assuming barely 30 per cent efficiency, converting this amount of energy (into heat or work or fat deposits) averages 2 millijoules per gram per second (2 mJ/g/sec) or about 2 milliwatts per gram—a factor of 10 000 greater than the sun. Some energetic bacteria, such as
Azotobacter
, generate as much as 10 joules per gram per second, out-performing the sun by a factor of 50 million.

At the microscopic level of cells, all life is animated, even the apparently sessile plants, fungi and bacteria. Cells whirr along, machine-like in the way that they channel energy into particular tasks, whether these are locomotion, replication, constructing cellular materials, or pumping molecules in and out of the cell. Like machines, cells are full of moving parts, and to move they need energy. Any form of life that can’t generate its own energy is hard to distinguish from inanimate matter, at least in philosophical terms. Viruses only ‘look’ alive because they are organized in a way that suggests the hand of a designer, but they occupy a shadowy landscape between the living and the nonliving. They have all the information they need to replicate themselves, but must remain inert until they infect a cell, as they can only replicate themselves using the
energy and cellular machinery of the infected cell. This means that viruses could not have been the first living things on Earth, nor could they have delivered life from outer space to our planet: they depend utterly on other living organisms and cannot exist without them. Their simplicity is not primitive, but a refined, pared-down complexity.

Despite its obvious importance to life, biological energy receives far less attention than it deserves. According to molecular biologists, life is all about information. Information is encoded in the genes, which spell out the instructions for building proteins, cells, and bodies. The double helix of DNA, the stuff of genes, is an icon of our information age, and the discoverers of its structure, Watson and Crick, are household names. The reasons for this status are a mixture of the personal, the practical, and the symbolic. Crick and Watson were brilliant and flamboyant, and unveiled the structure of DNA with the aplomb of conjurors. Watson’s famous book narrating the discovery,
The Double Helix
, defined a generation and changed the way that science is perceived by the general public; and he has been an outspoken and passionate advocate of genetic research ever since. In practical terms, sequencing the codes of genes enables us to compare ourselves with other organisms and to peer into our own past, as well as the story of life. The human genome project is set to reveal untold secrets of the human condition, and gene therapy holds a candle of hope for people with crippling genetic diseases. But most of all, the gene is a potent symbol. We may argue over nature versus nurture, and rebel against the power of the genes; we may worry about genetically modified crops and the evils of cloning or designer babies; but whatever the rights and wrongs, we worry because we know deep down, viscerally, that genes are important.

Perhaps because molecular biology is so central to modern biology we pay lip service to the energy of life in the same way that we acknowledge the industrial revolution as a necessary precursor of the modern information age. Electrical power is so obviously essential for a computer to function that the point is almost too banal to be worth making. Computers are important because of their data-processing capacity, not because they are electronic. We may only appreciate the importance of a power supply when the batteries run out, and there’s no plug to be seen. In the same way, energy is important to supply the needs of cells, but is plainly secondary to the information systems that control it and draw on it. Life without energy is dead, but energy without information to control it might seem as destructive as a volcano, an earthquake, or an explosion. Or is it? The flood of life-giving rays from the sun suggests an uncontrolled flow of energy is not inevitably destructive.

In contrast to our worries over genetics, I wonder how many people exercise themselves over the sinister implications of bioenergetics. Its terminology is what the Soviets used to call obscurantist, as full of mysterious symbols as a
wizard’s robes. Even willing students of biochemistry are wary of terms like ‘chemiosmotics’ and ‘proton-motive force’. Although the implications of these ideas may turn out to be as important as those of genetics, they are little known. The hero of bioenergetics, Peter Mitchell, who won the Nobel Prize for chemistry in 1978, is hardly a household name, even though he ought to be as well known as Watson and Crick. Unlike Watson and Crick, Mitchell was an eccentric and reclusive genius, who set up his own laboratory in an old country house in Cornwall, which he had renovated himself, following his own designs. At one time, his research was funded in part by the proceeds from a herd of dairy cows, and he even won a prize for the quality of his cream. His writings did not compete with Watson’s
Double Helix
—besides the usual run of dry academic papers (even more obscure than usual in Mitchell’s own case), he expounded his theories in two ‘little grey books’, published privately and circulated among a few interested professionals. His ideas can’t be encapsulated in a visually arresting emblem like the double helix, redolent of the standing of science in society. Yet Mitchell was largely responsible for articulating and proving one of the very greatest insights in biology, a genuine and bizarre revolution that overturned long-cherished ideas. As the eminent molecular biologist Leslie Orgel put it: ‘Not since Darwin has biology come up with an idea as counterintuitive as those of, say, Einstein, Heisenberg or Schrödinger… his contemporaries might well have asked “Are you serious, Dr Mitchell?”’