Photosystem I

Photosystem I (PSI) is one of two photochemical systems which is involved in the photosynthetic light reactions of algae, bacteria, and plants. Photosystem I is a multisubunit, transmembrane protein complex which is located in the thylakoid membrane.

As was mentioned in the first portion Photosystem I is linked in series with Photosystem II. In actuality Photosystem I follows that of Photosystem II. What happens is that a light-excited electron from the reaction center is transferred into an electron transport chain. The primary source of these electrons are water molecules. Once the electrons pass through the electron transport chain they end up in NADP⁺ (Nicotinamide adenine dinucleotide phosphate) which is later reduced to NADPH. At two different stages in the electron transport chain electrons are released into the thylakoid lumen.

Photosystem I comes into play once Photosystem II has completed the job of splitting the H₂O molecules and creating O₂ molecules which help to create a proton gradient across the thylakoid membrane. But up to this point the electrons have still not made it to NADPH. At the beginning of Photosystem I electrons, which have been excited from photons picked up by the antenna, are raised to an excited state of about –1.3 V. The excited electrons are then inserted into an electron transport chain. First, the electron is taken up by a chlorophyll acceptor (A₀ ). Secondly, it is transferred to a molecule of phylloquinone (A_1.) This particular molecule is also known as vitamin K_1. The electron then enters a series of three iron-sulfur proteins (F_x , F_b, F_A .) Finally the electron is transferred to another iron-sulfur protein, called soluble ferredoxin (Fd) which is found in the stroma. The enzyme ferredoxin then catalyzes the transfer of electrons to NADP^+, after the ferredoxin has been reduced by photosystem I.