Photosystems are specialized protein complexes that initiate the process of photosynthesis, the engine of energy conversion for nearly all life on Earth. This process takes light energy and transforms it into chemical energy, providing the foundation for the food chain and producing the oxygen we breathe. Photosystems capture photons of light and convert that energy into a flow of electrons, which drives the synthesis of energy-carrying molecules. This light-driven conversion is the first step in turning sunlight, water, and carbon dioxide into sugars.
Anatomy and Location
Photosystems are located within the chloroplasts, the organelles responsible for photosynthesis in plants and algae. These large protein complexes are embedded directly into the thylakoid membrane, which forms flattened, sac-like structures inside the chloroplast.
Each photosystem is composed of two main functional units: the antenna complex and the reaction center. The antenna complex acts as a light-harvesting funnel, containing hundreds of pigment molecules, primarily chlorophyll and carotenoids, which absorb photons across a range of wavelengths. This absorbed energy is then transferred between pigments until it reaches the reaction center, where the conversion to chemical energy begins.
The reaction center contains a pair of specialized chlorophyll molecules that are the site of the primary light-induced chemical reaction. The two photosystems, Photosystem I (PSI) and Photosystem II (PSII), are distinguished by the peak wavelength of light their reaction centers absorb. PSII’s reaction center is known as P680 (absorbing most effectively at 680 nanometers), while PSI’s is called P700 (absorbing maximally at 700 nanometers).
Photosystem II The First Step
Photosystem II (PSII), with its P680 reaction center, initiates the light-dependent process. When the antenna complex captures a photon, the energy is relayed to the P680 chlorophyll pair, exciting an electron to a higher energy state. This energized electron is immediately transferred to an acceptor molecule, leaving the P680 pair positively charged and unstable.
To replace the lost electron and stabilize the P680 chlorophyll, PSII performs photolysis, the splitting of water molecules. This reaction, carried out by a manganese-containing complex associated with PSII, strips electrons from water. The splitting of two water molecules yields four electrons, four protons (H$^+$), and one molecule of molecular oxygen (O$_2$).
The electrons are delivered to the P680 reaction center to replace those lost to the electron transport chain. This water-splitting mechanism is the source of nearly all the oxygen in the Earth’s atmosphere. The released protons are deposited into the thylakoid lumen, and the newly energized electrons are passed along the first segment of the electron transport chain.
Photosystem I The Energy Boost
The electrons travel down the first electron transport chain, a series of protein carriers, losing some of their initial energy. These lower-energy electrons are delivered to Photosystem I (PSI), where they replace the electrons that PSI has lost to its own light-driven reaction. The P700 reaction center in PSI absorbs a second photon of light, re-energizing the arriving electron to a very high energy state.
The highly energized electrons are then passed through a short, second electron transport chain within PSI to a small protein called ferredoxin. Ferredoxin transfers these electrons to an enzyme complex known as Ferredoxin-NADP$^+$ reductase.
This enzyme uses the high-energy electrons to reduce the electron carrier molecule NADP$^+$ into NADPH. The production of NADPH is the final destination for the electrons that began their journey in water, converting light energy into stored chemical energy. NADPH carries the reducing power needed for the subsequent stage of photosynthesis, where sugars are synthesized.
How Photosystems Produce Energy
The coordinated flow of electrons from PSII to PSI is often described as the Z-scheme. This linear electron transport through the two photosystems and the intermediate protein complexes generates both ATP and NADPH. The movement of electrons down the electron transport chain releases energy that is used to pump protons (H$^+$) from the stroma into the thylakoid lumen.
This pumping action, along with the protons released from water splitting in PSII, creates a high concentration of protons inside the thylakoid lumen. This difference in proton concentration establishes an electrochemical gradient, storing potential energy. This stored energy is referred to as the proton motive force.
The protons then flow back out of the lumen and into the stroma, moving down their concentration gradient through a specialized enzyme complex called ATP synthase. The movement of protons through the ATP synthase provides the energy to convert adenosine diphosphate (ADP) and inorganic phosphate into adenosine triphosphate (ATP). Both ATP and NADPH are then available to power the synthesis of sugars during the light-independent reactions.