= Solution
\b[Antennae collect photons; reaction centres convert excitation into charge separation; electron flow then generates reducing power and a proton gradient for carbon assimilation.] <Chlorophyll> and accessory pigments in <light-harvesting complexes> broaden light absorption and transfer excitation towards a <photosynthetic reaction centre>. Excitation-energy transfer between pigments is distinct from transferring an electron along the electron-transport chain. The reaction centre couples excitation to separation of an electron from its donor, creating an oxidizing partner and a reduced acceptor before the excitation is lost as heat or fluorescence.
In the <thylakoid membrane>, <photosystem II> uses light excitation of its reaction centre to drive electron transfer to acceptors. Its oxidized donor is regenerated using electrons extracted from water by the <oxygen-evolving complex>, which contains a manganese–calcium cluster. Four oxidizing equivalents permit the net reaction
$$
2\mathrm H_2\mathrm O\longrightarrow\mathrm O_2+4\mathrm H^++4e^-.
$$
The protons are released on the lumenal side. Electrons pass through <plastoquinone>, the <cytochrome b6f complex> and <plastocyanin> to <photosystem I>. Plastoquinone takes up protons on reduction and contributes to their net delivery into the lumen through the cytochrome-b6f pathway. The two photosystems operate in series because excitation at one centre alone does not bridge the full energetic span from water oxidation to efficient <nicotinamide adenine dinucleotide phosphate> reduction.
Excitation in photosystem I enables electron transfer to <ferredoxin>; <ferredoxin-NADP reductase> generates <NADPH> on the stromal side. For the four electrons corresponding to one <oxygen> molecule, the ideal minimum is eight photochemical excitations, four at each photosystem, yielding two <NADPH>. Real light requirements are higher because not every absorbed photon produces productive chemistry. The <proton-motive force> drives <ATP synthase> as protons return from the lumen to the <chloroplast stroma>. Counterion movements often allow a large pH component to accumulate without an equally large membrane voltage. In a classical test, an imposed acid-to-base transition in isolated <chloroplast> material supported <ATP> formation in darkness, separating the gradient's role from the immediate presence of light.<ATP formation caused by acid-base transition of spinach chloroplasts by A. T. Jagendorf and others>[Jagendorf and Uribe, 1966, pp. 170–177, acid–base transition experiment]{ref} Appropriate controls compare transitions that produce a transmembrane gradient with treatments that dissipate it.
In the stroma, the <Calvin cycle> uses <ATP> and <NADPH> to reduce fixed carbon and regenerate the carbon acceptor. <RuBisCO> adds <carbon dioxide> to ribulose bisphosphate; subsequent <phosphorylation> and reduction produce triose phosphate. Fixation of three <carbon dioxide> molecules to one net triose phosphate requires nine <ATP> and six <NADPH> in the standard accounting, with most intermediate carbon recycled to regenerate the acceptor. “Light-independent” chemistry is not physiologically independent of light: energy supply, stromal conditions and enzyme regulation coordinate it with photochemistry.
Linear electron flow need not exactly match the <ATP>/<NADPH> demand. <Cyclic electron flow around photosystem I> increases proton-gradient-driven <ATP> production without net <NADPH> generation or water oxidation. Excess excitation is dangerous; <non-photochemical quenching>, changes in antenna allocation and photosystem repair limit damage. \b[The <chloroplast> combines efficient collection with regulated conversion and protection, rather than maximizing photon capture regardless of metabolic demand.]
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