Photosynthesis enables plants, algae, and certain bacteria to convert light energy into chemical energy. This conversion happens in two main phases: the light-dependent reactions and the light-independent reactions, often called the Calvin cycle. The light-dependent reactions occur within the thylakoid membranes inside the chloroplasts, where specialized protein complexes capture photons. This process produces the energy-carrying molecules Adenosine Triphosphate (ATP) and Nicotinamide Adenine Dinucleotide Phosphate (NADPH). These molecules are necessary to power the subsequent synthesis of sugars from carbon dioxide in the light-independent stage.
The Z-Scheme Pathway
The flow of electrons that moves from Photosystem II (PSII) to Photosystem I (PSI) is known as Non-Cyclic Photophosphorylation. This process is commonly referred to as the “Z-Scheme” due to the characteristic shape formed when the redox potentials of the electron carriers are plotted. The electron’s energy level drops after leaving PSII, is re-energized by PSI, and then drops again to reduce its final acceptor, creating a zigzag or “Z” pattern. The primary purpose of this pathway is the concurrent creation of both ATP and NADPH, which are required for the Calvin cycle.
This non-cyclic path is the principal mechanism for oxygenic photosynthesis, where water is split to provide the necessary electrons. It is distinguished from cyclic photophosphorylation, which only involves PSI and recirculates the electron to produce only additional ATP.
Initiating the Electron Flow at Photosystem II
The electron flow begins at Photosystem II (PSII), a large protein complex embedded in the thylakoid membrane that contains the reaction center pigment P680. P680 is a pair of chlorophyll a molecules that absorb light at 680 nanometers. When P680 absorbs a photon, its energy level is boosted, and the excited electron is immediately transferred to a primary electron acceptor, pheophytin, initiating the electron transport chain.
The transfer of the electron leaves the reaction center oxidized, resulting in P680\(^+\). P680\(^+\) is the strongest biological oxidizing agent known, allowing it to pull electrons from water molecules in a process called photolysis. Two water molecules are oxidized to yield one molecule of molecular oxygen, four protons (H\(^+\)), and four electrons.
The electrons generated from water replenish P680\(^+\), allowing the photosystem to continue the cycle. The protons are released directly into the thylakoid lumen, contributing to the formation of a proton gradient. The excited electron then passes sequentially to mobile quinone acceptors, eventually reducing the mobile electron carrier Plastoquinone (PQ).
Generating Energy Through the Electron Transport Chain
The reduced Plastoquinone, now Plastoquinol (PQH\(_2\)), transfers the electrons to the next complex, the Cytochrome b\(_6\)f complex. This multi-subunit complex functions as a proton pump, analogous to Complex III in mitochondrial respiration. As the electrons move through the complex, the energy released is utilized to actively move protons from the stroma into the thylakoid lumen.
This translocation of protons concentrates hydrogen ions inside the thylakoid lumen, intensifying the electrochemical gradient across the membrane. The complex catalyzes the transfer of electrons from Plastoquinol to the soluble electron carrier Plastocyanin (PC). The proton gradient created across the thylakoid membrane is a form of stored potential energy.
This stored energy powers the ATP synthase enzyme, which is also embedded in the thylakoid membrane. As protons flow back out from the high concentration in the lumen to the low concentration in the stroma, they pass through the channels of the ATP synthase. This movement, known as chemiosmosis, drives the phosphorylation of Adenosine Diphosphate (ADP) to produce ATP. The electron is then handed off to Plastocyanin, which carries it to Photosystem I (PSI).
The Final Step Forming NADPH
The electron arrives at Photosystem I (PSI) via Plastocyanin, where it is accepted by the reaction center P700. P700, a chlorophyll a dimer, absorbs light at 700 nanometers. The absorption of a second photon re-excites the electron to a higher energy level, giving it the power to reduce the final electron acceptor.
The energized electron is transferred from PSI to the soluble protein Ferredoxin (Fd). Ferredoxin then interacts with the enzyme Ferredoxin-NADP\(^+\) reductase (FNR), the final enzyme in the non-cyclic pathway. FNR uses the high-energy electrons and a proton (H\(^+\)) from the stroma to reduce the coenzyme NADP\(^+\).
The resulting molecule is NADPH, a high-energy electron carrier. The simultaneous production of ATP from the proton gradient and NADPH completes the linear, non-cyclic electron flow. Both energy-carrying molecules are released into the stroma, where they are utilized to power the carbon fixation reactions of the Calvin cycle.