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Structural units of protein involved in photosynthesis

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. These membranes 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. PSII will absorb red light, and PSI will absorb far red light. Although photosynthetic activity will be detected when the photosystems are exposed to either red or far red light, the photosynthetic activity will be the greatest when plants are exposed to both wavelengths of light. Studies have actually demonstrated that the two wavelengths together have a synergistic effect on the photosynthetic activity, rather than an additive one. Each photosystem has two parts: a reaction center, where the photochemistry occurs, and an antenna complex, which surrounds the reaction center. The antenna complex contains hundreds of chlorophyll molecules which funnel the excitation energy to the center of the photosystem. At the reaction center, the energy will be trapped and transferred to produce a high energy molecule. The main function of PSII is to efficiently split water into oxygen molecules and protons. PSII will provide a steady stream of electrons to PSI, which will boost these in energy and transfer them to NADP and H to make NADPH. The hydrogen from this NADPH can then be used in a number of different processes within the plant. that can then be used by the chloroplast. 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.

A reaction center comprises several (about 25 30) protein subunits, which provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron sulfur clusters. Each photosystem has two main subunits: an antenna complex (a light harvesting complex or LHC) and a reaction center. The antenna complex is where light is captured, while the reaction center is where this light energy is transformed into chemical energy. At the reaction center, there are many polypeptides that are surrounded by pigment proteins. At the center of the reaction center is a special pair of chlorophyll molecules. Each PSII has about 8 LHCII. These contain about 14 chlorophyll a and chlorophyll b molecules, as well as about four carotenoids. In the reaction center of PSII of plants and cyanobacteria, the light energy is used to split water into oxygen, protons, and electrons. The protons will be used in proton pumping to fuel the ATP synthase at the end of an electron transport chain. A majority of the reactions occur at the D1 and D2 subunits of PSII.

Both photosystem 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. At the core of photosystem II is P680, a special chlorophyll to which incoming excitation energy from the antenna complex is funneled. One of the electrons of excited P680* will be transferred to a non fluorescent molecule, which ionizes the chlorophyll and boosts its energy further, enough that it can split water in the oxygen evolving complex (OEC) of PSII and recover its electron. At the heart of the OEC are 4 Mn atoms, each of which can trap one electron. The electrons harvested from the splitting of two waters fill the OEC complex in its highest energy state, which holds 4 excess electrons.