Power, Sex, Suicide: Mitochondria and the Meaning of Life (14 page)

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.

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 (O
₂
) 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 (H
₂
O). 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.

By the end of the nineteenth century, scientists knew that respiration took place in cells, and was the source of energy for every aspect of life. But how it actually worked—how the energy released by the oxidation of glucose was coupled to the energetic demands of life—was anybody’s guess.

Clearly glucose does not ignite spontaneously in the presence of oxygen. Chemists say that oxygen is thermodynamically reactive but
kinetically
stable: it doesn’t react quickly. This is because oxygen must be ‘activated’ before it is able to react. Such activation requires either an input of energy (like a match), or a catalyst, which is to say a substance that lowers the activation energy needed for the reaction to take place. For scientists of the Victorian era, it seemed likely that any catalyst involved in respiration would contain iron, for iron has a high affinity for oxygen—as in the formation of rust—but can also bind to oxygen reversibly. One compound that was known to contain iron, and to bind to oxygen reversibly, was haemoglobin, the pigment that imparts the
colour to red blood cells; and it was the colour of blood that gave the first clue to how respiration actually works in living cells.

Pigments such as haemoglobin are coloured because they absorb light of particular colours (bands of light, as in a rainbow) and reflect back light of other colours. The pattern of light absorbed by a compound is known as its absorption spectrum. When binding oxygen, haemoglobin absorbs light in the blue-green and yellow parts of the spectrum, but reflects back red light, and this is the reason why we perceive arterial blood as a vivid red colour. The absorption spectrum changes when oxygen dissociates from haemoglobin in venous blood. Deoxyhaemoglobin absorbs light across the green part of the spectrum, and reflects back red and blue light. This gives venous blood its purple colour.

Given that respiration takes place inside cells, researchers started looking for similar pigments in animal tissues rather than in the blood. The first success came from a practicing Irish physician named Charles MacMunn, who worked in a small laboratory in the hay loft over his stables, carrying out research in his spare time. He used to keep watch for patients coming up the path through a small hole in the wall, and would ring through to his housekeeper if he didn’t wish to be disturbed. In 1884, MacMunn found a pigment inside tissues, whose absorption spectrum varied in a similar manner to haemoglobin. He claimed that this pigment must be the sought-after ‘respiratory pigment’, but unfortunately McMunn could not explain its complex absorption spectrum, or even show that the spectrum was attributable to it at all. His findings were quietly forgotten until David Keilin, a Polish biologist at Cambridge, rediscovered the pigment in 1925. By all accounts Keilin was a brilliant researcher, an inspiring lecturer, and a kindly man, and he made a point of deferring priority to MacMunn. In fact, though, Keilin went well beyond MacMunn’s observations, showing that the spectrum was not attributable to one pigment, but to three. This enabled him to explain the complex absorption spectrum that had stumped MacMunn. Keilin named the pigments
cytochromes
(for cellular pigments) and labelled them a, b, and c, according to the position of the bands on their absorption spectra. These labels are still in use today.

Curiously, however, none of Keilin’s cytochromes reacted directly with oxygen. Clearly something was missing. This missing link was elucidated by the German chemist Otto Warburg (in Berlin) who received the Nobel Prize for his work in 1931. I say elucidated, because Warburg’s observations were indirect, and quite ingenious. They had to be, for the respiratory pigments, unlike haemoglobin, are present in vanishingly tiny amounts within cells, and could not be isolated and studied directly using the rough and ready techniques of the time. Instead, Warburg drew on a quirky chemical property—the binding of carbon monoxide to iron compounds in the dark, and its dissociation from them when illuminated—to work out the absorption spectrum of what he called the
‘respiratory ferment’.
¹
The spectrum turned out to be that of a haemin compound, similar to haemoglobin and chlorophyll (the green pigment of plants that absorbs sunlight in photosynthesis).

Interestingly, the respiratory ferment absorbed light strongly in the blue part of the spectrum, reflecting back green, yellow, and red light. This imparted a brownish shade, not red, like haemoglobin, nor green like chlorophyll. However, Warburg found that simple chemical changes could turn the ferment red or green, with spectra closely resembling those of haemoglobin or chlorophyll. This raised a suspicion, expressed in his Nobel lecture, that the ‘blood pigment and the leaf pigments have both arisen from the ferment… for evidently, the ferment existed earlier than haemoglobin and chlorophyll.’ His words imply that respiration had evolved before photosynthesis, a visionary conclusion, as we shall see.

Despite these great strides forward, Warburg still could not grasp how respiration actually took place. At the time of his Nobel Prize, he seemed inclined to believe that respiration was a one-step process (releasing all the energy bound up in glucose at once) and was unable to explain how David Keilin’s cytochromes fitted into the overall picture. Keilin, in the meantime, was developing the idea of a
respiratory chain
. He imagined that hydrogen atoms, or at least their constituent components, protons and electrons, were stripped from glucose, and passed down a chain of cytochromes, from one to the next, like firemen passing buckets hand to hand, until they were finally reacted with oxygen to form water. What advantage might such a series of small steps offer? Anyone who has seen pictures from the 1930s of the disastrous end of the Hindenburg, the largest zeppelin ever built, will appreciate the great amount of energy released by the reaction of hydrogen with oxygen. By breaking up this reaction into a number of intermediate steps, a small and manageable amount of energy could be released at each step, said Keilin. This energy could be used later on (in a manner then still unknown) for work such as muscle contraction.