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Light Harvesting and Its Regulation in Photosynthetic Grana Membranes


Photosystem II (PSII) initiates photosynthesis in plants by absorbing and converting light energy from the sun into chemical energy, a process called light harvesting. PSII is composed of proteins bound to pigment cofactors that can be grouped into antenna proteins, which absorb light and transfer excitation energy to other pigment-protein complexes, and reaction centers, which can convert excitation energy into chemical energy via a charge separation reaction. In plants, the proteins associated with PSII are located in the grana membrane, which is densely packed with photosystem II and major light harvesting complexes (LHCII). PSII reversibly binds with LHCII to form PSII supercomplexes. PSII supercomplexes and LHCIIs form a large, variably fluid array of pigment-protein complexes that gives rise to an energy transfer network that operates like a "smart" solar cell. In dim sunlight, the grana membrane harvests light with 90% efficiency. In response to light of fluctuating intensity and wavelength, the antenna proteins of PSII can regulate light harvesting. Understanding the design principles of light harvesting in grana membranes in different light conditions would be useful as a blueprint for designing robust solar cells.

This thesis presents measurements and models for understanding light harvesting in variable light conditions. We have developed the fluorescence lifetime snapshot technique to monitor changes in the energy transfer network of the grana membrane of green algae and plant leaves in response to changes in incident light. Using this technique, we suggest that there are two mechanisms for green algae to acclimate to changes in light intensity. To fully leverage the snapshot data, a structure-based model of energy transfer for the grana membrane is required. We constructed a detailed model of energy transfer and trapping in PSII supercomplexes and show how to use this model to construct an energy transfer model for the grana membrane. Together, the snapshot technique and membrane model will aid in the elucidation of the principles of light harvesting and its regulation in grana membranes.

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