Photosystem II is a protein complex central to photosynthesis, found in the thylakoid membranes of plants, algae, and cyanobacteria. Its primary job is to initiate the conversion of light into chemical energy. It captures energy from sunlight and uses it to start a chain of events that fuels the organism, beginning with the absorption of a single photon.
The Antenna Complex and Light Absorption
Within Photosystem II (PSII) is a network of pigment molecules known as the antenna complex. Composed of hundreds of chlorophyll and carotenoid molecules, this assembly acts like a satellite dish, increasing the surface area for capturing photons. This arrangement allows the photosystem to gather light energy more efficiently than a single pigment could. The different pigments absorb light at various wavelengths, ensuring a broad spectrum of sunlight can be utilized.
When a photon strikes a pigment, its energy is absorbed and passed to another pigment through resonance energy transfer. This rapid process funnels the captured energy inward from the outer pigments toward the heart of the photosystem. The purpose of the antenna pigments is to efficiently deliver this energy to a central pair of specialized chlorophyll molecules.
The Reaction Center and Electron Excitation
At the center of Photosystem II is the reaction center, where the collected light energy is put to work. A specialized pair of chlorophyll a molecules, designated P680, receives the energy funneled from the antenna complex. This pair’s uniqueness comes from its position within the protein environment, which alters its properties.
When energy arrives at P680, it excites an electron to a higher energy state. This energized electron is so unstable in its high-energy orbital that it is ejected from the P680 molecule. This event transforms the captured light energy into chemical potential energy in the form of the high-energy electron.
The departure of the electron leaves the P680 molecule in an oxidized state with a positive charge, designated P680+. This P680+ is a powerful oxidizing agent with a strong affinity for electrons. Its formation creates an “electron hole” that must be filled for the process to continue, enabling it to pull an electron from a nearby source.
Water Splitting and Oxygen Creation
The oxidizing nature of P680+ triggers the next event at the oxygen-evolving complex (OEC). This cluster of four manganese ions and one calcium ion is located near the P680 pair. The OEC acts as a catalytic center, using the pull from P680+ to perform the splitting of water molecules, a fundamental reaction for life on Earth.
To satisfy the electron debt of P680+, the OEC extracts electrons from water. For every four P680+ molecules generated, the OEC oxidizes two water molecules, yielding four electrons, four protons, and one molecule of diatomic oxygen (O2). The electrons are transferred one by one to replenish the P680+ molecules, resetting them to their P680 state to repeat the cycle.
The byproducts of water-splitting are also important. The protons are released into the thylakoid interior, contributing to a proton gradient used to generate ATP. The diatomic oxygen is released as a waste product into the atmosphere and is the source of nearly all breathable oxygen on Earth.
Passing Energy to the Electron Transport Chain
The high-energy electron ejected from P680 is immediately captured by an acceptor molecule, pheophytin, to prevent it from falling back. Pheophytin is a chlorophyll molecule that lacks the central magnesium ion. This capture is the first step in moving the electron away from the reaction center to harness its energy.
From pheophytin, the electron is passed to a more permanent carrier molecule named plastoquinone (PQ). After accepting two electrons and two protons from the surrounding environment, plastoquinone becomes reduced and mobile within the thylakoid membrane. It then detaches from Photosystem II and travels to deliver the electron to the next stage of photosynthesis.
This transfer to a mobile carrier marks the beginning of the photosynthetic electron transport chain. The energy from the electron, originally from light, will be used in subsequent steps to create ATP and NADPH. The work of Photosystem II is complete as it hands off the electron, which powers the synthesis of sugars for the organism.