Wonders of Life. Andrew Cohen
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Название: Wonders of Life
Автор: Andrew Cohen
Издательство: HarperCollins
Жанр: Прочая образовательная литература
isbn: 9780007452682
isbn:
Photosynthesis, therefore, has a dual job: to store energy and to make sugars. The rest of the equation – and in particular the oxygenic bit, which refers to the production of oxygen – is a largely irrelevant detail as far as a plant is concerned. This provides a clue as to how oxygenic photosynthesis evolved.
The molecular machinery of oxygenic photosynthesis in constructed from three distinct components known as photosystem I, photosystem II, and the Oxygen Evolving Complex, linked together by two electron transport chains. This linked molecular machine is known as the Z scheme. Photosystem I takes electrons and, using energy from the Sun collected by the pigment chlorophyll, forces them onto carbon dioxide to make sugars. Photosystem II functions in a different way. It uses another form of chlorophyll and, rather than forcing its energised electrons onto carbon dioxide, it cycles them around a circuit somewhat like a battery, syphoning off a little of the Sun’s captured energy and storing it in the form of ATP.
In order to make sugars and ATP, therefore, the plant needs sunlight, carbon dioxide and a supply of electrons. It doesn’t ‘care’ where those electrons come from. The plant may not care, but we certainly do, because plants get their electrons from water, splitting it apart in the process and releasing a waste gas (oxygen) into the atmosphere. This is the source of all the oxygen in the atmosphere of our planet, and so understanding the evolution of the Z scheme is of paramount importance if we are to understand how Earth came to be a home for complex animals like us. The story can be traced back over 3 billion years to a time when the only life on Earth were the single-celled bacteria and archaea.
CONVERSION OF WATER TO OXYGEN AND LIGHT TO ENERGY
This light micrograph shows cyanobacteria, or ‘blue-green algae’, which use phycocyanin to capture the energy of the Sun.
Take a look at this picture – it’s an image of a very particular type of bacteria. Look very closely at it because you have a lot to thank this particular kind of organism for. These are cyanobacteria – lowly bacteria that sit at the very bottom of the food chain. They’re the most numerous organisms on the planet. There are more of them on Earth than there are observable stars in the Universe, and these little creatures are what enabled you – and every other complex living thing that has ever lived on the planet, from dinosaurs to daffodils – to exist.
If you look at the picture carefully, you will see that, unlike the other monochromatic bacteria, this one is bursting with a kind of blue-green colour, which comes from a pigment known as phycocyanin – exactly the kind of pigment that would offer an organism protection from the Sun’s damaging UV radiation. But these bacteria don’t just use the pigment for protection, they use it to capture the energy of the Sun.
Today cyanobacteria are sometimes considered to be a problem. This image, although beautiful, is of a bloom of ‘blue-green algae’- or, more correctly, cyanobacteria – in Lake Atitlán in the Guatemalan Highlands. It provides a vivid example of bacteria reproducing at a ferocious rate, and, in some cases, this explosion of life can have a devastating effect on an ecosystem. Toxins produced by the bacteria can decimate water life and affect human health, so they are closely monitored by environmental agencies around the world. But we have cyanobacteria to thank for the oxygen we breathe, because it is a virtual certainty that oxygenic photosynthesis evolved in an ancient cyanobacterium.
The way to unravel the story of the evolution of the Z scheme is to look at how each individual part may have arisen. There is evidence that an early form of photosynthesis may have emerged as far back as 3.5 billion years ago in single-celled organisms that produced enigmatic mounds known as stromatolites (see Chapter 3), although the precise date is still an area of active debate and research. Whatever the date, there is general agreement that a simple form of photosynthesis, using energy from the Sun to synthesise sugars from carbon dioxide, just as photosystem I does in plants today, is very ancient. The pigment used today is chlorophyll, a member of a family of molecules known as porphyrins. Complex though they are, porphyrins have been found on asteroids, implying that they form naturally and are likely to have been around on Earth before the origin of life. There are still bacteria alive today that have only photosystem I. They take their electrons from easy targets, such as hydrogen sulphide or iron, and don’t therefore need much else in the way of machinery.
Over time, it is thought that some bacteria adapted this early photosynthetic machinery to perform a different task – the production of ATP. There are similarities between the two photosystems that strongly suggest a common origin and later specialisation.
Cyanobacteria are able to reproduce rapidly, and this can have a devastating impact on an ecosystem. This satellite image of Lake Atitlán in Guatemala shows blooms of cyanobacteria, caused by polluted runoff from the surrounding land.
The evolution of early versions of photosystems I and II in bacteria is therefore relatively well understood; their components are simple, and the chemistry reflects that occurring naturally on the early Earth. Things become more interesting, however, when we ask how these two machines came to be joined together in the Z scheme. While biologists don’t yet agree on the answer, one of the more elegant hypotheses, due to Professor John Allen at Queen Mary, University of London, and detailed in Nick Lane’s excellent book, Life Ascending, is as follows.
While some bacteria employed the precursor of photosystem I, and others used the precursor of photosystem II, there may also have been bacteria that possessed the genetic coding necessary to build both photosystems. This would allow them to switch between them, depending on environmental conditions and the availability of food. This is a relatively common thing for bacteria to do today; their genes can be switched on and off, allowing them to make hay while the sun shines – or at least, in this case, to use sunshine to make sugar or ATP, depending on whether the imperative is to reproduce or simply to survive. The possibility of an ingenious evolutionary adaptation now presents itself. What if it were possible to run these two machines at once, connecting the electron circuit from photosystem II into photosystem I, which would dutifully dispose of the cascade of electrons by pushing them onto carbon dioxide to form sugar? This would confer a great advantage on the organism in question, allowing it to make both food and ATP at the same time using sunlight as an energy source. This is certainly a plausible explanation for the separate evolution and then recombination of the two photosystems, but it leaves one remaining question: where does this machinery get its electrons? Here is where the Oxygen Evolving Complex enters the story and, with it, one of the most important evolutionary steps in the history of life on Earth.
The Oxygen Evolving Complex is an odd structure: more mineral than biological. It consists of four manganese atoms and a single calcium atom, held together in a lattice of oxygen. Manganese is locked away in vast mineral deposits on the ocean floor today, but in the early history of our oceans it would have been available in seawater for organisms to use. Bacteria use manganese to protect them from UV light, in much the same way as we use melanin – manganese is easily ‘photo-oxidised’, absorbing the potentially harmful UV photon and releasing an electron in the process. This may have been one of the ways in which electrons made their way into the primitive photosystem II in early bacteria. So manganese, at least, was already an important component of living things from the earliest СКАЧАТЬ