Coherent Processes in Photosynthetic Energy Transport and Transduction

The role of non-trivial quantum mechanical effects in biology, and especially photosynthesis, has been the subject of intense hype and scrutiny over the past decade. This is largely the product of an increase in temporal, spatial and energetic resolution afforded by modern crystallography and the advent of ultrafast laser spectroscopy. The latter technique has been able to resolve quantum coherent oscillations in the spectroscopic signals of light harvesting proteins, dubbed “quantum beats”, that persist for hundreds of femtoseconds (in the longest-lived cases). Quantum beats are symptomatic of general coherent phenomena and are thus indicative of non-trivial quantum coherent effects in photosynthetic systems. A brief introduction to quantum mechanics is given and the various coherence effects it produces are discussed in detail. Time, length and energy scales of light harvesting systems are presented, providing a context for understanding the possible extent of coherent effects ultimately leading to an understanding of quantum coherent energy transport and the proposal that quantum coherence may be responsible for the efficiency and robustness of energy transport in biological systems, especially in some algae. A consensus is emerging that most long-lived coherent phenomena are of vibrational or vibronic origin, where the latter may result in coherent excitation transport within an algal protein complex, however, it is unlikely that coherent behaviour is present between complexes. Broadly speaking, whether evolution has selected for these non-trivial quantum phenomena may be an unanswerable question as quantum processes come to prominence at the scales of interest for photosynthesis and quantum mechanics that cannot merely be switched on or off.