What a Wonderful World (3 page)

A rocket rises on a column of white smoke and orange flame. A baby kicks out in a moment of joy. These two things may appear to have nothing in common. But appearances are deceptive. Both are energised by essentially the same chemical reaction. Both are powered by rocket fuel.

A moment’s thought shows why this is not surprising. Boosting a heavy rocket into space requires the most powerful fuel – the one that, pound for pound, packs the biggest oomph. Life on Earth has been engaged for 3.8 billion years in trial-and-error experimentation. It would be odd if, in its efforts to power living organisms, it too had not stumbled on the most potent available energy source.

That energy source is the chemical reaction between hydrogen and oxygen. In the case of all animals, it is hydrogen extracted from food and oxygen extracted from the air. In the case of a rocket, it is liquid hydrogen and liquid oxygen.

So how does the reaction between hydrogen and oxygen work? And where exactly does the tremendous energy come from? That requires a little background knowledge.

All atoms, including those of hydrogen and oxygen, consist of a tiny nucleus and even tinier electrons. The electrons orbit the nucleus, snared by its powerful electric force in much the same way that planets, influenced by the force of gravity, orbit the Sun. There are many different ways the electrons can orbit
in a given atom. But, in general, they are happiest being as close to the nucleus as possible to minimise their energy.

This is a general principle of physics. For instance, a ball high on a hillside is said to have high gravitational energy. Given the slightest opportunity, it will try to minimise its energy – that is, roll down to the bottom of the hill where it has low gravitational energy. Similarly, the electrons in an atom, as surely as balls rolling downhill, will try to minimise their energy.

When two atoms come together, there may be new ways for their combined electrons to arrange themselves. If there is a configuration with a lower total energy than in the two separate atoms, then, as inevitably as a ball rolling downhill, the atoms will combine to form a molecule. This is all chemistry is: the rearrangement of electrons.

Since the energy of the molecule is less than the energy of the separate atoms that came together to make it, there is energy left over. It is a cornerstone of physics that energy can be neither created nor destroyed, only transformed from one form to another – for instance, from electrical energy into light energy. Consequently, the surplus energy becomes available to
do
things.
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In a rocket, for instance, the reaction between a hydrogen atom and an oxygen atom – actually,
two
hydrogen atoms react with each oxygen atom to make H
₂
O (water) – liberates a large amount of energy. This heats the water and expels the white vapour at great speed from the back of the rocket. Action and reaction being equal and opposite, the high-speed exhaust propels the rocket forward.

The liberation of so much energy by the reaction between hydrogen and oxygen is the reason it can lift a rocket to the edge of space.
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It is the reason why a marathon runner can go 26 miles
385 yards on a bowl of pasta. It is the reason why the reaction is exploited by every last animal on Earth.

Actually, the reaction between hydrogen and oxygen is not the only one that liberates energy. Before oxygen was present in substantial quantities in the Earth’s atmosphere, organisms gleaned their energy from much less efficient processes such as fermentation. Yeast cells make alcohol via fermentation. The muscles of sprinters, when they run short of oxygen, make lactic acid via fermentation. In the fermentation process only about 1 per cent of the surplus energy is made available to do work. This can be compared with a whopping 40 per cent in the case of the reaction of hydrogen with oxygen.

These two numbers tell us something interesting and profound about the biological world. In order to have carnivores, it is necessary to have at least three layers in the food chain: plants, animals that eat plants, and animals that eat animals that eat plants. But, if only 1 per cent of the energy of plants is available to the animals that feed on them, then only 1 per cent of 1 per cent – that is a mere 0.01 per cent – is available to animals that feed on those animals, and so on.

So until oxygen became available in more-or-less modern quantities about 580 million years ago, there could be no carnivores. (Actually, bacteria learned the oxygen trick more than 2 billion years ago but there were only tiny amounts of O
₂
available at the time.) In fact, it is estimated that the oxygen trick boosted the amount of biomass on Earth by an astonishing factor of about 1,000. Instead of two tiers, or trophic levels, levels in the food chain, suddenly it was possible to have five or six. The bewildering complexity of life on Earth today owes everything to the exploitation of oxygen.

But how exactly does the oxygen trick work? In a rocket, hydrogen and oxygen combine to make water, with the explosive release of a large amount of heat energy. Clearly, living organisms do not make use of such a violent process. They would be blown apart. Instead, they liberate the energy, step by step, in a far less destructive and subtle way.

What actually happens when hydrogen and oxygen react together in a rocket is what happens in all chemical reactions: the electrons play a game of musical chairs. Specifically, an oxygen atom grabs electrons from two hydrogen atoms.
3
In the process, the oxygen and hydrogen atoms fuse into a molecule of water.
4
However, say the hydrogen atoms supply electrons to an oxygen atom
but the hydrogen atoms and oxygen atom never actually meet
? This is the non-explosive twist on the oxygen–hydrogen reaction that is exploited by biology.

The first requirement is to obtain hydrogen. The gas does not exist in its free state on Earth. Being the lightest of all gases, if created in any quantity, it would float off into space. Inside a cell, however, an amazingly subtle and energy-efficient process called the Krebs cycle strips hydrogen atoms from food – that is, from either molecules of sugar (glucose – C
₆
H
₁₂
O
₆
) or fat. Two hydrogen atoms then donate their electrons to an oxygen atom. Only this does not happen directly, as in a rocket. Between the hydrogen atoms and the oxygen atom stretches a long wire made of protein complexes.
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And the donated electrons, bursting with excess energy, hop from location to location down the wire.

Focus on a single electron. As it hops down the wire, as inevitably as a ball rolling downhill, it drives hydrogen nuclei, or
protons,
6
through channels, or pores, in the cell membrane.
7
Since protons carry an electric charge – the opposite of electrons – this charges up one side of the membrane with respect to the other. Something like this happens in a battery; it creates an electric force field between the battery’s terminals. And, actually, this hints at what the super-energetic electron does as it thunders down the protein wire to an oxygen atom: it turns the cell membrane into a charged-up battery. The resulting electric force field across the membrane is stupendously powerful. It is comparable, in fact, to the electric field that, in a thunderstorm, breaks down the atoms in the air and unleashes a multimillion-volt bolt of lightning.
8

You might imagine that the cells in your body should crackle with lightning. However, the tremendous electric force field extends over only the tiny thickness of a cell membrane – about 5 millionths of a millimetre – and other molecules intervene to stop this force field having its way. Interestingly, however, in programmed cell death, or apoptosis, this protective mechanism is turned off and cells are in effect killed off by their own internal lightning bolts.

The powerful electric force field of the membrane battery drives a chemical reaction that creates adenosine triphosphate, or ATP. Such molecules are stores of energy; think of them as portable batteries. So, as the electron bounces down the protein wire, losing energy all the while, it leaves in its wake a large number of energy-packed ATP molecules. Released into the wild, these have the ability to power cellular processes wherever and whenever necessary.

In the final analysis, you are battery powered. There are about a billion ATP molecules in your body, and all of these are used
and recycled every 1–2 minutes. Toys may require a handful of batteries that become flat in a few hours. Contrast this with your body, which uses up 10 million power packs
every second
. Thank goodness that, for human beings, batteries are included.

Finally, the electron arrives at the end of the protein wire, exhausted of its energy. There it combines with the waiting oxygen atom. When a second electron from another hydrogen atom joins it, the oxygen atom achieves the highly desirable state of a filled outer shell of electrons. But this is not quite the end of the story. If the oxygen atom passes the electrons to a carbon atom – left behind when hydrogen was stripped from the food in the Krebs cycle – the result is a very stable molecule of carbon dioxide. And carbon dioxide, along with water vapour, is what oxygen-breathing animals exhale as waste.

So much for the chemistry of respiration; what about its physiology? Well, we breathe in air, of which about 20 per cent is oxygen. Only about a quarter of this actually gets used, so exhaled air still contains about 15 per cent oxygen. This is why it is possible to revive an unconscious person with exhaled breath via mouth-to-mouth resuscitation.

Our breath is drawn deep down into our lungs, the inner surfaces of which have a structure on the smallest scale rather like a branching tree. The branches, known as alveoli, run alongside fine blood vessels, and oxygen molecules pass from them to red blood cells. The tree-like structure of the alveoli maximises the area over which this oxygen transfer can occur, maximising the amount of oxygen that can enter the blood stream. Remarkably,
the surface area of a human lung is similar to that of a tennis court.

When an oxygen molecule is transferred to a blood cell, it is picked up by a giant protein called haemoglobin. It is then ferried to a cell where the oxygen is combined with hydrogen stripped from food to liberate energy for the cell. Crucially, haemoglobin changes its behaviour depending on the acidity of its surroundings. The acidity at its cellular destination changes the protein in a subtle way so that it
repels
rather than attracts its passenger. The protein therefore drops off its precious oxygen molecule. But the change in the haemoglobin means that it now
attracts
a molecule of carbon dioxide. As soon as one latches on, it is promptly ferried back to the lungs, where it passes from blood vessels to alveoli and is exhaled.

The oxygen we breathe and that powers all the biological processes in our bodies is essential to keep us alive. Whereas we can survive without food for a month, and without water for a week, we can survive with our air supply cut off for only about three minutes.
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Every instant of our lives we are a mere three minutes from death. This fact becomes shockingly apparent to a heart-attack victim whose heart stutters to a halt and stops pumping oxygen around the arteries and blood vessels of his or her body.
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But where does the oxygen we breathe come from? The answer, of course, is plants. Rather than breathing in oxygen and breathing out carbon dioxide, they take in carbon dioxide and pump out oxygen.

Pretty much all the energy used by life on Earth is therefore ultimately the energy of sunlight, which plants capture directly from the Sun.
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The trick is mind-bogglingly clever – otherwise we would long ago have found a way of mimicking it and powering human civilisation directly from sunlight. The energy of a particle of light – a photon – is transferred to an electron in a giant protein called chlorophyll. This is the molecule responsible for the green pigment of plants, although life also uses a second, non-green version. Bursting with energy, the electron can then energise chemical processes. Photosynthesis is a bewilderingly complex process but, in essence, it achieves the exact opposite of respiration.

Whereas respiration splits hydrogen from foods such as sugars and passes its electron to oxygen, ejecting as waste carbon dioxide, photosynthesis splits hydrogen from water and uses it with carbon from carbon dioxide to build sugars, ejecting as waste the leftover oxygen. Pause for a moment to think what an amazing trick this is. Using nothing more than water, carbon dioxide from the air and sunlight, plants are able to synthesise energy-rich food.

The sugars made by plants are in essence captured sunlight. And, whenever we eat plants, we in effect unleash the energy of this captured sunlight. But the miracle does not end there. Some plants such as trees, when they die, can become buried and transformed by heat and pressure deep down in the ground into fossil fuels such as coal. When we burn coal, we unleash yesterday’s sunlight. Ultimately, everything on Earth is powered by a beam of captured sunlight.

Photosynthesis is actually quite inefficient. The percentage of incoming light energy that is converted into sugar in most plants
is only about 1 per cent. The race is on, therefore, not only to create artificial photosynthesis but to make it
significantly better
than in nature – converting say 20 per cent of incident sunlight into hydrogen.