Universe a grand tour of modern science Phần 8 pot

particles, but also the various forces. All matter particles feel the weak force carried by W particles, made of an electron and antineutrino or vice versa, and by Z particles with a neutrino and antineutrino, or some other composition. Photons made of an electron and anti-electron carry the electric force, to which all charged particles respond. The strong nuclear force, felt only by proton-like matter particles and mesons, is carried by gluons (colour and anticolour) within the particles and by mesons (quark and antiquark) operating between the particles. Mathematically, all of these forces are described by so-called gauge theories, which give the same results wherever you start from. The electric force provides a simple example of indifference to the starting point, in a pigeon perching safely on a power line while being repeatedly charged to 100,000 volts. Signals of a fraction of a volt continue to pass in a normal manner through the bird’s nervous system. Indifference to circumstances is a requirement if the various forces are to operate in exactly the same way on and within a proton, whether it is anchored in a mountain or whizzing through the Galaxy close to the speed of light, as a cosmic-ray particle. In other words, gauge theories are compatible with high- speed travel and Albert Einstein’s special theory of relativity. The obligation that the force theories must be of this type strengthens the physicists’ confidence in them. Those four par agraphs sum up the Standard Model, a well-rounded theory and one of the grandest outcomes of 20th-century science. It was created and largely confir med in an era of unremitting excitement. Almost as fast as theorists plucked ideas from their heads, experimenters manufactured the corresponding particles literally out of thin air, in the vacuum of their big machines. It was as if Mother Nature was in a mood to gossip with the physicists, about her arcane ways of running the Universe. The frenzy lasted for about 20 years, bracketed by the materializations of the triply strange omega particle in 1964 and the Z carrier of the weak force in 1984. But after that climax came a period of hush on the subject of the fundamental particles and the forces operating between them. Most particle physicists had to content themselves with confirming the predictions of the existing theories to more and more decimal places. If the Standard Model were complete in itself, and arguably the end of the story, Mother Nature’s near-muteness at the end of the 20th century would have been unsurprising. Yet neither criterion was satisfied. As Chris Llewellyn Smith of CERN commented in 1998, ‘While the Standard Model is economical in concepts, their realization in practice is baroque, and the model contains many arbitrary and ugly features.’ 527 particle families I Hoping for flaws Two decades earlier, in the midst of all the excitement, Richard Feynman of Caltech played the party pooper. He put his finger on one of the gravest shortcomings of the-then emergent Standard Model. ‘The problem of the masses has been swept into a corner,’ he complained. Theorists have rules of thumb that work well in estimating the masses of expected new particles, by reference to those of known particles. Yet no one can say why quarks are heavier than electrons, or why the top quark is 44,000 times more massive than the up quark. According to the pristine versions of the theories all particles should have zero intrinsic mass, yet only photons and neutrinos were thought to conform. The real masses of other particles are an arbitrary add-on, supposedly achieved by introducing extraneous particles. By the start of the 21st century physicists were beefing up their accelerators to address the mass problem by looking for a particle called the Higgs, which might solve it. They were also very keen to find flaws in the Standard Model. Only then would the way be open to a superworld rich in other particles and forces. The physicists dreaded the thought of entering a desert with nothing for their machines to find, by way of particle discoveries, to match the great achievements of the previous 100 years. The first hint that they might not be so unlucky came in 1998, with results from an underground experiment in Japan. These indicated that neutrinos do not have zero mass, as required by the Standard Model. Hooray! E For more about the evolution of the Standard Model, see Electroweak force, Quark soup , and Higgs bosons. For theories looking beyond it, see Sparticles and Superstrings. Other related entries are Cosmic rays and Neutrino oscillations. 528 particle families ‘G reen plants spread the enormous surface of their leaves and, in a still unknown way, force the energy of the Sun to carry out chemical syntheses, before it cools down to the temperature levels of the Earth’s surface.’ Thus, in 1866, the Austrian physicist Ludwig Boltzmann related the growth of plants to recently discovered laws of heat. By stressing the large leaf area he anticipated the 21st-century view of greenswards and the planktonic grass of the sea as two-dimensional photochemical factories equipped with natural light guides and photocells. Botanists had been strangely slow even to acknowledge that plants need light. In 1688 Edmond Halley told the Royal Society of London that he had heard from a keeper of the Chelsea Physic Garden that a plant screened from light became white, withered and died. Halley was emboldened to suggest ‘that it was necessary to the maintenance of vegetable life that light should be admitted to the plant’. But why heed such tittle-tattle from an astronomer? The satirist Jonathan Swift came unwittingly close to the heart of the matter in 1726, in Voyage to Laputa, where ‘projectors’ were trying to extract sunbeams from cucumbers. Half a century later Jan Ingenhousz, a Dutch-born court physician in Vienna, carried out his Experiments on Vegetables, published in London in 1779. He not only established the importance of light, but showed that in sunshine plants inhale an ‘injurious’ gas and exhale a ‘purifying’ gas. At night this process is partially reversed. The medic Ingenhousz is therefore considered the discoverer of the most impor tant chemical reactions on Earth. In modern terms, plants take in carbon dioxide and water and use the radiant energy of sunlight to make sugars and other materials needed for life, releasing oxygen in the process. At night the plants consume some of the daytime growth for their own housekeeping. Animal life could not exist without the oxygen and the nutrition provided by plants. The fact that small communities on the ocean floor subsist on volcanic rather than solar energy does not alter the big picture of a planet where the chemistry of life on its surface depends primarily on combining atoms into molecules with the aid of light—in a word, on photosynthesis. Thereby more than 100 billion tonnes of carbon is drawn from the carbon dioxide of the air every year and incorporated into living tissue. 529 I Chlorophyll, photons and electrons The machinery of photosynthesis gradually became clearer, in the microscopic and molecular contents of commonplace leaves. During the 19th and early 20th centuries scientists found that the natural green pigment chlorophyll is essential. It concentrates in small bodies within the leaf cells, called chloroplasts. The key chemical reaction of photosynthesis splits water into hydrogen and oxygen, and complex series of other reactions ensue. Another preamble to further progress was the origin of photochemistry. It star ted with photography but was worked up by Giacomo Ciamician of Bologna into a broad study of the interactions of chemical substances and light. The physicists’ discovery that light consists of particles, photons, opened the way to understanding one-on-one reactions between a photon and an individual atom or molecule. Electrons came into the story too, as detachable parts of atoms. Chlorophyll paints the land and sea green. Its molecule is shaped like a kite, with a flat, roughly square head made mainly of carbon and nitrogen atoms, and a long wiggly tail of carbon atoms attached by an acetic acid molecule. In the centre of the head is a charged magnesium atom that puts out four struts— chemical bonds—to a ring of rings, each made of four atoms of carbon and one of nitrogen. Different kinds of chlorophyll are decorated with various attachments to the head and tail. From the white light of the Sun, chlorophyll absorbs mainly blue and red photons, letting green light escape as the pigment’s colour. Because the chlorophyll is concentrated in minute chloroplasts, leaves would appear white or transparent, did they not possess an optical design that forces light entering a leaf to ricochet about inside it many times before escaping again. This maximizes the chance that a photon will encounter a chloroplast and be absorbed. It also ensures that surviving green light eventually escapes from all over the leaf. The pace of discovery about photosynthesis quickened in the latter half of the 20th century. Using radioactive carbon-14 to label molecules, the chemist Melvin Calvin of UC Berkeley and others were able to trace the course of chemical reactions involving carbon. Contrary to expectation, the system does not act directly on the assimilated carbon dioxide but f irst creates energy-rich molecules, called NADPH and ATP. These are portable chemical coins representing free energy that the living cell can spend on all kinds of constructive tasks. Conceptually they link photosynthesis to the laws of heat, as Boltzmann wanted. Teams in Europe and the USA gradually revealed that two different molecular systems are involved. Somewhat confusingly they are called Photosystem II and Photosystem I, with II coming first in the chemical logic of the process, 530 photosynthesis although it was the second to be discovered. II is where incoming light has its greatest effect, in splitting molecules of water to make oxygen molecules and dismembering the hydrogen atoms into positively charged protons and lightweight, negatively charged electrons. Water, H 2 O, is a stable compound, and splitting it needs the combined energy of two photons of sunlight. But as you’ll not want highly reactive oxygen atoms rampaging among your delicate molecules, you’d better liberate two and pair them right away in a less harmful oxygen molecule. That doubles the energy required for the transaction. To accumulate the means to buy one oxygen molecule, by splitting two water molecules at once, you need a piggy bank. In Photosystem II, this is a cluster of four charged atoms of a metallic element, manganese. Each dose of incoming energy extracts another electron from one manganese atom. When all four manganeses are thus fully charged, bingo, the system converts two water molecules into one oxygen molecule and four free hydrogen nuclei, protons. The four electrons have already left the scene. The other unit in the operation, Photosystem I, then uses the electrons supplied by II, and others liberated by light within I itself, to set in motion a series of other chemical reactions. They convert carbon dioxide into energy-rich carbon compounds. Human beings are hard put to make sense of the jargon, never mind to understand all the details. Yet humble spinach operates its two systems without a moment’s thought, merrily splitting water in one and fixing carbon from the air in the other. I Pigments as a transport system Like other plants, spinach also runs molecular r ailways for photons and electrons. These are built of carefully positioned chains of pigment molecules, mainly chlorophyll. For light, they can act first like antennas to gather the photons, and then like glass fibres to guide their energy to the point of action. It is mildly surprising to have pigment chains relaying light, but much more remarkable that they also transport free electrons at an astonishing rate. The possibility was unknown to scientists until the 1960s. Then the Canadian-born theorist Rudolph Marcus, working in the USA, showed how electrons can leap from molecule to molecule. In photosynthesis, this trick whisk s the liberated electrons away along the molecular railway, before they can rejoin the wrong atoms. It delivers them ver y precisely to the distant molecules where their chemical action is required. The separation of electric charges achieved by this means is the most crucial of all the steps in the photosynthetic process. It takes place in a few million- millionths of a second. Ultrafast laser systems became indispensable tools in 531 photosynthesis studying photosynthesis, to capture events that are quicker than any ordinary flash. The production of oxygen within milliseconds seems relatively leisurely, while the reactions converting carbon dioxide into other materials can take several seconds. The layout of the high-speed pigment railways became apparent in the first complete molecular structure of a natural photocell, converting light energy into electrical energy. Its elucidation was a landmark in photosynthesis research. In 1981, at the Max-Planck-Institut fu ¨ r Biochemie, Martinsried, Hartmut Michel succeeded in making crystals of photosynthetic reaction centres from a purple bacterium, Rhodopseudomonas viridis. This opened the way to X-ray analysis, and by 1985 Johann Deisenhofer, Michel and others at Martinsried had revealed the most complex molecular 3-D assembly ever seen at an atomic level, up to that time. This photocell passes in rivet fashion through a membrane in the bacterium. When light falls on it, it creates a voltage across the membrane, sending a negative charge to the far side. The molecular analysis revealed how it works. Four protein molecules encase carefully positioned pigments, bacterial analogues of chlorophyll, which create a railway that guides the light energy to a place where two pigment molecules meet in a so-called special pair. There the light energy liberates an electron, which then travels via a branch line of the pigment railway to the dark side of the membrane. It settles with its negative charge on a ring-shaped quinone molecule that has a useful appetite for electrons. ‘Although it is a purple bacterium that has first yielded the secrets of the photosynthetic reaction centre,’ commented Robert Huber, who coordinated the work at Martinsried, ‘there is no need to doubt its relevance to the higher green plants on which human beings depend for their nourishment.’ I The gift of the blue-greens Whilst it was certainly encouraging that so complicated a molecule could be analysed, the photosystems of the higher plants, with two different kinds of reaction centres, were a tougher proposition. They would keep scientists busy into the 21st century. There are evolutionary reasons for the greater complexity. Purple bacteria live by scavenging pre-existing organic material, using light energy as an aid. This would be a dead end, if other organisms did not make fresh food from scratch, by reacting carbon dioxide with hydrogen. Some photosynthetic bacteria obtain their hydrogen by splitting volcanic hydrogen sulphide, but others took the big step to splitting water. ‘Think about it,’ said James Barber, a chemist at Imperial College London. ‘Water is the solvent of life. It was very odd that bacteria should start attacking 532 photosynthesis their solvent. That’s like burning your house to keep warm. Only the abundance of water on the Earth made it a sustainable strategy. And of course the first thing that plants do in a drought is to stop photosynthesizing.’ The key players in this evolutionary switch were blue-green algae, or cyanobacteria, first appearing perhaps 2.4 billion years ago. Their direct descendants are still among us. Blue-greens are commonplace in ponds and oceans, and on the shore of Western Australia they build mounds called stromatolites, with new layers growing on top of dead predecessors. Fossils of similar stromatolites are known in rocks 2 billion years old. Those remote ancestors of the present-day blue-greens possessed such an excellent kit for photosynthesis that other, larger cells, welcomed them aboard to make the first true algae. Whenever the cells reproduced themselves, they passed on stocks of blue-green guests to their daughters. Much later, some of the algae evolved into land plants. The green chloroplasts within the leaf cells of plants, where the photosynthesis is done, are direct descendants of the former blue-g reens. What was so special about them? Until the ancestral blue-greens appeared on the Earth, some photosynthetic bacteria, like the purples studied at Martinsried, had used quinones as the end-stations to receive electrons released by light. Others employed iron–sulphur clusters (Fe 4 S 4 ) for that purpose. The blue-greens beefed up photosynthesis by putting both systems together. As a result, their descendent chloroplasts possess Photosystems II (using quinones) and I (using iron–sulphur). Although there are many variants of photosynthesis, they are all related. Photosynthesis using chlorophyll seems to be a trick that Nature orig inated only once. Investigators of molecular evolution at Indiana and Kanagawa traced the whole story back in time, from the similarities and differences between proteins involved in photosynthesis, in plants, blue-greens and other photosynthetic bacteria alive today. Chlorophyll, the badge of sun-powered life, first appeared in an ancient form in a remote common ancestor of purple and green photosynthetic bacteria. Among the variants appearing later is chlorophyll a, which is exclusive to blue-greens and plants. I Engineering the photosystems Although it is a quinone user like the purple bacterium, Photosystem II generates a higher voltage. For its key job, it also has a special water-splitting enzyme—a protein molecule whose modus operandi remains elusive. Like the purple bacterium’s photocell, Photosystem II consists of a complex of protein molecules supporting pigment antennas and railways, but it is bigger, with about 45,000 atoms in all. 533 photosynthesis By 1995, at Imperial College London, James Barber’s team had isolated the Photosystem II complex from a plant—spinach. The material resisted attempts to crystallize it for full X-ray examination. Nevertheless, powerful electron microscopes operating at very low temperatures gave a first impression of its molecular organization. In Berlin, Wolfram Saenger and colleagues from the Freie Universita ¨ t and Horst Tobias Witt and colleagues from the Technische Universita ¨ t had better fortune with Photosystem II from a blue-green, Synechococcus elongatus, which lives in hot springs. Athina Zouni managed to grow small crystals. They were not good enough for very detailed analysis, but by 2001 the team had a broad-brush X-ray view of the complex. The blue-green’s Photosystem II was similar to what Barber was seeing in spinach, and reminiscent of the purple bacterium’s photosynthetic machine too. The team positioned about ten per cent of the 45,000 atoms, including key metal atoms and chlorophyll molecules. They pinpointed the piggy bank—the manganese cluster that accumulates electric charges for the break-up of water. The Berliners were also working on the blue-green’s Photosystem I, and strong similarities convinced them that I and II shared a common ancestry. The picture grew clearer, of a treasured reaction centre originating long ago, spreading throughout the living world, adapting to different modes of existence, but always preserving essential structures and mechanisms in its core. The Berlin group had better crystals of Photosystem I than they had of II. Ingrid Witt first managed to crystallize groups of three robust Photosystem I units from the blue-green S. elongatus, in 1988. That opened the possibility of X-ray analysis down to an atomic level. With so formidable a complex as Photosystem I, containing 12 different proteins and about 100 chlorophyll molecules, this was no small matter. Very powerful X-rays, available at the European Synchrotron Radiation Facility in Grenoble, were essential. The crystals had to be frozen at the temperature of liquid nitrogen to reduce damage to the delicate structures by the X-rays themselves. By 2001 the Berliners’ analysis of Photosystem I was triumphantly thorough. It showed the detailed arrangement of the proteins, of which nine are riveted through the supporting membrane. Six carefully placed chlorophyll molecules provide central transport links for light and electrons and make a special pair as in the Martinsried structure. Most impressively, a great light-harvesting antenna using another 90 chlorophylls surrounds the active centre of Photosystem I. Orange carotene pigments also contribute to the antenna. For outsiders who might wonder what value there might be in this strenuous pursuit of so much detail, down to the atomic level, Wolfram Saenger had an 534 photosynthesis answer. ‘We don’t just satisfy our curiosity about the mechanisms and evolution of this life-giving chemistry,’ he commented. ‘We have already gained a new and surprising appreciation of how pigments, proteins, light and electrons work together in living systems. And the physics, chemistry, biochemistry and molecular biology, successfully marshalled in the study of photosynthesis, can now investigate these and many other related molecular machines in living cells, and find out how they really work.’ I Can we improve on the natural systems? Practical benefits can be expected too. Growing knowledge of the genetics and molecular biology of the photosynthetic apparatus, and of its natural control mechanisms, may help plant breeders to enhance g rowth rates in crop plants. Other scientists use biomolecules to build artificial photosynthetic systems for generating electrical energy or for releasing hydrogen as fuel. In competition with them are photochemists who prefer metal oxides or compound metals, which are also capable of splitting water into hydrogen and oxygen when exposed to light, without any need for living things. In 1912 Ciamician of Bologna looked forward to a time when the secrets of plants ‘will have been mastered by human industry which will know how to make them bear even more abundant fruit than Nature, for Nature is not in a hurry and mankind is’. In that sense, two centuries spent grasping the fundamentals of photosynthesis may be just the precursor to a new relationship between human beings and the all-nourishing energy of the Sun. E For the geological impact of photosynthesis, see Global enzymes and Tree of life. For its present influences, see Carbon cycle and Biosphere from space. For more about proteins and their structures, see Protein shapes. The molecular biology of plants is dealt with more generally under Arabidopsis. For alternative sources of energy for life, see Extremophiles. 535 photosynthesis P otatoes are easy to grow, and when introduced into Ireland they meant that you could keep your family alive while spending most of your time labouring for the big landowners. This feudal social system worked tolerably until 1845, when an enemy of the potato arrived on the wind from the European continent. It was the potato blight Phytophthora infestans. Black spots and white mould on the leaves foretold that the potatoes would become a rotten pulp. The Great Irish Famine, which k illed and exiled millions, was neither the first nor the last case of a crop being largely wiped out by disease. The potato blight itself caused widespread hardship across Europe. Its effects reached historic dimensions in Ireland partly because landowners continued to export grain while the inhabitants starved. As Jane Francesca Wilde (Oscar’s mother) put it: T here’s a proud array of soldiers—what do they round your door? T hey guard our masters’ granaries from the thin hands of the poor. At least 20 per cent of the world’s crop production is still lost to pests, parasites and pathogens, and the figure rises to 40 per cent in Africa and Asia. Plant diseases can also devastate species in the wild, as when the bark-ravaging fungus Cryphonectria parasitica crippled every last stand of native American chestnut trees between 1904 and 1926. But cultivated crops are usually much more vulnerable to annihilating epidemics than wild plants are, because they are grown from varieties with a narrow genetic base. In the wars between living species that have raged since life began, human beings often think that their natural enemies are big cats, bears, sharks, crocodiles and snakes. In fact, the depredations of those large animals are insignificant compared with disease-causing microbes. They either afflict people directly or starve them by attacking their food supplies. There is no difference in principle between the conflicts of organisms of any size. All involve weapons of attack and defence, whether sharper canines versus tougher hides, or novel viruses versus molecular antibodies. Given the opportunities for improvements on both sides, biologists have called the interspecies war an evolutionary arms race. 536 [...]... upheaval in the Pacific weather is associated with a warming of the equatorial ocean and a faltering of the trade winds At such times a decent sailing canoe (not a balsa raft) could certainly make headway eastwards Other seagoing ethnologists, this time from the University of Hawaii, proved the point by building a replica of a Polynesian canoe and sailing eastwards from Samoa to Tahiti in 1 986 Meanwhile... see are literally skin deep.’ 561 p r e h i st o r i c g e n e s By 1 988 , Cavalli-Sforza and Piazza, with Paolo Menozzi of Parma, had constructed an evolutionary tree for 42 aboriginal populations worldwide, based on an analysis of variants of 120 genes It translated into migration routes, starting with departures from Africa perhaps 100,000 years ago One current of modern humanity headed to China and... lump Africans and Native Australians together, yet genetically they are the most widely separated of all human beings alive today An alleged kinship of Europeans, Chinese and Native Americans was also contradicted by the genetics The explanation for the discrepancies between the genetics and the physical anthropology is that superficial features are exactly those most likely to adapt to local climate... founder bands that crossed the Bering Strait from Asia at the end of the last ice age, and left the B blood group behind It is almost absent in Native Americans New mutations that survive in a population are so rare that they cannot have played much part in the genetic differences between human groups The primary message from Cavalli-Sforza’s analysis was that all the data fitted neatly with a single... planet, and by what are literally knock-on effects in the continents Seven large plates account for 94 per cent of the Earth’s surface In descending order of size they are the Pacific, African, Eurasian, Indo-Australian, North American, Antarctic and South American Plates Small plates make up the rest, the main ones being the Philippine, Arabian and Caribbean Plates, lying roughly where their names indicate,... charged atoms around it, which neutralizes the charge and removes an obstacle to the grains getting together Secondly comes the more remarkable idea that, as the dust grains approach one another in a plasma, they feel a mutual attraction Experts now call it the shadow force A racing yacht creates a shadow in the wind, which can thwart a rival trying to overtake it on the leeward side In a plasma, the... principle about a power supply for moving the plates about on the face of the Earth Internal heat first provided during the formation of the planet, by the amalgamation and settling of material under gravity, is sustained by energy released by radioactive materials present in the rocks of the Earth’s interior You can think of all activity at the surface as a direct or indirect result of heat trying to escape... Centre National d’Etudes Spatiales, urged the use of a radically different method of measuring gravity with a satellite Instead of gauging its variations by changes in the satellite’s behaviour, they said, let’s measure the gravity directly with an onboard instrument At first hearing that seems silly, because a satellite is in a weightless state of free fall But in fact the Earth’s gravity is not exactly...pl a n t d i s e a s e s Hereditary systems provide much natural resistance to diseases in plants, as well as animals Many herbal medicines are borrowed from the plant kingdom’s arsenal of chemical weapons An overview of the genetic system involved in fighting disease became available in 2000, when a European–US–Japanese consortium of labs in the Arabidopsis Genome Initiative read every gene in arabidopsis,... plenty of evidence of rapid adaptation of defences to meet new threats in the past, but also many antiques Although the R genes show a very wide range of ages, they are far from being typically young, as would be expected in a reliably progressive kind of arms race as described by Dawkins After the initial molecular verdicts concerning the plant’s armoury for resistance against disease, what survives of . like a kite, with a flat, roughly square head made mainly of carbon and nitrogen atoms, and a long wiggly tail of carbon atoms attached by an acetic acid molecule. In the centre of the head is a. per cent in Africa and Asia. Plant diseases can also devastate species in the wild, as when the bark-ravaging fungus Cryphonectria parasitica crippled every last stand of native American chestnut trees. possibility of retreat as well as advance, remains a valid basis for thinking generally about co-evolution and its weaponr y. Hamilton’s idea of sex as a means of sharing out 539 plant diseases responsibility