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Photosynthesis Richard Cogdell INSTANT EXPERT 30 Earth’s life support system You have photosynthesis to thank for every lungful of air you breathe. In fact, photosynthesis is probably the most important biochemical process on the planet. Besides pumping oxygen into the atmosphere, it is the energy source behind all our food and almost all the heat and power we use. Without it, the evolution of life on Earth would have followed a very different path. Yet unpicking the molecular details of photosynthetic chemistry, and understanding how the process shapes our environment, remains a key challenge ii | NewScientist | 2 February 2013 Anatomy of a chloroplast Photosynthesis: the basics CARBOHYDRATE LIPID THYLAKOID Plants and algae use the sun’s energy to convert water and carbon dioxide into sugars. This process – photosynthesis – takes place inside organelles called chloroplasts STROMA INNER MEMBRANE A plant leaf may contain 500,000 chloroplasts per square millimetre OUTER MEMBRANE SUN Chlorophyll pigments, which give plants their green colour, absorb a photon from the sun and pass its energy to a pair of chlorophylls in the reaction centre REACTION CENTRE The reaction centre then kicks out an electron – a process called charge separation CHARGE SEPARATION e– The electron is used to create NADPH, a chemical reducing agent, and ATP, the biological energy molecule e– OXYGEN EVOLVING CENTRE OXYGEN WATER NADPH The reaction centre is reset with an electron from the oxygen evolving vcentre which splits water molecules into electrons, hydrogen ions and oxygen gas NADPH and ATP are used in the Calvin cycle to convert a 3-carbon sugar (3-phosphoglycerate) into a 5-carbon sugar (ribulose 1,5 bisphosphate) Photosynthesis is the process by which plants, algae and some bacteria convert carbon dioxide and water into carbohydrates using energy from sunlight. In most cases, they achieve this by splitting apart the hydrogen and oxygen in water (H2O), giving off oxygen (O2) as a by-product. In many ways, photosynthesis is the reverse of respiration: when we animals respire, we use O2 to burn up carbohydrates, releasing CO2 and producing the energy we need to live. Photosynthesis consists of a complex series of reactions, but it can be divided into four key stages: light absorption, charge separation, carbon fixation and oxygen evolution. First, a photon of sunlight is absorbed by chlorophyll pigments and passed to a “reaction centre”, which contains a specially aligned pair of chlorophyll molecules. Here charge separation occurs: the chlorophyll pair uses the photon’s energy to spit out an electron. This triggers the final two stages. The ejected electron is passed along a chain of molecules until it is used to convert CO2 into carbohydrate, a process known as carbon fixation. Meanwhile the reaction centre is “reset” with a new electron stripped out of water. This replacement comes from part of the reaction centre complex called the oxygen evolving centre, which splits water molecules into electrons, hydrogen ions and oxygen gas. The complete process can be summarised in a simple equation: ATP 5-CARBON SUGAR An enzyme called rubisco adds carbon from CO2 onto a 5-carbon sugar, making two molecules of 3-phosphoglycerate Some of this carbon is returned to the Calvin cycle. The rest is converted into carbohydrates such as sucrose and used to build leaves and stems RUBISCO CALVIN CYCLE CO2 3-CARBON SUGAR SUCROSE Stack ‘em up: thylakoid “coins” in a chloroplast (above) are linked by thin lamellae H2O + CO2 + light ➝ C(H2O) + O2 All this chemistry, from light absorption to the synthesis of carbohydrates, occurs in a structure called a chloroplast. Chloroplasts have two membranes. The smooth outer membrane holds the whole structure together. The inner membrane is folded into a series of stacked discs called thylakoids that contain the pigments and Justin Guariglia/Corbis In the chloroplast, the photosynthetic reactions that depend on light are physically separated from those that do not. These are called the “light” and “dark” reactions, respectively. All the components of the light reactions are arranged in or on proteins held in the thylakoid membrane. Light harvesting antenna, for instance, are proteins that contain chlorophyll pigments arranged to absorb light and pass the energy to nearby reaction centres. While some bacteria contain one kind of photosynthetic reaction centre, algae and plants contain two types – photosystem one and photosystem two. Charge separation in PS2 pulls electrons from the oxygen evolving centre and passes them to PS1. PS1 is activated by a second photon and the electrons it produces are passed out of the thylakoid membrane and onto molecules involved in the dark reactions. The dark reactions occur in the stroma. Here, enzymes drive a cyclic reaction that converts CO2 and a sugar containing five carbon atoms into molecules of 3-phosphoglycerate, a 3-carbon sugar. A proportion of these sugars are fed back into the cycle. The rest are used as building blocks to form carbohydrates such as sucrose, cellulose or starch. The details of this reaction, known as the Calvin Cycle, were discovered in 1950 by Melvin Calvin, James Bassham, and Andrew Benson at the University of California in Berkeley. The enzyme responsible for fixing carbon from CO2 is called rubisco. It is probably the most abundant protein on the planet. Every atom of carbon in your body has been captured from the atmosphere by rubisco yet remarkably it is a rather inefficient enzyme; it has a low affinity for CO2, and also reacts with O2 in a process called photorespiration, with the result that about a third of the carbon it fixes is released back into the atmosphere. Driven by light energy, photosynthetic chemistry in the thylakoid membrane produces ATP, a molecular source of energy, along with a reducing agent called NADPH. These molecules are then consumed in the dark reactions. NADPH is formed during the final stage of the electron transport chain while ATP is created when energy from photons is used to pump protons across the thylakoid membrane. This sets up an electrochemical gradient that pushes the protons back out, releasing energy and generating ATP. Peter Fakler/Alamy BIOLOGY PICS/spl Ashley Cooper/Corbis The light and dark reactions Powerhouses: every year photosynthesis pumps 300 billion tonnes of oxygen into the biosphere protein complexes required to capture solar energy and release oxygen. The enzymes and other components involved in converting CO2 into sugars are located in the stroma, the fluid-filled space inside the chloroplast (see diagram, left). The central role played by chloroplasts was highlighted 75 years ago, when Robert Hill, a biochemist at the University of Cambridge, discovered that these organelles can generate oxygen when illuminated in the absence of CO2. This finding was a key discovery because it provided one of the first indications that the ultimate source of electrons is water and not CO2. 2 February 2013 | NewScientist | iii Shaping the planet Present: Fifty shades of green MATTHEW OLDFIELD/spl Despite more than 2 billion years of evolution, the core reactions of photosynthesis have remained remarkably similar across species. Yet a variety of subtle physical and biochemical modifications have also evolved, each optimised to suit conditions in specific ecological niches. Plants, for example, have evolved slightly different forms of photosynthesis. Some 85 per cent of plant species are known as C3 plants. These use the enzyme rubisco to fix carbon from CO2 to form 3-carbon sugar molecules that provide the building blocks for sucrose. Another group of plants has evolved a way to get round the inefficiency of rubisco. C4 plants, including tropical grasses such as sugar cane, supercharge Not so sluggish: playing host to chloroplasts gives Elysia an energy boost carbon fixation by using an additional enzyme, PEP carboxylase, to fix CO2 into malic acid, a 4-carbon molecule. The malic acid is then pumped into specialised cells where it is broken down to release CO2. Inside these cells, rubisco is exposed to high concentrations of CO2 which helps it work more efficiently. Although this process requires energy, it allows photosynthesis in C4 plants to be up to 50 per cent more efficient than C3 plants, giving them a competitive advantage in hot sunny conditions. A different adaption is found in dinoflagellates such as Amphidinium carterae. These live in the sea at depths where the only available light is in the blue-green part of the spectrum. They have evolved a unique light harvesting complex that uses pigments called carotenoids, which absorb blue-green light. Chlorophyll pigments in plants absorb weakly at these wavelengths. A number of creatures including jellyfish, flatworms, bivalve molluscs and salamanders also make use of photosynthesis, thanks to a symbiotic relationship with photosynthetic algae. The sea slug Elysia, for example, eats green algae and keeps their chloroplasts alive in its body, supplementing its diet using the carbohydrates they create. When photosynthetic organisms first began to generate oxygen, it marked the start of a transformation of our world. Oxygen provided access to a more efficient source of energy through respiration, a process that would ultimately allow multicellular animals to evolve. Now, more than 2 billion years after these changes began, the world is transforming again. Our emissions are causing damaging climate change. So how will photosynthetic organisms adapt to a warmer planet, and what are the implications for our biosphere? Past: First breath When the first bacteria began to harness light energy some 3.4 billion years ago, Earth’s atmosphere was mainly composed of nitrogen and CO2. These anaerobic photosynthetic organisms relied on hydrogen, or organic or sulphur compounds as a source of electrons. Then, about 2.4 billion years ago, the “great oxygenation event” kicked off. Photosynthetic organisms evolved – probably ancestors of presentday cyanobacteria – that were capable of splitting water to produce oxygen. Now oxygen levels in the biosphere began to rise (see timeline, below). The changes that occurred over the next 2 billion years resulted in the extinction of many anaerobic organisms – it would have been a dramatic and cataclysmic transformation. Yet the arrival of oxygen was not entirely bad news. Ultraviolet radiation from the sun hit oxygen in the upper layers of the atmosphere, and the subsequent reaction created ozone (O3). The layer of it in the stratosphere filtered out this harmful ultraviolet light, Since oxygenic photosynthesis evolved some 2.8 billion years ago, the proportion of the atmosphere that is made up of CO2 has dropped from about 20 per cent to just 0.04 per cent. The levels started to rise again at the start of the industrial revolution, and continue to go up. This is happening because the capacity of photosynthesis to soak up the huge volumes of gas we release by burning fossil fuels has been exceeded. What will this change mean for plants and for other life forms like us that ultimately depend on photosynthesis? Studies show that some trees are already growing bigger and faster, but perhaps the only certainty is that the effects on the composition of plant populations will be highly unpredictable. To find out more about how elevated CO2 concentrations will influence crop growth, researchers are simulating future atmospheric conditions in field experiments that expose plants to different compositions of air. These experiments show that C4 and C3 plants respond differently. C4 plants such as maize (corn) slightly increase their rate of photosynthesis but there is little effect on growth, even when CO2 levels reach 0.06 per cent. However at this level of CO2, the rate of photosynthesis in C3 plants increases by about 40 per cent. This is reflected in the crop, with wheat, rice and soybean showing increases in yield of up to 14 per cent. There may also be an impact on water use. Plants regulate their CO2 uptake using tiny pores in their leaves called stomata. As CO2 levels rise, stomata will stay closed for longer. These pores also allow water vapour to escape, so higher CO2 levels may reduce plants’ water losses, meaning farmers would not have to water their crops as often. These benefits might come at a price. Increasing rates of photosynthesis mean plant growth may then be limited by the availability of key nutrients such as phosphorus and nitrogen. This could prove most serious for crops such as pulses that have seeds with high protein content and these may need extra fertiliser. Farmers can probably mitigate gradual changes to the atmosphere by altering farming practices – applying extra fertiliser, say, or changing crop varieties. What will be harder to deal with are sudden extremes of weather. Extensive drought in the US and heavy rain in parts of Europe drastically reduced grain yields in 2012, and global warming is expected to bring even wilder fluctuations in weather patterns. Open and shut case: plants control the uptake of CO2 via tiny pores in their leaves iv | NewScientist | 2 February 2013 ”The release of oxygen permitted the evolution of new life forms that obtained energy from respiration” Relics: stromatolites are fossilised deposits formed by ancient cyanobacteria Nearly all the oxygen on our planet comes from photosynthesis. This gas first began to appear when photosynthetic cyanobacteria evolved. These bacteria could split water to give them energy, rather than using hydrogen or sulphur compounds 300 mya 40 Atmospheric oxygen levels (%) right: Frans Lanting/Corbis far right: Mint Images/Rex below: DR JEREMY BURGESS/spl Future: Plants in a changing world which damages DNA, helping life spread out of the deep oceans. The earliest land dwellers were mosses and liverworts, descended from green algae that thrived in warm, shallow water. Oxygen also permitted the evolution of new life forms that obtained their energy from respiration. Aerobic respiration is very efficient. The increase in energy available to support life allowed a great expansion in the number of species on our planet and, in particular, the evolution of large multicellular creatures. By 400 million years ago, oxygen levels had begun to stabilise at close to current levels, and plants such as ferns, grasses and cacti had colonised the land. The release of oxygen by photosynthetic organisms also altered Earth’s geology. For instance, oxygen in the oceans triggered the formation of iron oxide, eventually producing the red bands of iron ore deposits in sedimentary rocks. Oxygen also generated thousands of other minerals in the crust, helping to create the rich variety of materials we exploit today. 30 High oxygen peak during the Carboniferous 3.4 bya First photosynthetic bacteria evolve 20 Great oxygenation event begins 2.8 bya Red and brown algae evolve 1.9 bya Oxygen levels drop 500 mya First land plants evolve 750 mya Green algae evolve Photosynthetic cyanobacteria begin to release oxygen 10 0 1.2 bya 2.4 bya 4 Billion years ago (bya) 3 2 1 0 2 February 2013 | NewScientist | v Waste not: sugar cane, the world’s largest crop, is turned into biofuel, with left-over bagasse burned to make heat vi | NewScientist | 2 February 2013 The complexity of photosynthesis is a huge challenge for those trying to unpick its details. And with the twin threats of climate change and food shortages, scientists are looking to photosynthesis for help. Can plants and algae be the key to new carbon-neutral fuels, for example? Might we even be able to supercharge the photosynthetic process and increase the yields of vital food crops? New sources of fuel Global carbon emissions are rising steadily and if our planet is to avoid catastrophic warming, we must work rapidly to replace fossil fuels. Can photosynthesis help? Plant power has already been harnessed for biofuel. US distilleries produce more than 50 billion litres of bioethanol annually, mainly from fermented corn. Most is blended with conventional petrol and used to power vehicles. Yet questions remain over the sustainability of this biofuel. The conversion of solar energy into bioethanol is very inefficient, meaning huge areas of land are needed if production is to be scaled up. Another way that photosynthesis can offer us fuels is if we can mimic the way in which plants and algae use light to split water, to generate Derek Lovley (above) H2 as well as O2 (see “From ocean hopes to modify a to atmosphere”, page vii). bacteria so it produces Scientists already do this in the hydrocarbon fuel from lab – photovoltaic cells connected sunlight and CO2 to a pair of platinum electrodes immersed in water will generate bubbles of H2 fuel. However, this technique would be prohibitively expensive on a large scale because of the high cost of platinum. The challenge is to mass-produce electrodes at lower cost. One contender is a system devised by Daniel Nocera and colleagues at the Massachusetts Institute of Technology. Their oxygen evolving electrode uses a structure inspired by the plant’s oxygen evolving centre, but with cobalt instead of manganese. This splits water to release oxygen, creating hydrogen ions that combine with electrons at the other electrode – an alloy of nickel, molybdenum and zinc – to form hydrogen gas. Electricity is provided by a special silicon-based solar cell. Still further off is the “electric leaf”. This is a concept for a hybrid fuel generation system, using photovoltaic panels that supply electricity to living cells. These cells will be engineered to create not H2, but energy-rich hydrocarbons. A bacteria called Geobacter might provide the basis for the biological half of this double act. Geobacter isn’t photosynthetic. Instead, it extracts electrons from minerals and uses them to power its metabolism. Derek Lovley at the University of Massachusetts Amherst, near Boston, has shown that Geobacter can grow using electrons provided by a photovoltaic cell and that the bacteria can extend wire-like hairs called pili to make electrical connections. This raises an intriguing question: could we modify Geobacter so it turns electrons into hydrocarbon fuel? Jay Keasling, of the University of California, Berkeley, has shown that the metabolic pathway required to synthesise hydrocarbons called terpenes can be engineered into E. coli. In principle, the same thing could be done with Geobacter, creating a hybrid system that converts sunlight into a petrol substitute. From ocean to atmosphere ”Geobacter bacteria could provide a hybrid system that converts sunlight into a petrol substitute” Photosynthesis, and therefore all life on Earth, has been able to proliferate because of oxygen evolution. Plants, algae and cyanobacteria release oxygen by splitting water using a protein structure called the oxygen evolving centre. At its heart are four manganese ions, held in specific orientations by a protein scaffold. Although scientists can mimic the way the centre splits water using electricity and a platinum catalyst, this requires roughly twice the energy used by photosynthetic organisms. The oxygen evolving centre reduces its energy needs by dividing up the chemistry into a series of small steps. In particular, the manganese ions have four oxidation states and give up their electrons one at a time, steadily increasing their oxidation power until molecular oxygen is formed. Yet despite decades of study, scientists have so far failed to unravel every fine detail of the way the centre functions. We know the key features of its structure, thanks to X-ray measurements. However, these only provide a static snapshot: how the metal ions and the protein’s amino acids cooperate as the reactions proceed is still unknown. To find out more, researchers are using techniques such as time-resolved X-ray crystallography, in which ultrashort X-ray pulses are beamed through a sample of oxygen evolving centres. These measurements can An “electric leaf” constructed from a genetically engineered bacterium could convert sunlight and carbon dioxide into vehicle fuel LIGHT PHOTOVOLTAIC PANELS CO2 + – GEOBACTER BACTERIA HYDROCARBON FUEL A false colour image shows how CO2 emission (red) gives way to CO2 absorption (green) as photosynthesis switches on at dawn Jamison Daniel, ORNL/NCCS A plant’s efficiency at turning CO2, water and light into biomass is extremely low – typically around 4 to 5 per cent at best. But where do these limits come from? Can they be overcome to produce crops with higher yields? Plants rely on chlorophyll molecules to collect light, yet these pigments don’t absorb over the entire spectrum – light at wavelengths above 750 nanometres is not used. This means plants waste about half of the energy in the solar spectrum, so researchers are attempting to tackle this by combining plant reaction centres with light harvesting antenna from purple bacteria which absorb light wavelengths from 800 to 1000 nanometres. Plants also make far more chlorophyll than they need. This is a survival mechanism: with extra chlorophyll in their leaves, little light will reach competitors growing below. But this also means that in strong sunshine, plants absorb more light than they can use. Under these conditions up to 80 per cent of the light collected is wasted, with excess energy dissipated as heat. Researchers hope that reducing a plant’s light harvesting capacity will increase its photosynthetic efficiency, so several teams are making mutant green algae with reduced pigment content. Another major inefficiency occurs during carbon fixation, thanks to the enzyme rubisco. C4 plants, such as sugar cane and sorghum have partially solved this thanks to a CO2 concentration mechanism (see “The light and dark reactions”, page ii) that raises the efficiency of photosynthesis to about 6 per cent. Cyanobacteria have a similar strategy: they contain carboxysomes, protein assemblies containing rubisco where CO2 can be concentrated while O2 is excluded. To improve crop yield, researchers are trying to convert C3 plants into C4 ones, and see if C3 plants can be modified to make their own carboxysomes. One team based at the University of Cambridge is attempting to change the leaf anatomy of a C3 plant so that it produces carboxysomes in its chloroplasts. To do this, the plant must not only synthesise all the components needed but they must also be delivered and assembled inside the chloroplast. Even if these efforts succeed, it is hard to envisage photosynthetic efficiency rising above 10 per cent. FRONTIERS OF PHOTOSYNTHESIS Daniel Nocera (below) is developing a solar cell (right) to make H2 fuel bottom: VOLKER STEGER/spl Isaac Hernandez/IsaacHernandez.com top: Dominick Reuter Jason Larkin/Panos Improving crop yields reveal structural changes on a picosecond timescale, with a resolution of a few nanometres. Such experiments could allow us to follow the detailed changes that occur in the centre’s structure as a single molecule of oxygen is released, solving a scientific riddle and perhaps pointing the way to new kinds of photovoltaic technology. 2 February 2013 | NewScientist | vii Richard Cogdell Richard Cogdell FRS is the Hooker Professor of Botany and Director of the Institute of Molecular, Cell and Systems Biology at the University of Glasgow, UK. His research focuses on artificial photosynthesis and the light reactions of bacterial photosynthesis Next INSTANT EXPERT Michael O’Shea The human brain 6 April The emergence of quantum biology Despite decades of research, fundamental questions about photosynthesis remain unanswered. One important issue is whether we can use it to develop artificial systems capable of turning solar energy into carbon-neutral fuels (see “New sources of fuel”, page vi). The world has plenty of fossil fuels, but our unrestrained use of them will have severe consequences for the planet. Progress here may require a step change in current approaches. Strange as it may seem, deciphering the quantum properties of the pigments involved may be the key we will need to master photosynthesis. Recent measurements have shown that when the pigments in lightharvesting antenna are excited by the energy of a photon, their electrons can jump into a quantum superposition of excited states. This “coherent” quantum state can last hundreds of femtoseconds or so. Although this effect may seem subtle, it raises the intriguing possibility that such quantum states play a fundamental role in the early stages of the light reactions. It may also help to explain why viii | NewScientist | 2 February 2013 photosynthetic antenna are so efficient at transferring light energy to the reaction centres. Within any light-harvesting antenna complex, the protein structure will be changing constantly because of unavoidable thermal effects. These fluctuations cause the precise energies of the chlorophyll pigments bound to the protein to change, influencing the “energy landscape” of the system and either enhancing energy transfer processes or making them less efficient. However a coherent quantum state could, in principle, smooth out the effects of these fluctuations so that energy transfer always remains highly efficient. If this hypothesis proves correct, it raises a key question: can we learn to harness the power of these quantum effects and use them to improve the performance of devices such as photovoltaic cells? Might similar quantum states play a key role elsewhere in photosynthetic systems or in other places such as olfactory receptors? These questions lie at the heart of the emerging field of quantum biology. Further READING Molecular Mechanisms of Photosynthesis by Robert E. Blankenship (Blackwell, 2002) Photosynthesis by P. J. Weaire, (The Biochemical Society, 1994) “Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å” by Yasufumi Umena et al. Nature, vol 473, p 55 “Powering the planet: chemical challenges in solar energy utilization” by Nathan S. Lewis and Daniel G. Nocera, PNAS, vol 103, p 15729 “Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement,” by Robert E. Blankenship et al. Science, vol 332, p 805 “Lessons from nature about solar light harvesting” by Gregory D. Scholes et al. Nature Chemistry, vol 3, p763 “Rubisco: structure, regulatory interactions and possibilities for a better enzyme” by Robert J. Spreitzer and Michael E. Salvucci, Annual Review of Plant Biology, vol 53, p 449 Cover image Biology Pics/SPL