A photosynthetic reaction centre is a protein that is the site of the light reactions of photosynthesis. The reaction centre contains pigments such as chlorophyll and phaeophytin. These absorb light, promoting an electron to a higher energy level within the pigment. The free energy created is used to reduce an electron acceptor, and is critical for the production of chemical energy during photosynthesis.
Reaction centres are present in all green plants and in many bacteria and algae. Green plants have two reaction centres known as photosystem I and photosystem II and the structures of these centres are complex, involving a multisubunit protein. The reaction centre found in Rhodopseudomonas bacteria is currently better understood since it has fewer proteins than the examples in green plants.

Capturing light energy

Bacteria
The bacterial photosynthetic reaction centre has been an important model to understand the structure and chemistry of the biological process of capturing light energy. In the 1960s, Roderick Clayton was the first to purify the reaction centre complex from purple bacteria. However, the first crystal structure was determined by Hartmut Michel, Johann Deisenhofer and Robert Huber for which they shared the Nobel Prize in 1988. This was also significant since it was the first structure for any membrane protein complex.
Four different subunits were found to be important for the function of the photosynthetic reaction centre. The L and M subunits, shown in blue and purple in the image of the structure both span the plasma membrane. They are structurally similar to one another, both having 5 transmembrane polypeptide helices. Four bacteriochlorophyll b (BChl-b) molecules, two bacteriophaeophytin b molecules (BPh) molecules, two quinones (Q_A and Q_B), and a ferrous ion are associated with the L and M subunits. The H subunit, shown in gold, lies on the cytoplasmic side of the plasma membrane. A cytochrome subunit, shown in green, contains four c-type hemes and is located on the periplasmic surface (outer) of the membrane.
The reaction centre contains two pigments that serve to collect and transfer the energy from photon absorption: BChb and Bph. BChb roughly resembles the chlorophyll molecule found in green plants, but due to minor structural differences, its peak absorption wavelength is shifted into the infrared, with wavelengths as long as 1000nm. Bph has the same structure as BChb, but the central magnesium ion is replaced by two protons.

Structure
The process starts when light is absorbed by two BChl-b molecules that lie near the periplasmic side of the membrane. This pair of chlorophyll molecules, often called the "special pair", absorbs photons at roughly 960nm, and thus is called P960 (with P standing for "pigment"). Once P960 absorbs a photon it ejects an electron, which is transferred through another molecule of Bchl to the BPh in the L subunit. This initial charge separation yields a positive charge on P960 and a negative charge on the BPh. This process takes place in 10 picoseconds (10 is reduced to P960) from a heme in the cytochrome subunit above the reaction centre.
The high-energy electron which resides on the tightly bound quinone molecule Q_A is transferred to an exchangeable quinone molecule Q_B. This molecule is loosely associated with the protein and is fairly easy to detach. Two of the high-energy electrons are required to fully reduce Q_B to QH₂ taking up two protons from the cytoplasm in the process. The reduced quinone QH₂ diffuses through the membrane to another protein complex (cytochrome bc₁-complex) where it is oxidised. In the process the reducing power of the QH₂ is used to pump protons across the membrane to the periplasmic space. The electrons from the cytochrome bc₁-complex are then transferred through a soluble cytochrome c intermediate, called cytochrome c₂, in the periplasm to the cytochrome subunit. Thus, the flow of electrons in this system is cyclical.

Green plants
In 1772, the chemist Joseph Priestly carried out a series of experiments relating to the gasses involved in respiration and combustion. In his first experiment, he lit a candle and placed it under an upturned jar. After a short period of time, the candle burned out. He carried out a similar experiment with a mouse in the confined space of the burning candle. He found that the mouse died a short time after the candle had been extinguished. However, he could revivify the foul air by placing green plants in the area and exposing them to light. Priestly's observations were some of the first experiments that demonstrated the activity of a photosynthetic reaction centre.
In 1779, Jan Ingenhousz carried out more than 500 experiments spread out over 4 months in an attempt to understand what was really going on. He wrote up his discoveries in a book entitled 'Experiments upon Vegetables'. Ingenhousz took green plants and immersed them in water inside a transparent tank. He observed many bubbles rising from the surface of the leaves whenever the plants were exposed to light. Ingenhousz collected the gas which was given off by the plants and performed several different tests in attempt to determine what the gas was. The test which finally revealed the identity of the gas was placing a smoldering taper into the gas sample and having it relight. This test proved it was oxygen, or as Joseph Priestly had called it, 'de-phlogisticated air'.
In 1932, Professor Robert Emerson and an undergraduate student, William Arnold, used a repetitive flash technique to precisely measure small quantities of oxygen evolved by chlorophyll in the algae Chlorella. Their experiment proved the existence of a photosynthetic unit. Gaffron and Wohl later interpreted the experiment and realized that the light absorbed by the photosynthetic unit was transferred.

Oxygenic photosynthesis
Photosystem II is the photosystem that generates the electron that will eventually reduce NADP. Manganese also forms strong bonds with oxygen-containing molecules such as water.
Every time the P680 absorbs a photon, it emits an electron, gaining a positive charge. This charge is neutralized by the extraction of an electron from the manganese centre which sits directly below it. The process of oxidizing two molecules of water requires four electrons. The water molecules which are oxidized in the manganese centre are the source of the electrons which reduce the two molecules of Q to QH₂.

Photosystem I

Light harvesting complex
Photosynthesis
Photosystem
Phycobilisome