Eating the Sun: How Plants Power the Planet

Oliver Morton

Editor's Note: Today, we run excerpts from the upcoming book by Oliver Morton, Chief News and Features Editor at Nature, EATING THE SUN: How Plants Power the Planet, published by Harper Collins (additional excerpts available at the Harper Collins website).

Eating the Sun CoverIntroduction

Here's what happened today. What really happened.

Dawn broke first in the Pacific: because our international dateline is in the middle of our largest ocean that's where the day's dawn always breaks first, its tangential light reflected from a million waves and a few container ships into an empty sky. What wasn't reflected lit up the upper layers of the ocean, a soft new light for the fish and that which they feed on.

When it made landfall in the north, the sun swept over the tundra like water up a beach; a couple of hours later, at the other end of the world, it broke like a wave against the mountains and pastures of New Zealand. Soon it was filling the rice paddies of the Philippines and the shallows of the South China Sea. And every time the sunlight hit something green - something truly green, not something painted green or dyed green: something with a greenness that grew - the most important process on the planet began again.

When the light shone on the greenness, the greenness welcomed it, and comprehended it, and put it to use. The greenness was chlorophyll, a pigment. It was arranged in pools and the sunlight's energy bounced from one molecule to the next like a frog across lily pads before reaching the subtle trap at the pool's centre, the three-billion-year-old trap where the light of the sun becomes the stuff of the earth. As the trap's jaws snapped shut on the sunlight, the spring that powered those jaws pulled electrons from a nearby water molecule, breaking it up into hydrogen and oxygen. The hydrogen was used, along with the stream of electrons that flowed up through the trap, to turn carbon dioxide into organic matter. The oxygen was discarded.

In every plant reached by the dawn this extraordinary mechanism came to life millions of times over. There are hundreds of thousands of pigment pools and sunlight traps in every green cell, hundreds of thousands of cells in full-grown leaves. And once awakened by the light, the flow of electrons through the leaves did not stop until darkness fell. The carbon dioxide to which those electrons were channelled was turned first into a sugar and then into all sorts of other molecules. Some of them were used to thicken the plants' stems, to lengthen their leaves, to enrich the soil beneath them and to colour the flowers still held tight in their buds. The rest were used to fuel the processes that make such growth possible. Light made life: that is what photosynthesis means.

If the light-driven flow of electrons stopped, on this day or any day, so would everything else that you care about. So would almost everything that evolution has wrought. If this process stopped, so would the world. The planet wouldn't stop turning: dawns would still arrive with impressive regularity. But they wouldn't matter. No more datelines. No more dates.

The manufacture of life from light is not the only thing going on as our dawn sweeps over Asia. For every leaf that greets the dawn there's an ant that eyes the leaf; for the growing grass there are hungry calves, and for the fattening calves there are hungry men; for the sugar-swollen root there's a sugar-sucking fungus. For growth there's decay. All over the world, by day and night, animals and bacteria and fungi and the plants themselves are using the oxygen which photosynthesis spits out into the atmosphere to turn organic material back into carbon dioxide and water. In so doing, they liberate the energy the plants stored away. The two processes come close to cancelling each other out.

Today, though, things are out of balance. Today is a spring day - at least it is spring in the northern hemisphere, which is home to most of the world's plants, as well as most of its people. And on spring days, photosynthesis wins. Carbon is pulled out of the air and into the green faster than it can be returned. Today, the world is growing.

Oliver MortonBy the time it reaches me in England, the dawn has come halfway around the planet. Between the sloped roof of my neighbours' house and the high, unbroken side elevation of the ugly hall that sometimes plays host to my wife's dance troupe, the sun lights upon a line of four young sycamores, the tallest of them maybe ten metres high. I don't think anyone told the sycamores to be where they are, on a sliver of soil by the ramp down to the hall's car park. I think they just grew there. Twenty years ago, at a guess, they fluttered there as seeds. Since then they've grown into full-blown trees. And they've done it just by doing what every other plant that's been lit up over the past twelve hours has been doing: eating the sun.

They're not eating as greedily today as they will in a month from now. It is still spring, and they have only a few leaves. A month ago the branches were bare; a couple of weeks ago, they were in bud, their black winter lines touched with a slight fuzziness, a dream of the green to come. Now, though the shortest of the four is still only in bud, the tallest has small leaves held close to its upraised branches. And on the other two trees the leaves have opened out quite fully, especially at the far ends of the branches, where they flop into the air like spindly five-fingered flags. Sycamores, especially small sycamores, come into leaf early, to get a jump on the competition, eager to pull themselves a little higher into the sky and out of others' shade.

I mention the differences between these four sibling trees to point out that there is individuality here. For the most part, this book will be about the universal - about the physics and physiology of trap- ping sunlight and putting it to use, about the structure of the molecular machinery shared by everything that's green, about the historical role that the photosynthesizing plants play en masse in the planet's life. But it's worth remembering that the organisms that embody these universalities all have their little quirks - a particular pattern of branches, a slight suddenness in coming into bud, a shape xiv that reflects the shadow of an ugly building, an accommodation to the drainage offered by a parking ramp. Within the universal, the unique.

On the leaves of each of these variously developing trees, and on every other leaf that greets the sun, there are tiny openings through which the air that will be strip-mined of its carbon is let in. With the sugars built from this carbon, and with the energy stored in those sugars, the little leaves are made bigger; with sugars stored in the roots over winter and brought upwards in the sap new leaves are begun. Within a couple of months these sycamores, which a few weeks ago were just sets of sticks pointing upwards, will have increased their surface area a hundredfold by covering themselves with leaves. The trees will absorb many times more sunlight than the little patch of land on which they stand. Next year they will do the same - but they will spread a little wider and start off a little taller, having turned another year of air and sunshine into new rings of wood in their trunks and branches.

In the Atlantic, to the southwest of me, plankton are blooming off the coast of Africa, floating in cloudy eddies large and distinct enough to be seen from space, like Florentine marbling on the ocean's blue parchment. The plankton eat the sun in just the same way that the tundra and the forests and the rice paddies and the sycamores do, using inorganic carbon dissolved in the water that surrounds them. Unlike the trees, though, the plankton will not survive for year after year; they will not build up an architecture of trunk and branch and twig and leaf. The only growth they know is multiplication - which they carry out with great alacrity. For all that the bloom is a passing fancy of wind and current, rather than a solid landscape like a forest or savanna, it will end up weighing tens of thousands of tonnes. Transient as their lives may be, over the course of the year the plankton in the sea will suck up almost as much carbon dioxide as all the trees, grasses and other green things on the land.

By the time dawn has reached America, most of the people on the earth have woken up and gone to work; some in the east have retired for the night. During their working day, almost all of those who can see will have seen something green. In built-up London I can count well over fifty trees on the hundred-metre walk from my flat to the Light Rail station. We might not give a conscious thought to the ever-present green, but at some level we will enjoy it. The greenness of life is so important and all-pervading that evolution has tuned our eyes to discriminate among its various hues more pre- cisely than among those of any other colour, and so shaped our brains that we take solace in it. The green, we know without thinking, is good.

We don't just enjoy seeing the green. It shapes the possibilities of our lives. More than two billion of us will have tended to the eaters of the sun in some way today. We will have hoed the ground for them, planted them, fed them fertilizers. We will have picked their fruits, dug up their nutritious roots, fed them to our livestock and ourselves. We will have made their carcasses into fabrics and furni- ture and firewood. We will have tended to some of them simply for their beauty - and to others because we know no finer surface over which to run while kicking a ball.

And even if we ignore today's plants completely, if we cut our- selves off in concrete and steel, we will still rely on yesterday's. On this day we will burn over thirty million tonnes of fossil fuel to generate our electricity and drive our cars and fire our factories and warm our homes. And all that power and warmth comes from sunlight eaten long ago. Energy trapped 300 million years ago by trees simpler but grander than my sycamores ended up stored in coal; plankton like those now blooming off the Azores were trans- formed into oil and gas. The carbon in the carbon dioxide we give off by burning them is carbon taken from the ancient atmosphere they breathed.

There's a catch, though. The rate at which we reclaim energy from the distant past has produced an accounting error; the profits and loss in our carbon accounts no longer balance. Which brings us to the last place, more or less, to see our dawn; Hawaii. Set apart from all the continents, Hawaii is one of the best places on the xvi planet from which to measure the average composition of the atmos- phere. Such measurements have been made there almost every day for nearly fifty years. While the measurements may jump around a little from one day to the next, over time there are trends. Today's measurement will probably be a little lower than the one made a week ago, because today - and every day for the past few weeks - the great greenness of the northern hemisphere has been taking in carbon dioxide through photosynthesis much faster than it is released by respiration, or the burning of fossil fuel. The world is breathing in.

Over the course of the spring, the carbon-dioxide level will drop and drop as billions of tonnes of carbon are taken from the air and put into plants. Only in the autumn, when the leaves fall and the grasses lose their song, will the carbon-dioxide level start to grow again, the great exhalation of a world eating its stores of food while waiting for the sun's return. From Hawaii, you can see the carbon rise and fall in this annual cycle. You can watch the breathing of the world, year in and year out. And you can also see that, each summer, after the plants have had their fill, there is a bit more carbon dioxide left in the atmosphere than there was the year before. That's the residue of the fossil fuels: ash from energy stored away long ago. Ash building up faster than it can be swept away.

As the dawn moves past Hawaii, the day is almost done. On this day, and the next day, and every day, a scarcely conceivable 4000 trillion kilowatt hours of energy reached the top of the earth's atmos- phere as sunshine. Some was reflected back into space and some was absorbed by the atmosphere. Some warmed the land and the sea, its warmth driving the winds and the ocean currents. Only a small fraction of one percent of that sunlight was captured by the pools of chlorophyll. But this tiny fraction of a vast number is still vast: the scrap of sunlight eaten by the plants today represented a similar amount of energy to that stored in all the world's nuclear weapons put together. And over the course of the day, that energy served to turn hundreds of millions of tonnes of carbon dioxide into food and living tissue.

And as a result the world stayed alive.

That's what really happened today. 

The case for artificial photosynthesis

In a park outside Paris, I watch a line of poplars shimmering in the wind, their leaves scintillating like the sound of a shaken tambourine; I listen as Bill Rutherford leads a cosmopolitan table of earnest men in a discussion of the mechanisms underlying photosystem II; and I think of Los Alamos.

There are two models for the direct harvesting of sunlight-the photovoltaic cell and the leaf-and they are vastly different. A photovoltaic cell is a pure, peculiar, and unnatural type of stone, fashioned with skill in impeccably clean industrial foundries. It is contrived, but not complex; two or more pieces of silicon, or some other similar material, with an extremely carefully controlled level of impurities. When a photon excites an electron within this semiconductor, the electron becomes free to travel, and so does the "hole" that it has left behind. The flow of electrons in one direction and holes in the other will generate useful current for as long as the light keeps shining. There are no mechanisms, no features, no moving parts save the conducting electrons themselves.

A leaf, by way of contrast, is remarkably complex, from the ribs providing structure to the cells that open and close the stomata to the chloroplasts themselves and their magnificent membrane-bound machinery of photosystems and twirling ATP-makers and cytochromes. But it is nothing like so pure, nor so fixed in its properties. It is edible, not mineral. It grows on its own from dirt and air, which is good. But it is not long for this world. One is made in foundries, one grows from seed. One lasts decades, one a season. One is expensive, one is cheap. One is efficient, one is not. One makes current, one makes sugar. And though the semiconductor definitely has the edge, neither really fits the bill as a way of powering a civilisation. Existing photovoltaics are too expensive, and they produce electric power only some of the time. If they are to be broadly useful, they have to be made much cheaper, and some way has to be found to store the energy they produce. Leaves are already cheap, but they are inefficient; run-of-the-mill photovoltaics can turn 15% of the solar energy hitting them into electrical energy for our use, while the best plantations struggle to do a tenth as well.

As we have seen, to run even today's world on biofuels, let alone tomorrow's, would take enough space and water to be a hugely disruptive undertaking. And leaves don't make electricity, which much of the world works off: to get them to do so, you must burn them and drive turbines with the heat, losing even more in terms of efficiency, and introducing a new need for capital expenditure. Yet that trick of making themselves from scratch, of putting together all that complex machinery without being told to-that is undeniably neat. The challenge that faces us is to find new technologies that sit in the space between the photovoltaic cell and the leaf-new hybrids of industry and nature. To make leaf-like things that generate alternative fuels, or even, conceivably, electricity. To make industrial systems that capture the benefits of structural details as small as the fold of a protein-ideally learning some of those neat self-assembly tricks in order to produce such details spontaneously. We need to work on a whole gamut of solar conversion technologies: from those that make oils to those that make hydrogen to those that make electric currents; from those that need a lot spent on raw materials but pay it back through their efficiency to those that can be grown any time and anywhere for minimal cost; from those that power a city to those that power a telephone. We need choices.

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