In photosynthesis, light is collected and transported along a structural complex of chlorophyll and protein scaffolding.

The paper described below demonstrated that energy is transported along the complex via superpositioned electron states and quantum coherent excitons (to the amazement of the epxerimenters). If plants utilize quantum coherence (avoidning decoherence), it seems likely that animal cells are able to utilize similar processes for information processing.

Nature 446, 782-786 (12 April 2007)

Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems

Gregory S. Engel1,2, Tessa R. Calhoun1,2, Elizabeth L. Read1,2, Tae-Kyu Ahn1,2, Tomá Manal1,2,5, Yuan-Chung Cheng1,2, Robert E. Blankenship3,4 & Graham R. Fleming1,2

1. Department of Chemistry & QB3 Institute, University of California, Berkeley
2. Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Department of Biology,
4. Department of Chemistry, Washington University, St Louis, Missouri 63130, USA
5. Present address: Institute of Physics of Charles University, 12116 Prague 2, Czech Republic.

Correspondence to: Graham R. Fleming1,2 Correspondence and requests for materials should be addressed to G.R.F. (Email: grfleming@lbl.gov).

(From Abstract)

In the present experiment, we use two-dimensional electronic spectroscopy to observe oscillations caused by electronic coherence evolving during the population time in FMO [bacteriochlorophyll complex]. Such quantum coherence, a coherent superposition of electronic states analogous to a nuclear wavepacket in the vibrational regime, is formed when the system is initially excited by a short light pulse with a spectrum that spans multiple exciton transitions. Theoretical predictions indicate that both the amplitudes and shapes of peaks will contain beating signals with frequencies corresponding to the differences in energy between component exciton states17.

To observe the quantum beats, two-dimensional spectra were taken at 33 population times T, ranging from 0 to 660 fs. Representative spectra are shown in Fig. 1 and a video of the spectral evolution is included in the Supplementary Information. In these spectra, the lowest-energy exciton gives rise to a diagonal peak near 825 nm that clearly oscillates: its amplitude grows, fades, and subsequently grows again. The peak's shape evolves with these oscillations, becoming more elongated when weaker and rounder when the signal amplitude intensifies. The associated cross-peak amplitude also appears to oscillate. Surprisingly, the quantum beating lasts for 660 fs. This observation contrasts with the general assumption that the coherences responsible for such oscillations are destroyed very rapidly, and that population relaxation proceeds with complete destruction of coherence2 (so that the transfer of electronic coherence between excitons during relaxation is usually ignored2, 18, 19).

(from Discussion)

The FMO light-harvesting complex provides an opportunity to apply more complete energy transfer theories that invoke non-markovian dynamics and include coherence transfer. Such theories need to include wavelike energy motion owing to long-lived coherence terms, alongside the population transfer included in current models. Further, the observed preservation of coherence in this photosynthetic system requires us to redefine our description of the role of electron–phonon interactions within photosynthetic proteins. In particular, the protein may not only enforce the structure that gives rise to the couplings, but also modulate those couplings with motions of charged residues and changing local dielectric environments, which will change exciton energies and promote coherence transfer