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How does photosystem I contribute to photosynthesis?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Photosystem I (PSI) is a critical component of the light-dependent reactions in photosynthesis, functioning within the thylakoid membranes of the chloroplast. Its primary contributions include:123

NADPH Production: PSI contains a specialized reaction center pigment known as $P_{700}$ . When $P_{700}$ absorbs light energy, it becomes excited and transfers electrons through a series of iron-sulfur proteins to ferredoxin. These electrons are ultimately used to reduce $N A D P^{+}$ to NADPH, a high-energy molecule essential for the Calvin-Benson cycle to convert carbon dioxide into sugars.134
Electron Replacement: After losing an electron, the oxidized $P_{700}^{+}$ recovers electrons from plastocyanin, which receives them from intermediate carriers linked to the activity of photosystem II.1
Cyclic Electron Flow: PSI can operate independently of photosystem II through a process called cyclic electron flow. In this pathway, electrons are recycled from ferredoxin back to intermediate carriers, a mechanism that helps generate the proton gradient required for ATP synthesis without producing NADPH.15

Together with photosystem II, PSI facilitates the conversion of light energy into the chemical energy necessary to fuel plant metabolism.3