Question

Explain how photosystems I and II together carry out the light reactions, resulting in energy generation.

Short answer

Photosystems I and II, found in the thylakoid membranes of chloroplasts, absorb light energy through their light-harvesting complexes and transfer it to their reaction centers. In Photosystem II, the excited electron is passed through an electron transport chain, pumping protons and creating a proton gradient used to generate ATP. Meanwhile, Photosystem I participates in both cyclic and non-cyclic electron flow, producing ATP and NADPH, respectively. ATP and NADPH generated by these processes are then utilized in the light-independent reactions of photosynthesis (Calvin cycle).

Step-by-step solution

Understand the Photosystems

Photosystems I and II are two protein complexes found in the thylakoid membranes of chloroplasts, which carry out the light-dependent reactions of photosynthesis. Each photosystem contains light-harvesting complexes (LHCs) to absorb light energy and a reaction center (RC) where energy is transferred and used to drive electron transport. The main difference between the two photosystems is their absorption peaks; Photosystem I (PSI) absorbs light at a wavelength of 700 nm, while Photosystem II (PSII) absorbs at 680 nm.

Light Absorption

The process begins with the absorption of light by the LHCs, which contain chlorophyll and carotenoid pigments. These pigments capture light energy and transfer it from molecule to molecule until it reaches the RC. Each photosystem absorbs light energy independently, resulting in the excitation of an electron within the RC chlorophyll molecule.

Photosystem II Electron Transport

In PSII, the excited electron is transferred to the primary electron acceptor, pheophytin. From pheophytin, the electron is passed through an electron transport chain (ETC), which consists of plastoquinone, cytochrome b6f, and plastocyanin. As the electron moves through the ETC, protons are pumped into the thylakoid lumen, creating a proton gradient that will be used later to generate ATP.

Water Splitting

Since the electron has been removed from the RC, PSII must replace it to continue functioning. This is accomplished through the process called water splitting or photolysis. An enzyme within PSII extracts electrons from H₂O molecules, producing O₂ and H⁺ ions. The electrons from water are then used to refill the gap in PSII's RC, allowing it to absorb another photon and restart the process.

Photosystem I Electron Transport

Simultaneously, PSI absorbs light energy and excites its own electron, which is then passed to a primary electron acceptor, ferredoxin. Unlike PSII, however, PSI does not lose its electron permanently. Instead, the electron is returned to the RC in a process called cyclic electron flow. This process is important for producing ATP without producing NADPH.

Generating NADPH

In certain cases, instead of returning to the RC, the electron from PSI is passed to NADP⁺, an electron carrier molecule. After receiving the electron and a proton, NADP⁺ is reduced to form NADPH. This process, known as non-cyclic electron flow, is essential for the production of NADPH.

ATP Synthesis

As mentioned earlier, the transport of electrons through the ETC in PSII creates a proton gradient. This gradient is maintained by the constant pumping of protons into the thylakoid lumen, and the diffusion of protons back into the stroma via ATP synthase. The movement of protons through ATP synthase provides enough energy to phosphorylate ADP into ATP. In conclusion, the light reactions carried out by photosystems I and II involve the absorption of light energy, the transfer of electrons through ETC, and the production of both ATP and NADPH. These products are then used to power the subsequent components of photosynthesis, specifically the light-independent reactions (Calvin cycle).