Light energy is captured by chlorophyll and used to drive the assembly of sugars from carbon dioxide and water. The energy from light is first stored in the energy carriers ATP and NADPH during the light-dependent reactions, and those carriers power the subsequent steps that build glucose. The energy ends up stored in the chemical bonds of the sugar molecules, so photosynthesis transfers light energy into chemical energy. While some energy may be released as heat, the main outcome is chemical energy stored in carbohydrates.

In the light-dependent reactions, NADPH is formed in the stroma of the chloroplast. After Photosystem I absorbs light, electrons are transferred to ferredoxin and then to the enzyme ferredoxin–NADP+ reductase (FNR) on the stromal side of the thylakoid membrane. FNR uses those electrons to reduce NADP+ to NADPH, so the molecule is produced in the stroma. Water splitting at the oxygen-evolving complex happens on the lumen side and supplies electrons, but NADPH formation itself occurs in the stroma, not in the lumen or mitochondria. This NADPH (along with ATP) powers carbon fixation in the Calvin cycle within the stroma.

Pigments beyond the main chlorophyll a are there to broaden the range of light that plants can use and to keep the photosynthetic machinery safe. Chlorophyll a does the core job of capturing light and driving the chemical reactions, but it doesn’t absorb every wavelength equally. Carotenoids step in as accessory pigments, soaking up light in wavelengths where chlorophylls are weaker and then transferring that energy to the reaction centers. They also help protect the system by quenching excess energy and neutralizing reactive oxygen species that can form under bright light. This diversity matters because natural light varies a lot—cloud cover, shade, and leaf age change the spectral quality reaching the chloroplasts. Having multiple pigments ensures more of the available light is harvested efficiently and that the damage from too much light is mitigated. In higher plants, chlorophylls stay central for the actual photochemistry, while carotenoids support and safeguard that process. Phycobilins, meanwhile, are used by some algae and cyanobacteria, not by higher plants.

Non-cyclic electron flow uses both photosystems to move electrons from water all the way to NADP+. Light energy excites PSII, which splits water and provides electrons to the chain. The electrons travel through the chain (plastoquinone, cytochrome b6f, plastocyanin) to PSI, where another round of excitation lifts them to ferredoxin and then to NADP+ reductase to form NADPH. As electrons are drawn from water at PSII, oxygen is released. The movement of electrons powers proton pumping across the thylakoid membrane, and the resulting proton gradient drives ATP synthase to make ATP. So the outputs are NADPH, ATP, and O2. This is distinct from cyclic flow, which involves only ATP production around PSI without NADPH formation or oxygen evolution, and from scenarios where photosystems would act independently without the connected electron transport chain.