Journal of Energy Bioscience 2024, Vol.15, No.4, 255-266 http://bioscipublisher.com/index.php/jeb 257 2.3 Mechanisms of energy transfer and electron transport The mechanisms of energy transfer and electron transport in photosynthetic protein complexes are highly efficient and tightly regulated: Excitation Energy Transfer: Light energy absorbed by the antenna pigments is transferred to the reaction centers through a process known as excitation energy transfer. This process involves the transfer of excitation energy from one pigment molecule to another until it reaches the reaction center, where it drives the primary charge separation (Chukhutsina et al., 2015; Qin et al., 2015). Electron Transport Chain: The electron transport chain consists of a series of redox reactions that transfer electrons from water to NADP+, forming NADPH. In PSII, the oxidation of water generates electrons, protons, and oxygen. The electrons are transferred to plastoquinone, which carries them to the cytochrome b6f complex. From there, electrons are transferred to plastocyanin and then to PSI, where they are used to reduce NADP+ to NADPH (Nelson and Yocum, 2006; Suga et al., 2014; Chukhutsina et al., 2015). Proton Gradient and ATP Synthesis: The transfer of electrons through the cytochrome b6f complex is coupled with the translocation of protons across the thylakoid membrane, creating a proton gradient. This gradient drives the synthesis of ATP by ATP synthase, providing the energy required for the Calvin cycle and other cellular processes (Nelson and Yocum, 2006). These structural and functional insights into photosynthetic protein complexes highlight the intricate mechanisms that plants, algae, and cyanobacteria use to convert light energy into chemical energy efficiently. Understanding these processes at a detailed level can inform efforts to optimize photosynthesis for improved energy conversion efficiency. 3 Current Challenges in Photosynthetic Efficiency 3.1 Limitations in natural photosynthetic systems Natural photosynthetic systems, while highly efficient in their native environments, face several inherent limitations. One significant challenge is the selective spectral coverage due to the specific natural pigmentation of photosynthetic proteins, which restricts the range of light wavelengths that can be effectively utilized for energy conversion (Liu et al., 2020). Additionally, the structural complexity and variability of photosystem complexes, such as PSI and PSII, across different organisms can lead to inefficiencies in energy transfer and charge separation processes (Croce and Amerongen, 2020). The need for a highly organized assembly of photofunctional chromophores and catalysts within proteins to optimize solar energy conversion further complicates the development of fully functional artificial systems (Wasielewski, 2009). 3.2 Factors affecting the efficiency of light energy conversion Several factors influence the efficiency of light energy conversion in photosynthetic systems. The arrangement and connectivity of pigments within light-harvesting complexes play a crucial role in determining energy transfer rates and overall efficiency (Croce and Amerongen, 2020). For instance, the dynamic regulation of light-harvesting systems through state transitions in plants and green algae helps balance energy distribution between PSI and PSII, optimizing photosynthetic activity under varying light conditions (Shang et al., 2023) (Figure 1). Moreover, the structural design of photosystems, such as the PSI-LHCII supercomplex, facilitates efficient light harvesting and energy transfer by forming specific pigment networks (Su et al., 2017). The quantum coherence observed in some photosynthetic proteins also suggests that long-range quantum coherence can enhance light-harvesting efficiency by enabling more efficient energy transfer between molecules (Collini et al., 2010). 3.3 Impact of environmental stressors on photosynthetic performance Environmental stressors, such as high light intensity, temperature fluctuations, and nutrient availability, can significantly impact photosynthetic performance. High light conditions, for example, can lead to the detachment of moderately bound LHCIIs in PSII to down-regulate light harvesting and prevent photodamage (Su et al., 2017).
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