Light-harvesting complexes

85262_largeLight harvesting complexes

Mohan Sarovar – Quantum mechanical photosynthetic light harvesting machinery (Google Tech Talks) & Tessa Calhoun et al  – Quantum coherence enabled determination of the energy landscape in light-harvesting complex II  –  Journal of Physical Chemistry B, 2009

Photosynthesis provides the most convincing evidence for the existence of quantum functionality in biological systems. Functionality here means that the organism could not do what it does without the quantum feature.

The argument against quantum features in biological systems tended to rest on an assumption that systems were in thermal equilibrium, but biological systems are far  from thermal equilibrium, and the state of the non-equilibrium environment may actually perpetuate quantum states.

Photosynthetic antennae absorb and transmit light to a reaction centre. This process is at least 95% efficient and happens in a picosecond timescale, which is much better than is normally observed in nature.

The 2007 study showed that the FMO, which links the main antennae to the reaction centre, did not work by particles hopping from molecule to molecule, but involved them moving across the system in a wavelike manner, sampling energy levels. Initial studies involved bacteria at 77K, but since then quantum states in photosynthesis have been demonstrated at room temperature (Collini et al, 2010) and have involved multicellular green plants such as spinach (Calhoun et al, 2009). Initially it might be possible to have seen quantum states in photosynthesis as an outlier in extreme conditions, but it is now apparent that it is a feature of mainstream plant life. The (Calhoun, 2009) study, for instance, observed quantum coherence in light-harvesting complex II, the form of antennae complex found in multicellular green plants that contain about 50% of the world’s chlorophyl.

The Mohar talk focused on quantum entanglement to a greater extent than has been the case with most of this type of research, Quantum coherence could account for the functionality of the photosynthetic quantum states by itself, while it is still not clear whether entanglement has a role to play, or is simply a by-product of quantum states.

The FMO complex is important with respect to entanglement. The FMO is a ‘wire’ between the main antennae and the reaction centre in photosynthesis. It has seven chlorophyll molecules or chromophores. Light enters through chromophores one to six, and leaves through chromophore three.

Entanglement in the FMO is in the form of correlations between spatially separated chromophores. The chromophores are embedded in a protein cage, which is part of the environment of the chromophore. Chromophores one and two are entangled and are close to one another, but one and three are also entangled, although they are as far away from one another as is possible within the FMO. P. At the time of the 2007 study, it was surmised that the system carried out a form of quantum computing analogous to Grover’s algorithm in order to find the most efficient path when transferring energy. Subsequent research seems to have led to the conclusion that this is not the case. The efficiency advantage of quantum states may lie in avoiding energy minima, robustness or uni-directionality.

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