Vibrating molecules help plants make use of quantum effects

It’s part of the emerging research field of quantum biology, which is looking at the role that quantum mechanics plays in biological systems.

Until recently, it was thought that quantum effects could only be observed in a system where there was minimal interference from the environment, for example a system that was cooled to near absolute zero to reduce the thermal vibrations of molecules. Biological systems were considered too complex for quantum descriptions to apply.

However in 2007, scientists in the US discovered that quantum mechanics can help explain how plants photosynthesise light so efficiently. EU researchers are now investigating exactly how this works.

Dr Yasser Omar from the University of Lisbon, Portugal, explains that during the first part of photosynthesis, light energy from the sun is absorbed by the plant. This energy is then transported from molecule to molecule along the biological structure of the plant to the so-called reaction centre, after which it is transformed into chemical energy.

The efficiency of this energy transfer process is helped by a quantum phenomenon known as the superposition principle, which means that the energy is able to travel down every route towards the plant’s reaction centre at the same time.

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‘Quantum superposition is very strange and shocking, even for the experts.’

‘Quantum superposition is very strange and shocking, even for the experts,’ said Dr Omar. ‘We don’t really understand it, but if we accept this principle, we have quantum theory, which successfully describes and predicts the behaviour of atomic and subatomic matter.’

Dr Omar leads the EU-funded PAPETS project, which is investigating how quantum effects can be maintained in plants, which are complex systems with lots of environmental interference.

‘Our project is trying to understand how come these quantum effects can be there when they shouldn’t be, because you would expect all the environmental effects would kill the quantumness,’ he said.

By using a combination of experimentation and theoretical models, the PAPETS team have found that quantum effects are preserved in photosynthesis precisely because of the natural vibrations of molecules in the plant.

Contrary to expectations, these vibrations actually work to enhance the quantum effects, rather than destroy them.

As part of its work, PAPETS is also investigating whether quantum mechanics play a role in our olfactory (smelling) system.

Artificial light harvesting

Dr Omar says that understanding how plants convert light energy so efficiently can contribute to the design of more efficient artificial light harvesting systems.

‘One of the goals of the project is (to find out) can we also do this with artificial light-harvesting systems?’ he said.  ‘We are not close to commercial products yet but the guiding principles could ultimately lead to the design of more efficient solar cells.’

Although photosynthesis is so far the only biological system for which there is hard evidence of quantum behaviour, the idea that quantum mechanics may also be used to explain other biological phenomena is leading to the emergence of a new research field: quantum biology.

Professor Benedetta Mennucci from the University of Pisa, Italy, says that interest is growing in quantum biology partly because scientists have developed more sophisticated tools.

‘This is quite a new way of looking at biological systems,’ she said. ‘Until now, the systems were considered to be too complex to be explained via quantum mechanical descriptions. The analysis of the real systems was too expensive.

‘Now we have developed more accurate theoretical methods and efficient computational approaches. We can verify what are the quantum mechanical effects and how they are effective.’

Prof. Mennucci leads the EU-funded ENLIGHT project, which is working to develop a computerised model of how light-harvesting works at a molecular level in plants, bacteria and algae.

‘The main process is known. What is not known is how this process has been optimised to organisms living in completely different conditions. To reach this goal we have to describe the systems as completely as possible.’

She also believes that quantum effects might be seen in other biological systems.

‘We have developed the system for light-harvesting processes but it can be used to study more biological systems. I have the feeling that these methods will help us understand very complex phenomena and show that is it exactly the complexity of the system that makes it work.’