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Sciences Research: Harnessing nature’s ‘power plant’

Imagine a technology that would not only provide a green and renewable source of energy, but could also help scrub the atmosphere of the excessive amounts of carbon dioxide that result from the burning of fossil fuels. That is the promise of artificial versions of photosynthesis, the process by which green plants have been converting solar energy into electrochemical energy for billions of years. Roger Highfield investigates

Photosynthesis facts

Photosynthesis is probably the most important chemical reaction on Earth because it enables plants to capture energy from the Sun and convert it into a form that can sustain living cells.

The key product of photosynthesis, which takes place in structures called chloroplasts, is the chemical adenosine triphosphate, the fuel for all the cell’s activities. In this way, plants use sunlight to build complex structures, from wood to fruit. Eventually, that energy fuels creatures higher up the food chain, including people.

Scientists have attempted to recreate photosynthesis in the lab for over a century because it is such an efficient way to harvest light from the Sun.

In one of the more fanciful proposals, some have suggested that ‘electric trees’ could be created by combining photosynthesis with the same biological machinery that is already exploited by the electric eel to generate up to 600 volts to stun its prey. Anywhere the seeds of an electrical tree are planted, electricity could be tapped to run a fridge or a household.

Even better, this is an innovation with one very obvious side effect: oxygen. We owe our lives to oxygen, and virtually all of the oxygen we breathe is produced by plants and some bacteria through photosynthesis.

Almost all the food we eat and all the fossil fuel we use are also derived from photosynthetic organisms. These range from plants and algae to phytoplankton, some species of bacteria and other single-celled organisms called archaea.

Photosynthesis in plants, which mostly occurs in leaves, involves an elaborate array of chemical reactions requiring dozens of protein enzymes and other chemical components. Yet, for the past few decades, most success has been seen harnessing another form of solar energy, which is called photovoltaic energy.

Photovoltaic energy: pros and cons
The good news is that this does appear to offer a technology for large-scale energy generation that avoids the pitfalls of existing fossil-fuel methods. In the latter, coal, oil or gas, when burnt, generate huge amounts of carbon dioxide that in turn drives climate change and can, in the case of high-sulphur fuels, also lead to acid rain. Even better, many different ways have been found to harvest sunlight. They can capture sunbeams using crystalline or amorphous silicon or gallium arsenide solar cells. These solar cells can be made very efficient.

But the bad news is that these conventional photovoltaic cells are expensive, both in monetary terms and in terms of the energy used to produce them. This leads to longer ‘pay-back’ times for these types of technology.

Taking note of nature
By comparison, the leaves around us are the lowest-cost, largest-scale harvesters of solar energy on Earth, and make the impact of man-made solar cells seem woefully inadequate by comparison. Although the greenery around us converts beams of sunlight into the stuff of plant life at around 5% efficiency, the ease with which it can be grown means that the only manufacturing cost is the land it requires to bud and sprout.

One of the many teams now trying to take a leaf out of nature’s notebook can be found at Bangor University, where scientists are investigating whether it is possible to harness plant life’s remarkable ability to turn sunlight into energy. 

Nothing new under the sun
What is striking about this work and similar efforts around the planet is that it sounds like something once used by Jonathan Swift to lampoon apparently useless scientific research. In Gulliver’s Travels (1726), Swift talks of the Grand Academy, his parody of London’s Royal Society, which is today the oldest learned society on the planet. There, the hero of Swift’s satire meets men devoting their lives to absurd experiments, such as extracting sunlight from cucumbers. Today, however, this feat would provide some fascinating insights into the basic biology that powers the planet. Swift would be amazed.

Biosolar energy
The Bangor team, led by Dr Peter Holliman from the School of Chemistry, has just begun a feasibility study into ‘biosolar energy’ – or harnessing the sun’s energy potential – using the same methods as used by plant life – photosynthesis – in newly designed solar cells.

The first working devices are now being readied for test to see if Dr Holliman’s team has found a way to combine the low-cost benefits of plant life with the reproducibility and dependability required of commercial devices.

Through photosynthesis, green plants are able to harvest energy from sunlight and convert it to chemical energy at an energy-transfer efficiency rate of approximately 97%, so that the 5% of photons successfully captured are converted with almost no loss to heat and so on.

If Dr Holliman and his team can create artificial versions of photosynthesis, the dream of solar power as the ultimate green and renewable source of electrical energy could be realised. The potential for this type of technology is substantial – photosynthesis is sufficiently effective to support a huge range of plant-based life on earth. The technology would be substantially different in approach to current solar technologies, and has the potential to be more cost-effective.

The ‘superleaf’
The question is how practical is it to replicate this process outside the plant cell and to scale it up to produce what Dr Holliman calls a ‘superleaf’ that would produce more energy?

To answer this fundamental question, a major programme of research is now being undertaken by Dr Holliman in a project funded by a one-year grant from the UK’s Engineering and Physical Sciences Research Council, one of the government-backed councils that supports research in Britain.

To investigate the potential of a superleaf, Dr Holliman has been joined by another solar energy expert, Dr Udaya Ketipearachchi from Sri Lanka, along with a group of experts in biological chemistry, Dr Lorrie Murphy, Dr Graham Ormondroyd and Gwenda Davies.

Harnessing photosynthesis
Sunset image Many teams around the world are engaged in a race to understand the fundamentals of photosynthesis to make Mother Nature’s work easier to perform in a laboratory and thus to exploit in a commercial process. Some, like the Bangor team, are trying to generate electricity this way, others to generate clean fuel, such as hydrogen.

In the beginning stages of photosynthesis, the absorption of light by chlorophyll – a molecule responsible for the green colour in plants – drives the complex photosynthetic reaction. ‘We remember from our schooldays that photosynthesis operates by light being absorbed by the leaf. The process then captures solar energy when water is split into oxygen, protons and electrons. This takes place in the chloroplast of cells, which are held in plant leaves. Within these chloroplasts lie membranes where photosynthesis takes place,’ explains Dr Holliman.

The energy of sunlight is transferred in the form of electrons and positive charges throughout a pathway of various steps before the final products – carbohydrates (the plant’s food) and oxygen – are produced. Speed is the key – the transfer of the solar energy takes place almost instantaneously, so little energy is wasted as heat.

How photosynthesis achieves this near-instantaneous energy transfer is thought to be down to quantum effects, strange effects that are only predicted to occur at atomic dimensions: Mother Nature is a gifted quantum mechanic.
The photosynthetic trick for transferring energy from one molecular system to another should make any shortlist of Mother Nature’s most spectacular accomplishments.

If we can learn enough to emulate this process, we might be able to create artificial versions of photosynthesis that would help us effectively tap into the sun as a clean, efficient, sustainable and carbon-neutral source of energy. However, because the components of natural photosystems do not work properly outside their normal environment, scientists are investigating other catalysts that could be used to replicate these natural functions.

An answer
Dr Holliman’s team is taking a more pragmatic approach in this research. Why reinvent the wheel? Or, in this case, why reinvent the leaf? Instead, they propose to extract the sunlight-harvesting machinery from several hundred leaves and attach them to electrodes to create a superleaf. They are focusing on the structures found within the chloroplasts, called thylakoid membranes, where the photosynthesis takes place.

These membranes contain the ‘photosystems’ – blends of protein and pigment that catch light, rather like a dish capturing signals from a satellite. Anything from 100 to 5,000 pigment molecules (called collectively the ‘antenna’) are clustered around a central receiving unit, the reaction centre. Here is where the real ‘magic’ of photosynthesis occurs: this is where the energy of electrons excited by light is converted to chemical energy.

But the details are fiendishly complex. The light-harvesting system of plants actually consists of two protein complexes, Photosystem I and Photosystem II. And each complex features antennae made up of chlorophyll and carotenoid molecules that gain extra ‘excitation’ energy when they capture photons. This excitation energy is funnelled through a series of molecules into the reaction centre where it is converted to chemical energy.

The aim of the project
The aim of the work at the School of Chemistry in Bangor is to test the feasibility of removing photosynthetic thylakoid membranes from leaves and attaching them to electrodes to create solar-energy harvesters. Dr Holliman does not want to give away too much of how he is doing this (‘it might be patentable’), but the key is to design the special electrodes – complex chemicals – that are able to tap into the thyakoid membranes and to draw off the power in an efficient way to a conventional electrode.

‘We aim to test the feasibility of extracting the membranes from within the cell and attaching them to electrodes to create efficient low-cost solar-energy harvesters. It is already known that such solar cells are capable of generating very small currents from sunlight. Here, we are aiming to increase these currents in order to develop a viable commercial technology.’

Steady progress
The team has not yet created a working superleaf, but is making steady progress. ‘Things are going okay so far,’ says Dr Holliman ‘We have our initial photo-electrodes and thylakoids, and we are currently working on the linkage part. As soon as we have this (which hopefully should be quite soon now), we will start making and testing devices.’

Dr Holliman hopes that his feasibility study will show that biosolar technology could have commercial applications. One key objective is to show that it can undercut the cost of a domestic solar cell, which, to power a typical house, would cost anything from as little as £5,000 to around £10,000. The aim is to come up with a leaf-inspired system that could potentially deliver similar energy outputs, but at much lower cost.

The future
When this next revolution in solar power pays off, the impact would be apparent even to the untrained eye. Rather than the conventional silvery grey or marine blue solar panels that are now ubiquitous, a new colour would appear: green. The biosolar cells would be thin and verdant. They would indeed look like giant, rectangular leaves, as vivid and green as the local park. Imagine a new town development where every home is powered by a cucumber-coloured roof. That would give a whole new meaning to the term ‘power plant’.

Roger Highfield is Editor of New Scientist magazine.

Read other science research papers, including 2006 research about a New Zoonosis Centre and 2007 research about The Space Weather Forecast.