I'm confused about the oxygen evolving complex step in photosynthesis..From what I can understand so far, a P680 pigment molecule gets excited in PSII forming P680* which then loses an electron to form P680+ which is a strong oxidant. This then splits 2 water molecules forming 4 electrons. How are these electrons used in PSII?? I think I'm mixing up several steps here..can anyone clarify?
Of course! Understanding the oxygen-evolving complex (OEC) and its role in photosynthesis can be complex, but I'll do my best to explain it step by step.
1. Excitation of P680: In Photosystem II (PSII), light energy is absorbed by pigment molecules, including P680. When P680 absorbs a photon, it becomes excited and forms P680*.
2. Electron transfer: P680* is a highly reactive molecule and quickly loses an electron to a primary electron acceptor, called pheophytin. This electron transfer occurs within a fraction of a second.
3. Electron transport chain: The electron is then passed through a series of electron carriers (plastoquinone, cytochrome b6f complex) within the thylakoid membrane. This process generates energy by creating a proton gradient across the membrane.
4. Water splitting: The crucial step happens at the OEC, which is a cluster of manganese and calcium ions bound to PSII. The OEC facilitates the oxidation of water molecules (H2O). The breakdown of two water molecules releases four electrons, four protons (H+), and one molecule of molecular oxygen (O2). This process is known as water oxidation.
5. Electron replenishment: The four electrons released from water splitting are used to replace the electrons lost by P680. These electrons are passed one by one from the OEC through the electron transport chain until they reach P680+, reducing it back to its neutral state.
6. Replenishment of the OEC: After donating its electrons, the OEC is re-energized by extracting more electrons from water. This cycle of water oxidation, electron transfer, and electron replenishment at the OEC is vital for the continuous functioning of PSII.
In summary, the oxygen evolving complex is responsible for splitting water molecules in PSII, releasing electrons that are used to replenish the missing electron from the excited P680, and generating molecular oxygen. This whole process is fundamental for producing oxygen and providing electrons to continue the light-dependent reactions of photosynthesis.