Light harvesting in photosystem II

Abstract

Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some “extra” Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.

The lowest-energy chlorophyll of photosystem II is adjacent to the peripheral antenna: Emitting states of CP47 assigned via circularly polarized luminescence

“…To understand the mechanism of light-energy capture in photosynthesis, an accurate description of the kinetics of energy transfer in the light harvesting system is vital, because several important processes take place in the thylakoid membrane on similar timescales to energy transfer [5,6]. These processes include the primary charge separation in the PSII RC, whose kinetics and mechanism remain a matter of intense research [7][8][9].…”

Differential Roles of Carotenes and Xanthophylls in Photosystem I Photoprotection

“…34 Thus, the specific interaction between Chls and Cars in the LHCI antenna system of PSI might play a more relevant role in PSI photoprotection than is generally believed. Supporting this view is the fact that Cars bound to the LHCI system are effective in quenching 3 Chl*, thus suggesting that pigments bound to these complexes are well-protected from EL damage. 35,36 Only recently has the presence of a specific binding site for β-Car in LHCI subunits been confirmed, 12,37 whereas differences in the de-epoxidation of Vio in the LHCI subunits suggest different accessibility to the respective Car-binding sites.…”

“…1 In each PS, the first stage of solar energy conversion consists of photon absorption by an array of chlorophylls (Chl), which funnel excitation energy to the reaction center where charge separation occurs. 2,3 Plants are highly effective at photosynthesis, and under steady-state conditions, the singlet excited states of the primary pigment chlorophyll ( 1 Chl*) are efficiently quenched by the photochemical reaction centers. 4 However, the photosynthetic apparatus can easily experience photooxidative damage: excess light (EL) shifts the balance of the captured to utilized energy ratio, increasing the probability of the intersystem crossing to the Chl triplet state ( 3 Chl*) and releasing the highly reactive oxygen species (ROS) singlet oxygen ( 1 O 2 ).…”

Dynamic photosynthesis under a fluctuating environment: a modelling-based analysis

“…The antenna complexes are composed of proteins and pigments (chlorophylls and carotenoids) located in the core and periphery of the photosystems. The distinction between inner and outer (or core and peripheral) antenna complexes is often used (Croce and van Amerongen, 2013;van Amerongen and Croce, 2013;Papageorgiou and Govindjee, 2014). While this distinction is important when studying energy transfer among the different components of PSIac and PSIIac, these processes may be assumed to be in rapid equilibrium at the scales of seconds (Croce and van Amerongen, 2013;van Amerongen and Croce, 2013).…”

“…The distinction between inner and outer (or core and peripheral) antenna complexes is often used (Croce and van Amerongen, 2013;van Amerongen and Croce, 2013;Papageorgiou and Govindjee, 2014). While this distinction is important when studying energy transfer among the different components of PSIac and PSIIac, these processes may be assumed to be in rapid equilibrium at the scales of seconds (Croce and van Amerongen, 2013;van Amerongen and Croce, 2013). The energy absorbed by PSIIac is used to oxidise a molecule of chlorophyll in the reaction centre, which drives the oxidation of water into O2, H % and electrons, through a series of intermediate reactions that constitute the oxygen evolving complex, located on the lumenal side of the thylakoid membrane.…”