Practice question

Photosynthetic light reactions convert photon energy into chemical energy through vectorial electron and proton transport generating ATP and reducing power. This machinery is embedded exclusively in the thylakoid membrane system of chloroplasts, forming flattened sac-like discs that stack into grana interconnected by unstacked stroma lamellae, vastly increasing surface area for photon capture. Integral complexes include Photosystem II with Mn4CaO5 oxygen-evolving complex, plastoquinone pool, cytochrome b6f dimer, plastocyanin luminal electron shuttle, Photosystem I, ferredoxin and ferredoxin-NADP+ reductase. Light excites P680 and P700 special-pair chlorophylls, driving electrons from water splitting, which releases O2 and protons into lumen, through the chain to NADP+, producing NADPH in stroma. Simultaneously, proton translocation from stroma to lumen via water oxidation and cytochrome b6f Q-cycle proton pumping generates steep pH gradient of about 3 pH units and membrane potential that powers chloroplast ATP synthase, CF0CF1, to synthesize ATP by chemiosmosis as per Peter Mitchell theory. Antenna light-harvesting complexes LHCII and LHCI augment absorption and regulate state transitions for balanced energy distribution and photoprotection via non-photochemical quenching.