Summary

Photosynthesis is the process by which chlorophyll-bearing autotrophs convert light energy into the chemical energy of carbohydrate, fixing atmospheric CO2 using water as the ultimate source of electrons and releasing O2 as a by-product -- a fact first correctly predicted by van Niel and built up experimentally through Priestley's, Ingenhousz's, Engelmann's and Hill's classic work. The process occurs chiefly within the chloroplast, whose stacked thylakoid membranes (grana, interconnected by stroma lamellae) house the light reaction, and whose surrounding fluid stroma houses the dark, biosynthetic reaction.

Light is captured by pigments -- chlorophyll a as the primary, universally reaction-centre pigment; chlorophyll b and the carotenoids (carotene and xanthophyll) as accessory pigments; and, in various algal and bacterial groups, chlorophylls c and d and bacteriochlorophyll, each with its own characteristic empirical formula, alongside the rare and poorly characterised "chlorophyll e." These pigments are organised into Photosystem I (P700) and Photosystem II (P680), whose reaction centres, once photoactivated, eject high-energy electrons that split water at PS II (releasing O2), pass down an electron transport chain to PS I, and ultimately reduce NADP+ to NADPH -- the non-cyclic, Z-scheme route that also generates ATP; alternatively, PS I alone can run in a closed loop, producing ATP only, in cyclic photophosphorylation. Both routes build a proton gradient across the thylakoid membrane that, per Mitchell's chemiosmotic hypothesis, drives ATP synthase to synthesise ATP. …