P700, or Photosystem I primary donor, (where P stands for pigment) is the reaction-center chlorophyll a molecule in association with photosystem I. Its absorption spectrum peaks at 700 nm. When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll. The resulting P700 with excited electron is designated P700*, and is the strongest biological reducing agent (in contrast to P680⁺ of photosystem II, the strongest biological oxidizing agent). The electron is subsequently captured by the primary electron acceptor. Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors. Photosystem I is more complex than photosystem II (P680), as it has a more complex antenna system, has more subunits in its overall structure, and may exhibit cyclic or non-cyclic electron transfer from the excited P700* to the electron acceptor.

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