How are excited electrons transferred between carriers in the thylakoid membrane?

SouravNovember 9, 2024

Answer

Excited electrons are transferred between carriers in the thylakoid membrane through a series of well-coordinated steps involving various protein complexes and mobile electron carriers. This process is essential for converting light energy into chemical energy during the light-dependent reactions of photosynthesis.

Mechanism of Electron Transfer in the Thylakoid Membrane

1. Excitation of Electrons:
   • The process begins when photons are absorbed by chlorophyll molecules in Photosystem II (PSII), specifically by a special pair of chlorophyll molecules known as P680. This absorption excites electrons to a higher energy state, allowing them to be transferred to a primary electron acceptor, pheophytin.
2. Electron Transport Chain:
   • After being accepted by pheophytin, the excited electron is passed along a series of proteins that make up the electron transport chain (ETC) in the thylakoid membrane. This chain includes several key components:
     • Plastoquinone (PQ): This mobile electron carrier shuttles electrons from PSII to the cytochrome b6f complex, facilitating the movement of electrons while also contributing to proton pumping across the thylakoid membrane.
     • Cytochrome b6f Complex: This complex receives electrons from plastoquinone and transfers them to another mobile carrier, plastocyanin (PC). During this process, it also pumps protons into the thylakoid lumen, contributing to the proton gradient necessary for ATP synthesis.
3. Transfer to Photosystem I:
   • Plastoquinone transfers its electrons to the cytochrome b6f complex, which then passes them on to plastocyanin. Plastocyanin carries these electrons to Photosystem I (PSI), where they are re-energized by another photon absorbed by P700, another special pair of chlorophyll molecules.
4. Final Electron Transfer:
   • The re-excited electrons from PSI are transferred to ferredoxin (Fd), a soluble electron carrier. From ferredoxin, the electrons are ultimately transferred to NADP+ via NADP+ reductase, forming NADPH, which is used in the Calvin cycle for synthesizing sugars.
5. Cyclic Electron Flow:
   • In addition to linear electron flow (the path described above), there is also cyclic electron flow that occurs primarily in PSI. In this pathway, electrons from PSI can return to the cytochrome b6f complex instead of being transferred to NADP+, allowing for additional ATP production without generating NADPH. This mechanism helps balance ATP and NADPH levels according to cellular needs