Photosystem



Photosystems (ancient Greek: phos = light and systema = assembly) are protein complexes involved in quinone terminal electron acceptor. One has to note that both reaction centers types are present in chloroplasts and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, evolving oxygen as a byproduct.

Structure

A reaction center comprises several (> 10 or >11) protein subunits, providing a scaffold for a series of cofactors. The latter can be pigments (like carotenoids), quinones or iron-sulfur clusters. Because chlorophyll a can only absorb light of a narrow wavelength, it works with the antenna pigments to gain energy from a larger part of the spectrum. The pigments absorb light of various wavelengths and pass along their gained energy to the reaction center chlorophyll. When the energy reaches the chlorophyll a, it releases two electron transport chain.

Though chlorophyll a normally has an optimal absorption wavelength of 660 nanometers, it associates with different proteins in each type of photosystem to slightly shift its optimal wavelength, producing two distinct photosystem types. Other proteins serve to support the structure and electron pathways in the photosystem.

Relationship between Photosystems I and II

  Historically photosystem I was named I since it was discovered before photosystem II, but this does not represent the order of the electron flow.

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. To replenish the deficit of electrons, electrons are extracted from water (either through photolysis or enzymatic means) and supplied to the chlorophyll.

Photoexcited electrons travel though the chemiosmosis), to transport hydrogen (H+) through the membrane to provide a proton-motive force to generate ATP. 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. The deficit is due to photo-excitation of electrons which are again trapped in an electron acceptor molecule, this time that of photosystem I.

These 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 glyceraldehyde 3-phosphate, the basic building block from which plants can make a variety of substances.

See also

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E1, E2, E3) - Sucrase-isomaltase complex -Tryptophan synthase
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Photosystem". A list of authors is available in Wikipedia.