How does light absorption by photosystems generate excited electrons?

Sourav PanNovember 9, 2024

Answered

Photosystems play a crucial role in the process of photosynthesis by absorbing light energy and generating excited electrons, which are essential for converting light energy into chemical energy. This process occurs primarily in the thylakoid membranes of chloroplasts and involves two distinct photosystems: Photosystem II (PSII) and Photosystem I (PSI).

Mechanism of Light Absorption and Electron Excitation

1. Photon Absorption:
   • When light photons strike chlorophyll pigments within the photosystems, they excite electrons to a higher energy state. This process begins in Photosystem II, where chlorophyll absorbs light most effectively at a wavelength of 680 nm (referred to as P680).
2. Electron Excitation:
   • The absorbed energy is transferred among various pigments until it reaches a specific chlorophyll molecule known as the reaction center. Here, the energy is sufficient to eject an electron from the chlorophyll, creating a high-energy electron . This electron is then transferred to a primary electron acceptor, initiating the electron transport chain .
3. Water Splitting:
   • To replenish the lost electrons in PSII, water molecules are split in a process called photolysis. This reaction produces oxygen as a byproduct and releases additional electrons that replace those lost by chlorophyll .
4. Electron Transport Chain:
   • The high-energy electrons are passed through a series of proteins embedded in the thylakoid membrane, known as the electron transport chain (ETC). As electrons move through this chain, they lose energy, which is harnessed to pump protons into the thylakoid lumen, creating a proton gradient. This gradient drives ATP synthesis via ATP synthase.
5. Re-excitation in Photosystem I:
   • The electrons eventually reach Photosystem I (absorbing light at 700 nm, or P700), where they are re-excited by additional photons. The re-energized electrons are then transferred to another set of carriers, ultimately leading to the reduction of NADP+ to NADPH, which is another form of stored chemical energy