Photosynthesis converts light energy from the sun into chemical energy used by plants, algae, and cyanobacteria. This conversion begins with Photosystem II (PSII), a large, complex protein machine embedded in membranes. PSII acts as the initial power generator, capturing solar energy to perform the first chemical reactions of oxygenic photosynthesis. The actions within this complex set the entire process in motion, ultimately resulting in the production of the oxygen we breathe.
The Structure and Location of Photosystem II
Photosystem II is located within the thylakoid membranes, the flattened sac-like structures found inside the chloroplasts of plant cells. In cyanobacteria, this protein complex is situated in the internal cell membranes. The complex is a massive assembly, comprising a core of over 20 different protein subunits and numerous cofactors that execute the light-driven reactions.
Two distinct functional regions make up the complex: the Antenna Complex and the Reaction Center. The Antenna Complex is a peripheral light-harvesting system composed of various pigment-protein complexes. These complexes contain chlorophyll and accessory pigments that maximize light capture across a broad spectrum. The Reaction Center is the functional core, built around two proteins, D1 and D2, which hold the specialized chlorophyll pair known as P680. P680 is the site where light energy is first converted into chemical energy.
Initial Event: Light Absorption and Energy Transfer
The process begins when photons of light strike the pigments within the Antenna Complex. Upon absorbing a photon, each pigment enters a higher-energy excited state. This excitation energy is then passed rapidly from one pigment molecule to the next in a process called resonance energy transfer.
This energy transfer is highly efficient, acting like a funnel that directs the energy inward to the Reaction Center. The transfer occurs without the physical movement of electrons; it is a wave-like transfer of excitation energy between closely spaced molecules. This mechanism ensures that very little of the captured solar energy is lost as heat or fluorescence.
The energy is finally delivered to the P680 chlorophyll pair, causing it to become excited (P680). This excited P680 is now poised to initiate the chemical phase of photosynthesis. The rapid energy transfer is crucial because the excited state is short-lived, demanding immediate utilization of the energy.
The Core Mechanism: Water Splitting and Electron Movement
Once P680 is formed, it immediately transfers one high-energy electron to an acceptor molecule, pheophytin. This primary charge separation creates the highly oxidized P680\(^{+}\) and a reduced pheophytin. P680\(^{+}\) is one of the strongest biological oxidizing agents known, meaning it has an immense drive to regain an electron.
To replace the lost electron, P680\(^{+}\) pulls an electron from a nearby tyrosine residue, which acts as an electron relay. This tyrosine is re-supplied with an electron from the Oxygen Evolving Complex (OEC), the site of water splitting. The OEC is a specialized metallo-oxo cluster containing manganese and calcium ions. These ions are capable of storing the four oxidizing equivalents needed to split two water molecules.
The OEC cycles through five distinct oxidation states, known as S-states. The absorption of four separate photons is required to complete one full cycle. This four-step accumulation of energy allows the OEC to strip four electrons and four protons from two water molecules, releasing molecular oxygen (\(\text{O}_2\)).
The four electrons extracted from the water molecules are used to sequentially reduce the P680\(^{+}\) and keep the process running. Meanwhile, the initial high-energy electron is passed from pheophytin to a chain of electron carriers. The electron first moves to a tightly bound plastoquinone (\(\text{Q}_A\)), and then to a second, more loosely bound plastoquinone (\(\text{Q}_B\)).
After accepting two electrons and two protons from the surrounding environment, \(\text{Q}_B\) is fully reduced to plastoquinol (\(\text{PQH}_2\)). This reduced plastoquinol detaches from PSII and moves into the lipid membrane. It carries the energized electrons to the next protein complex in the photosynthetic electron transport chain. The protons released from the water-splitting reaction are deposited directly into the thylakoid lumen.
The Immediate Products and Significance
Photosystem II produces three products that drive the remainder of the photosynthetic process. The first is molecular oxygen (\(\text{O}_2\)), which is released into the atmosphere as a byproduct of water oxidation. The splitting of water is the source of nearly all the atmospheric oxygen on Earth.
The second product is a stream of energized electrons carried by plastoquinol. These electrons are destined for the rest of the electron transport chain, including cytochrome \(b_6f\) and Photosystem I. They ultimately provide the reducing power necessary for the synthesis of complex organic molecules.
The third product is the concentration of protons (\(\text{H}^+\)) inside the thylakoid lumen. The release of these protons, combined with the action of subsequent electron carriers, establishes a high concentration gradient across the thylakoid membrane. This difference in proton concentration represents stored energy used by ATP synthase to generate ATP, the cell’s energy currency.