Photosynthesis converts light energy into chemical energy, a process occurring in plant cells and certain bacteria. At its core are photosystems, assemblies of proteins and pigments embedded in the thylakoid membranes of chloroplasts. Photosystems capture photons from sunlight, initiating the energy and electron transfers that power the entire process.
The Two Primary Components of a Photosystem
Every photosystem is organized into two parts: an antenna complex and a reaction center. The antenna complex, or light-harvesting complex, contains a vast array of pigment molecules like chlorophylls and carotenoids bound to proteins. This arrangement functions like a biological satellite dish, capturing light energy over a broad surface area.
When a photon strikes a pigment molecule in the antenna complex, the energy is passed from one pigment to another through resonance energy transfer. This process funnels the excitation energy toward the core of the photosystem. It is the energy, not electrons, that moves between these pigment molecules, directing it to its destination.
The destination for this energy is the reaction center, where the conversion of light to chemical energy occurs. The reaction center contains a unique pair of chlorophyll a molecules called the “special pair.” When energy from the antenna complex reaches this pair, it excites an electron to a higher energy state. This energized electron is then transferred to a primary electron acceptor, initiating the chain of electron flow.
Photosystem II (PSII) and Water Oxidation
The first photosystem in the electron transport sequence is Photosystem II (PSII). Its reaction-center chlorophyll, designated P680, best absorbs light at a 680-nanometer wavelength, a property determined by its surrounding protein environment. The energy funneled to P680 initiates an electron transfer, leaving the special pair oxidized.
A unique feature of PSII is the oxygen-evolving complex (OEC). This structure is a cluster of four manganese ions, one calcium ion, and oxygen atoms held by a protein scaffold. The OEC is responsible for splitting water molecules, a process known as photolysis.
The OEC extracts four electrons from two water molecules to replenish those lost by the P680 special pair. As a result of splitting water, oxygen gas is released into the atmosphere. Protons (hydrogen ions) are also released into the thylakoid interior, contributing to a proton gradient that drives ATP synthesis.
Photosystem I (PSI) and Energy Storage
Following PSII, Photosystem I (PSI) continues the light-dependent reactions. Its reaction-center chlorophyll, P700, optimally absorbs light at a 700 nm wavelength. Unlike PSII, PSI does not split water; instead, it receives electrons that have traveled from PSII via an electron transport chain.
The electron arriving at P700 has lost some energy it gained in PSII. PSI’s function is to use a new input of light to re-energize this electron. The P700 special pair then transfers the high-energy electron through its internal series of cofactors.
This energized electron is passed to a protein called ferredoxin on the outer side of the thylakoid membrane. Ferredoxin transfers the electron to the enzyme NADP+ reductase. This enzyme uses the electron and a proton from the stroma to reduce NADP+ into NADPH. NADPH is a stable energy-carrying molecule used in the next stage of photosynthesis, the Calvin cycle.
Assembly Within the Thylakoid Membrane
Photosystems are organized within the thylakoid membranes, which are composed of stacked regions (grana) and unstacked connecting regions (stroma lamellae). This structure leads to a spatial separation of the two photosystem types.
Photosystem II is located mainly in the tightly packed membranes of the grana stacks. In contrast, Photosystem I is found in the stroma lamellae and at the edges of the grana. This placement gives PSI direct access to the stroma, where its final products are needed.
This segregation creates an organized workflow. PSII passes an energized electron to a mobile carrier called plastoquinone. This carrier travels through the membrane to the cytochrome b6f complex and then to plastocyanin, another mobile carrier. Plastocyanin delivers the electron to PSI in the stroma lamellae, ensuring an efficient path from water splitting to NADPH production.