How do excited electrons from Photosystem II contribute to forming a proton gradient?

Sourav PanNovember 9, 2024

Answered

Excited electrons from Photosystem II (PSII) play a vital role in creating a proton gradient across the thylakoid membrane during the light-dependent reactions of photosynthesis. This gradient is essential for ATP synthesis. Here’s how this process unfolds:

Mechanism of Proton Gradient Formation

1. Excitation and Electron Transfer:
   • When PSII absorbs light, the chlorophyll molecule (P680) becomes excited and loses an electron to a primary electron acceptor, pheophytin. This process initiates the electron transport chain (ETC) as the excited electron is passed along various carriers, including plastoquinone (PQ) and cytochrome b6f complex.
2. Energy Release and Proton Pumping:
   • As the electrons move through the ETC, they lose energy at each step. This released energy is harnessed to actively pump protons (H++) from the stroma into the thylakoid lumen through the cytochrome b6f complex. This action increases the concentration of protons inside the thylakoid lumen compared to the stroma, creating a proton gradient.
3. Water Splitting:
   • Concurrently, PSII splits water molecules to replenish the lost electrons from chlorophyll. This photolysis of water produces additional protons and oxygen as byproducts. The protons generated from water splitting further contribute to the proton accumulation in the lumen, enhancing the proton gradient.
4. Proton Motive Force:
   • The accumulation of protons in the thylakoid lumen creates an electrochemical gradient, known as a proton motive force (PMF). This gradient represents stored potential energy that can be utilized for ATP synthesis.
5. ATP Synthesis:
   • The proton gradient drives protons back into the stroma through ATP synthase, a membrane-bound enzyme that synthesizes ATP from ADP and inorganic phosphate (Pi) as protons flow down their concentration gradient. This process is known as chemiosmosis