Photosynthesis occurs in two stages:
1) Light reactions
2) Light-independent reactions
In the light reactions, there are four parts: photosystem II, electron transport chain, photosystem I, and another electron transport chain. Each photosystem is a protein molecule embedded into the thylakoids membrane; photosystems are the light-absorbing units.
Photosystem II contains antennae complex that absorb photons from light and transfer it to reaction center chlorophylls. These special chlorophylls become oxidized and transfer their electrons to a primary electron acceptor. The special chlorophylls cannot remain without electrons, so they obtain electrons from the breaking down of a water molecule.
Then the electrons (which were accquired by the primary electron acceptor) are transferred to the first electron transfer chain. This chain creates a proton gradient and ATP is produced by chemiosmosis from ADP and inorganic phosphate.
At photosystem I, photons from light are absorbed by antennae complex and are transferred to the reaction center chlorophylls of photosystem I. This center becomes oxidized when it transfers its electrons to a primary electron acceptor, The reaction center is refilled with electrons from the electron transport chain following photosystem II. Next, the electrons that are now held by the primary electron acceptor are transferred to another electron transport chain. This, in turn, transfers the electrons to NADP+ to reduce it to NADPH.
Now, you have NADPH and ATP as products of the light reactions.
In the second stage (independent-light reactions), NADPH and ATP are used to reduce CO2 to a 3-carbon sugar (G3P) in a process called the Calvin Cycle.
Hope this helps
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