Two Photosystems

Plants evolved to use electrons from water and transfer them to a electron carrier like NADPH for useful biochemistry. The problem is that a single photon does not contain sufficient energy to pump the electron to the required higher energy state in one step. As a consequence, plants use two different photosystems coupled in a series to excite the electrons with two consecutive photons (Figure: Energy Capture). A photosystem is an array of light-harvesting pigments containing a reaction center that carries out the necessary photochemistry. The reaction centers of the photosynthetic photosystems are pigments termed P-680 and P-700 .

Using this greatly simplified scheme, a photon is first captured by the pigments of Photosystem II and the excitation energy transferred to an electron in P-680, a pigment in the reaction center. The high-energy electron in excited P-680 (P-680*) is next transferred to plastoquinone (PQ) via a protein component termed D1. This electron is eventually transferred to Photosystem I. A second photon is similarly captured by Photosystem I and the energy transferred to the pigment P-700 in its reaction center. The electron excited in P-700 is transferred to NADP⁺ through the protein ferredoxin (Fd) to produce NADPH. When P-680* and P-700* donate their high-energy electrons, they become oxidized (lacking an electron). 

To bring the system back to neutrality so that it can cycle through the process again, electrons are channeled from water to P-680, thereby forming O₂, and electrons from PQ (originally donate by P-680) are transferred to P-700 via cytochrome (Cyt) and plastocyanin (PC) intermediates. The net effect is that the energy in photons is captured in pigments and used to take the electrons from water and pump them to an energy level high enough to form NADPH. The high energy of the electrons in NADPH are used by the cell for many reduction reactions. Watch the simulated animation below to better understand this process.  

*This animation has no audio.*