by Nick Lane
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.
PART 2
The Vital Force
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 × 1026 watts and its total mass is 2 × 1030 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?”’
Part 2 of this book is broadly about Mitchell’s discovery of the way that life generates its energy, and the implications of his ideas for the origin of life. In later chapters, these ideas will enable us to see what the mitochondria did for us: why they are essential for the evolution of all higher forms of life. We’ll see that the precise mechanism of energy generation is vital: it constrains the opportunities open to life, and it does so very differently in bacteria and eukaryotic cells. We’ll see that the precise mechanism of energy generation precluded bacteria from ever evolving beyond bacteria—from ever becoming complex multicellular organisms—while at the same time it gave the eukaryotes unlimited possibilities to grow in size and sophistication, propelling them up a ramp of ascending complexity to the marvels that we see all around us today. But this same mechanism of energy generation constrained the eukaryotes, too, albeit in utterly different ways. We’ll see that sex, and even the origin of two sexes, is explained by the constraints of this same form of energy generation. And beyond that we’ll see that our terminal decline into old age and death also stems from the small print of the contract that we signed with our mitochondria two billion years ago.
To understand all this, we first need to grasp the importance of Mitchell’s insights into the energy of life. His ideas are simple enough in outline, but to feel their full force we’ll need to look a little deeper into their details. To do this, we’ll take a historical perspective, and as we go along we can savour the dilemmas, and the great minds that wrestled with them in the golden age of biochemistry, littered with Nobel Prizes. We’ll follow the shining path of discovery, which showed how cells generate so much energy that they put the sun in the shade.
4
The Meaning of Respiration
Metaphysicians and poets used to write earnestly about the flame of life. The sixteenth century alchemist Paracelsus even explicitly declared: ‘Man dies like a fire when deprived of air.’ While metaphors are supposed to illuminate truths, I suspect that the metaphysicians would have been contemptuous of Lavoisier, the ‘father of modern chemistry’, who argued that the flame of life was not merely a metaphor, but exactly analogous to a real flame. Combustion and respiration are one and the same process, Lavoisier said, in the kind of literal scientific spoiler that poets have protested about ever since. In a paper addressed to the French Royal Academy in 1790, Lavoisier wrote:
Respiration is a slow combustion of carbon and hydrogen, similar in every way to that which takes place in a lamp or lighted candle and, in that respect, breathing animals are active combustible bodies that are burning and wasting away… it is the very substance of the animal, the blood, which transports the fuel. If the animal did not habitually replace, through nourishing themselves, what they lose through respiration, the lamp would very soon run out of oil and the animal would perish, just as the lamp goes out when it lacks fuel.
Both carbon and hydrogen are extracted from the organic fuels present in food, such as glucose, so Lavoisier was correct in saying that the respiratory fuels are replenished by food. Sadly, he never got much further. Lavoisier lost his head to the guillotine in the French Revolution four years later. In his book, Crucibles, Bernard Jaffe assigns the ‘judgement of posterity’ to this deed: ‘Until it is realized that the gravest crime of the French Revolution was not the execution of the King, but of Lavoisier, there is no right measure of values; for Lavoisier was one of the three or four greatest men France has produced.’ A century after the Revolution, in the 1890s, a public statue of Lavoisier was unveiled. It later transpired that the sculptor had used the face, not of Lavoisier, but of Condorcet, the Secretary of the Academy during Lavoisier’s last years. The French pragmatically decided that ‘all men in wigs look alike anyway’, and the statue remained until it was melted down during the Second World War.
Though Lavoisier revolutionized our understanding of the chemistry of respiration, even he didn’t know where it took place—he believed it must happen in the blood as it passed through the lungs. In fact, the site of respiration remained controversial through much of the nineteenth century, and it was not until 1870 that the German physiologist Eduard Pflüger finally persuaded biologists that respiration takes place within the individual cells of the body, and is a general property of all living cells. Even then, nobody knew exactly whereabouts in the cell respiration took place; it was commonly ascribed to the nucleus. In 1912, B. F. Kingsbury argued that respiration actually took place in the mitochondria, but this was not generally accepted until 1949, when Eugene Kennedy and Albert Lehninger first demonstrated that the respiratory enzymes are located in the mitochondria.
The combustion of glucose in respiration is an electrochemical reaction—an oxidation to be precise. By today’s definition, a substance is oxidized if it loses electrons. Oxygen (O2) is a strong oxidizing agent because it has a strong chemical ‘hunger’ for electrons, and tends to extract them from substances such as glucose or iron. Conversely, a substance is reduced if it gains electrons. Because oxygen gains the electrons extracted from glucose or iron, it is said to be reduced to water (H2O). Notice that in forming water each atom of the oxygen molecule also picks up two protons (H+) to balance the charges. Overall, then, the oxidation of glucose equates to the transfer of two electrons and two protons—which together make up two whole hydrogen atoms—from glucose to oxygen.
Oxidation and reduction reactions are always coupled, because electrons are not stable in isolation—they must be extracted from another compound. Any reaction that transfers electrons from one molecule to another is called a redox reaction, because one partner is oxidized and the other is simultaneously reduced. Essentially all the energy-generating reactions of life are redox reactions. Oxygen isn’t always necessary. Many chemical reactions are redox reactions, as electrons are transferred, but they don’t all involve oxygen. Even the flow of electricity in a battery can be regarded as a redox reaction, because electrons flow from a source (which becomes progressively oxidized) to an acceptor (which becomes reduced).
Lavoisier was chemically correct, then, when he said that respiration was a combustion, or oxidation, reaction. However, he erred not just about the site of respiration, but also about its function: he believed that respiration was needed to generate heat, which he thought of as an indestructible fluid. But clearly we don’t function like a candle. When we burn fuel, we don’t simply radiate the energy as heat, we use it to run, to think, to build muscles, to cook a meal, to make love, or for that matter, candles. All these tasks can be defined as ‘work’, in the sense that they require an input of energy to take place—they don’t occur spontaneously. An understanding of
respiration that reflected all this awaited a better appreciation of the nature of energy itself, which only came with the science of thermodynamics in the mid nineteenth century. The most revealing discovery, by British scientists James Prescott Joule and William Thompson (Lord Kelvin), in 1843, was that heat and mechanical work are interchangeable—the principle of the steam engine. This led to a more general realization, later referred to as the first law of thermodynamics, that energy can be converted from one form into another, but never created nor destroyed. In 1847, the German physician and physicist Hermann von Helmholtz applied these ideas to biology, when he showed that the energy released from food molecules in respiration was used partly to generate the force in the muscles. This appliance of thermodynamics to muscle contraction was a remarkably mechanical insight in an age still besieged by ‘vitalism’—the belief that life was animated by special forces, or spirits, which could not be reproduced by mere chemistry.
The new understanding of energy eventually fostered an appreciation that the bonds of molecules contain an implicit ‘potential’ energy that can be released when they react. Some of this energy can be captured, or conserved in a different form, by living things, and then channelled into work, such as the contraction of muscles. For this reason, we can’t talk about ‘energy generation’ in living things, although it is such a convenient phrase that I have occasionally transgressed. When I say energy generation I mean the conversion of potential energy, implicit in the bonds of fuels like glucose, into the biological energy ‘currencies’ that organisms use to power the various forms of work; in other words, I mean the generation of more working currency. And it is to these energy currencies that our story now turns.