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Photosystems are made of several protein subunits, as well as hundreds of cofactors, and act as large complex. In the process of photosynthesis, light is absorbed by a photosystem (ancient Greek: phos = light and systema = assembly) to begin an energy-producing reaction. The photosytems are contained within the chloroplasts in the leaves of plants. Two types of photosystems exist: photosystem I (P700) and photosystem II (P680). Each photosystem is differentiated by the wavelength of light to which it is most reactive (700 and 680 nanometers, respectively), and the type of terminal electron acceptor. Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuffle electrons to a quinone terminal electron acceptor.
Structure
A photosystem is made up of a reaction center pigment of chlorophyll a and numerous antenna pigments and proteins. 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 electrons into an electron transport chain.
Though chlorophyll a normally has an optimal absorption wavelength of 660 nanometers, it associates with different proteins in each type of photosytem 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 one 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-centre 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 to photosystem I through an electron transport chain set in the thylakoid membrane. This energy fall is harnessed, (the whole process termed chemiosmosis), to transport H+ ions 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-centre 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 go through the aforementioned electron transport chain again in which case it is termed cyclic photophosphorylation. Otherwise the electrons pass down another electron transport chain which ends at an 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.
See also
External links
- Photosystems I + II: Imperial College
- Photosystem I: Molecule of the Month in the Protein Data Bank
- Photosystem II: Molecule of the Month in the Protein Data Bank
"Photosystem"