Q.Describe the light-dependent steps of photosynthesis. How are they linked to the dark reactions?

In the light reaction, chlorophyll captures solar energy and converts it into the chemical energy of ATP and the reducing power of NADPH2, releasing oxygen as water is split in the process. Absorbing a photon of at least the minimum required (quantum) energy boosts one of chlorophyll's electrons to a higher-energy excited state; once light is removed, the electron falls back to its original low-energy orbital as the molecule returns to its stable ground state. The light-harvesting pigments in the thylakoid membrane are organised into clusters built around a special reaction-centre pigment pair — either P680 or P700 (named for the wavelength, in nm, at which each absorbs maximally) — with the surrounding accessory ('antenna') pigments funnelling the light energy they capture to these centres at very high speed. P680 together with its antenna pigments forms Photosystem II (PS-II); P700 with its own antenna pigments forms Photosystem I (PS-I). PS-II carries out the photolysis of water: illuminated P680 ejects an electron (ultimately captured by P700/PS-I) and regains it by splitting water — $4H_2O \rightarrow 4H^+ + 4OH^-$; $4OH^- \rightarrow 4(OH) + 4e^-$; $4OH \rightarrow 2H_2O + O_2$; net $4H_2O \rightarrow 2H_2O + O_2\uparrow + 4H^+ + 4e^-$ — releasing oxygen and building up a proton gradient across the thylakoid membrane whose dissipation (via ATP synthase) drives ATP formation. PS-I, when illuminated, emits energy-rich electrons that flow down a carrier chain to NADP+, which, combined with protons from water-splitting, is reduced to NADPH2 — the reducing power later used to fix CO2 in the dark reaction. In the overall electron-transport sequence linking the two photosystems, electrons released at PS-II pass, in order, through plastoquinone (PQ), the cytoch …