Quantum phenomena in biology are receiving the attention of more and more researchers, with photosynthesis being the process getting the most attention. Back in 2007, it was apparent that quantum effects were effective for "explaining the extreme efficiency of photosynthesis". Then, in 2010, the photosynthetic apparatus of cryptophyte algae was the focus of research, because its pigments are farther apart than was expected for efficient functioning. In a News & Views article in Nature, van Grondelle & Novoderezhkin discussed evidence suggesting that a process known as quantum coherence is part of the explanation. They added: "This is the first time that this phenomenon has been observed in photosynthetic proteins at room temperature, rather than at much lower temperatures, bolstering the idea that quantum coherence influences light harvesting in vivo." The most recent study has provided a theoretical argument that quantum effects must be present and that classical physics does not provide an explanation. It is claimed to be "the first unambiguous theoretical evidence of quantum effects in photosynthesis". The Press Release describes the work in this way:
"Often, to observe or exploit quantum mechanical phenomena, systems need to be cooled to very low temperatures. This however does not seem to be the case in some biological systems, which display quantum properties even at ambient temperatures. Now, a team at UCL have attempted to identify features in these biological systems which can only be predicted by quantum physics, and for which no classical analogues exist. "Energy transfer in light-harvesting macromolecules is assisted by specific vibrational motions of the chromophores," said Alexandra Olaya-Castro (UCL Physics & Astronomy), supervisor and co-author of the research. "We found that the properties of some of the chromophore vibrations that assist energy transfer during photosynthesis can never be described with classical laws, and moreover, this non-classical behaviour enhances the efficiency of the energy transfer."" (Source here)
New frontiers for understanding the natural world (image source here)
Most light-gathering macromolecules are composed of chromophores (the light-absorbing pigments) attached to proteins. These are responsible for the first step of photosynthesis, which is to capture light and transfer its energy to another system that can store it. Earlier work showed that energy is transferred in a wave-like manner (the quantum coherence model). However, theoreticians were of the opinion that classical physics could still find a way of explaining the observations.
"Molecular vibrations are periodic motions of the atoms in a molecule, like the motion of a mass attached to a spring. When the energy of a collective vibration of two chromophores matches the energy difference between the electronic transitions of these chromophores a resonance occurs and efficient energy exchange between electronic and vibrational degrees of freedom takes place. Providing that the energy associated to the vibration is higher than the temperature scale, only a discrete unit or quantum of energy is exchanged. Consequently, as energy is transferred from one chromophore to the other, the collective vibration displays properties that have no classical counterpart. The UCL team found the unambiguous signature of non-classicality is given by a negative joint probability of finding the chromophores with certain relative positions and momenta. In classical physics, probability distributions are always positive." (Source here)
Bear in mind that considerable resources have already been spent on trying to develop a biomimetic system that captures solar energy like plants - only to find that photosynthesis is extraordinarily complex and the research has not yet delivered any commercial outputs. It is a reminder that the Darwinian vision of ultimate simplicity has been repeatedly falsified. Photosynthesising microorganisms are among the earliest to appear in the Precambrian fossil record - and yet these organisms have chemical and physical pathways that are only beginning to be understood within the research community. What is emerging are processes and structures that carry the hallmarks of design, with complex specified information at every level of analysis. We are at the beginning of a journey into quantum effects in biology. It is the design paradigm that is best equipped to guide our thoughts and keep us on the right path.
"Other biomolecular processes such as the transfer of electrons within macromolecules (like in reaction centres in photosynthetic systems), the structural change of a chromophore upon absorption of photons (like in vision processes) or the recognition of a molecule by another (as in olfaction processes), are influenced by specific vibrational motions. The results of this research therefore suggest that a closer examination of the vibrational dynamics involved in these processes could provide other biological prototypes exploiting truly non-classical phenomena." (Source here)
Non-classicality of the molecular vibrations assisting exciton energy transfer at room temperature
Edward J. O'Reilly & Alexandra Olaya-Castro
Nature Communications, 9 January 2014, 5, Article number:3012 | doi:10.1038/ncomms4012
Abstract: Advancing the debate on quantum effects in light-initiated reactions in biology requires clear identification of non-classical features that these processes can exhibit and utilize. Here we show that in prototype dimers present in a variety of photosynthetic antennae, efficient vibration-assisted energy transfer in the sub-picosecond timescale and at room temperature can manifest and benefit from non-classical fluctuations of collective pigment motions. Non-classicality of initially thermalized vibrations is induced via coherent exciton-vibration interactions and is unambiguously indicated by negativities in the phase-space quasi-probability distribution of the effective collective mode coupled to the electronic dynamics. These quantum effects can be prompted upon incoherent input of excitation. Our results therefore suggest that investigation of the non-classical properties of vibrational motions assisting excitation and charge transport, photoreception and chemical sensing processes could be a touchstone for revealing a role for non-trivial quantum phenomena in biology.
Cartwright, J. Quantized vibrations are essential to photosynthesis, say physicists, physicsworld.com (22 January 2014)
Tyler, D. Explaining the extreme efficiency of photosynthesis. ARN Literature Blog (16 April 2007)
Tyler, D. The latest thinking on how photosynthesis evolved. ARN Literature Blog (11 February 2007)
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