Photosystem

Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae and cyanobacteria, which are located inside the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: PSI and PSII.
Reaction centers.
At the heart of a photosystem lies the reaction center, which is an enzyme that uses light to reduce molecules (provide with electrons). This reaction center is surrounded by light-harvesting complexes that enhance the absorption of light.
Two families of reaction centers in photosystems exist: type I reaction centers (such as photosystem I (P700) in chloroplasts and in green-sulphur bacteria) and type II reaction centers (such as photosystem II (P680) in chloroplasts and in non-sulphur purple bacteria). The two photosystems originated from a common ancestor, but have since diversified.
Each of the photosystem can be identified by the wavelength of light to which it is most reactive (700 nanometers for PSI and 680 nanometers for PSII in chloroplasts), the amount and type of light-harvesting complex present, and the type of terminal electron acceptor used.
Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. Both reaction center types are present in chloroplasts and cyanobacteria, and work together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct.
Structure of PSI compared to PSII.
A reaction center comprises several (about 25-30) protein subunits, that provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters.
In oxygenic photosynthesis.
Both photosystems I and II are required for oxygenic photosynthesis. Oxygenic photosynthesis can be performed by plants and cyanobacteria; cyanobacteria are believed to be the progenitors of the photosystem-containing chloroplasts of eukaryotes. Photosynthetic bacteria that cannot produce oxygen have only one photosystem, which is similar to either PSI or PSII.
When photosystem II absorbs light, electrons in the reaction-center chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors.
Photoexcited electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain set in the thylakoid membrane. This energy fall is harnessed, (the whole process termed chemiosmosis), to transport hydrogen (H+) through the membrane, into the thylakoid lumen, to provide a potential energy difference between the thylakoid lumen space and the chloroplast stroma, which amounts to a proton-motive force that can be used to generate ATP. The protons are transported by the plastoquinone. If electrons only pass through once, the process is termed noncyclic photophosphorylation.
When the electron reaches photosystem I, it fills the electron deficit of the reaction-center chlorophyll of photosystem I.
ATP is generated when the ATP synthase transports the protons present in the lumen to the stroma, through the membrane. The electrons may either continue to go through cyclic electron transport around PS I or pass, via ferredoxin, to the enzyme NADP+ reductase. Electrons and hydrogen ions are added to NADP+ to form NADPH. 
This reducing agent is transported to the Calvin cycle to react with glycerate 3-phosphate, along with ATP to form glyceraldehyde 3-phosphate, the basic building-block from which plants can make a variety of substances.