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Photosynthesis, pigment–protein complexes and electronic energy transport: simple models for complicated processes

Posted on 18. December, 2017.

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Essentially all the accessible energy for life in the Earth’s biosphere is made available through the process of photosynthesis, the conversion of light energy from the Sun into storable chemical energy for metabolism. The process itself has been the subject of remarkable evolution throughout Earth’s history.

The early Archean Earth atmosphere was without oxygen, meaning photosynthetic life was predominantly anoxygenic; indeed fossil records suggest the presence of anoxygenic bacteria some 3 Ga ago (1 Ga = 109 years). Somewhat paradoxically, it has been suggested that before the great oxidation event (ca 2.45 Ga ago) which gave oxygen prevalence in the atmosphere as observed today, some bacteria evolved oxygenic photosynthetic machinery (ca 2.7 Ga ago) likely making use of trace amounts of oxygen derived from atmospheric hydrogen peroxide (H2O2). Today, our oxygen-rich atmosphere means oxygenic photosynthesis is by far the dominant form of photosynthesis, as evidenced in a wide variety of photosynthetic organisms including green plants, algae, and many bacteria. Photosynthetic machinery has continued to evolve, with each organism striving to achieve photosynthesis with greater efficiency. This efficiency, in particular, is the focal point of this review; for decades the hope of mimicking the capture of solar energy as an untapped renewable energy source for electrical power generation on a mass scale has been a significant driving force in understanding photosynthesis and its related chemical machinery.

The article is organised as follows. First, the authors highlight the conserved processes which are found across all types of photosynthesis. They discuss electron energy transport (EET) in pigment–protein complexes (PPCs) as the main process which is attributed to the observed transport efficiencies, with reference to the present questions raised about the role of quantum mechanical effects which may aid transport. Next, in Section 2, they describe a common simulation approach for modelling EET in photosynthetic systems, namely density matrix propagation, and discuss how this approach can be extended to include a more precise description of the environment; furthermore, the core idea of treating the pigment–protein complex as a network of connected nodes is introduced. In Sections 3 and 4, simulations of two exemplar pigment–protein complexes are presented, namely the Fenna–Matthews–Olson (FMO) complex and light-harvesting complex-II (LHC-II), where recent work has used simulations to investigate the source of efficiency from two perspectives: (i) the efficiency of EET; and (ii) the robustness of energy transport to perturbations in the system. The article closes with a short discussion on where this general network approach might find use in artificial settings, such as optimising solar cells, as well as the limitations of this model in describing some of these energy transport networks.

 

Read the full article in Science Progress (2017), 100(3), pp. 313–330 https://doi.org/10.3184/003685017X14967574639964