Photosystem II (PSII) initiates photosynthesis in plants through the absorption of light and subsequent conversion of excitation energy to chemical energy via charge separation. The pigment binding proteins associated with PSII assemble in the grana membrane into PSII supercomplexes and surrounding light harvesting complex II trimers. To understand the high efficiency of light harvesting in PSII requires quantitative insight into energy transfer and charge separation in PSII supercomplexes. We have constructed the first structure-based model of energy transfer in PSII supercomplexes. This model shows that the kinetics of light harvesting cannot be simplified to a single rate limiting step. Instead, substantial contributions arise from both excitation diffusion through the antenna pigments and transfer from the antenna to the reaction center (RC), where charge separation occurs. Because of the lack of a rate-limiting step, fitting kinetic models to fluorescence lifetime data cannot be used to derive mechanistic insight on light harvesting in PSII. This model will clarify the interpretation of chlorophyll fluorescence data from PSII supercomplexes, grana membranes, and leaves.

Oxygen-evolving photosynthetic organisms possess nonphotochemical quenching (NPQ) pathways that protect against photoinduced damage. The majority of NPQ in plants is regulated on a rapid timescale by changes in the pH of the thylakoid lumen. In order to quantify the rapidly reversible component of NPQ, called qE, we developed a mathematical model of pH-dependent quenching of chlorophyll excitations in Photosystem II. Our expression for qE depends on the protonation of PsbS and the deepoxidation of violaxanthin by violaxanthin deepoxidase. The model is able to simulate the kinetics of qE at low and high light intensities. The simulations suggest that the pH of the lumen, which activates qE, is not itself affected by qE. Our model provides a framework for testing hypothesized qE mechanisms and for assessing the role of qE in improving plant fitness in variable light intensity.regulation of photosynthesis | nonlinear differential equations | biological feedback | chlorophyll fluorescence | photoprotection P hotosynthetic organisms are highly efficient at absorbing photons and transferring energy to a reaction center, where charge separation takes place. However, when the rate of energy consumption by the reaction center is slower than the rate of energy transfer to the reaction center, long-lived chlorophyll excited states build up in the Photosystem II (PSII) antenna. These longlived states present a significant hazard to the organism because the energy contained in excited chlorophyll is sufficient to generate singlet oxygen, which is highly reactive and can break bonds in the proteins essential for photosynthesis (1). Because sufficient light harvesting is necessary for fueling growth, but too much is harmful, plants face a challenge in balancing light harvesting and photoprotection, especially when light intensity rapidly fluctuates between levels that limit photosynthesis and levels that exceed the plant's capacity for photosynthesis (2).The mechanisms of regulated dissipation of excess absorbed energy in the PSII antenna are collectively known as nonphotochemical quenching (NPQ) (3). NPQ mechanisms dissipate excitation energy harmlessly as heat, reducing the extent of photoinhibition (4). There are multiple mechanisms for NPQ and these mechanisms respond on different timescales (3). The most rapid component of NPQ is called qE, and it responds to fluctuations in light intensity on the timescale of seconds to minutes (5, 6).The qE quenching pathway is activated by a decrease in the pH of the thylakoid lumen (3). The low pH of the lumen activates qE by protonating the proteins PsbS (7) and violaxanthin deepoxidase (VDE) (8, 9), and possibly other light harvesting complexes (10, 11). VDE goes on to convert the carotenoid violaxanthin to zeaxanthin in the xanthophyll cycle, which includes the intermediate antheraxanthin (12). The presence of zeaxanthin and the xanthophyll lutein, along with PsbS, is necessary for full expression of qE in vivo. In addition to the protonation of PsbS and the formation of zeaxanthin...

Photosynthetic organisms are capable of harvesting solar energy with near unity quantum efficiency. Even more impressively, this efficiency can be regulated in response to the demands of photosynthetic reactions and the fluctuating light-levels of natural environments. We discuss the distinctive design principles through which photosynthetic light-harvesting functions. These emergent properties of photosynthesis appear both within individual pigment-protein complexes and in how these complexes integrate to produce a functional, regulated apparatus that drives downstream photochemistry. One important property is how the strong interactions and resultant quantum coherence, produced by the dense packing of photosynthetic pigments, provide a tool to optimize for ultrafast, directed energy transfer. We also describe how excess energy is quenched to prevent photodamage under high-light conditions, which we investigate through theory and experiment. We conclude with comments on the potential of using these features to improve solar energy devices.