Photosystem
Encyclopedia
Photosystems are functional and structural units of protein complex
es involved in photosynthesis
that together carry out the primary photochemistry
of photosynthesis
: the absorption of light
and the transfer of energy and electrons
. They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the chloroplast
s), or in the cytoplasmic membrane of photosynthetic bacteria.
, which is an enzyme
that uses light to reduce
molecules. In a photosystem, this Reaction Center is surrounded by light-harvesting complexes
that enhance the absorption of light and transfers the energy to the Reaction Centers. Light-Harvesting and Reaction Center complexes are membrane protein
complexes that are made of several protein-subunits and contain numerous cofactors
. In the photosynthetic membranes, reaction centers provide the driving force for the bioenergetic electron and proton transfer chain. When light
is absorbed by a reaction center (either directly or passed by neighbouring pigment-antennae), a series of oxido-reduction reactions is initiated, leading to the reduction of a terminal acceptor. 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). Each photosystem can be identified by the wavelength
of light to which it is most reactive (700 and 680 nanometers, respectively for PSI and PSII in chloroplasts), the amount and type of light-harvesting complexes 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. One has to note that both reaction center types are present in chloroplasts and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct.
, pheophytin
, carotenoids), quinones, or iron-sulfur clusters.
The 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 by a cluster of four Manganese ions in photosystem II and supplied to the chlorophyll via a redox-active tyrosine.
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, to the lumen, to provide a proton-motive force 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. The deficit is due to photo-excitation of electrons that are again trapped in an electron acceptor molecule, this time that of photosystem
ATP is generated when the ATP synthetase 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.
Protein complex
A multiprotein complex is a group of two or more associated polypeptide chains. If the different polypeptide chains contain different protein domain, the resulting multiprotein complex can have multiple catalytic functions...
es involved in photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
that together carry out the primary photochemistry
Photochemistry
Photochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...
of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
: the absorption of light
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
and the transfer of energy and electrons
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...
. They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s), or in the cytoplasmic membrane of photosynthetic bacteria.
Reaction centers
At the heart of a photosystem lies the Reaction CenterPhotosynthetic reaction centre
A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis...
, which is an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
that uses light to reduce
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
molecules. In a photosystem, this Reaction Center is surrounded by light-harvesting complexes
Light-harvesting complex
A light-harvesting complex is a complex of subunit proteins that may be part of a larger supercomplex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction...
that enhance the absorption of light and transfers the energy to the Reaction Centers. Light-Harvesting and Reaction Center complexes are membrane protein
Membrane protein
A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes.-Function:...
complexes that are made of several protein-subunits and contain numerous cofactors
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....
. In the photosynthetic membranes, reaction centers provide the driving force for the bioenergetic electron and proton transfer chain. When light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
is absorbed by a reaction center (either directly or passed by neighbouring pigment-antennae), a series of oxido-reduction reactions is initiated, leading to the reduction of a terminal acceptor. Two families of reaction centers in photosystems exist: type I reaction centers (such as photosystem I (P700
P700
P700, or Photosystem I primary donor, is the reaction-center chlorophyll a molecule in association with photosystem I. Its absorption spectrum peaks at 700 nm. When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll...
) in chloroplasts and in green-sulphur bacteria) and type II reaction centers (such as photosystem II
Photosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
(P680
P680
P680, or Photosystem II primary donor, refers to any of the 2 special chlorophyll dimers , PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule...
) in chloroplasts and in non-sulphur purple bacteria). Each photosystem can be identified by the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of light to which it is most reactive (700 and 680 nanometers, respectively for PSI and PSII in chloroplasts), the amount and type of light-harvesting complexes present and the type of terminal electron acceptor used. Type I photosystems use ferredoxin
Ferredoxin
Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...
-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...
terminal electron acceptor. One has to note that both reaction center types are present in chloroplasts and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, creating 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 chlorophyllChlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
, pheophytin
Pheophytin
Pheophytin or phaeophytin is a chemical compound that serves as the first electron carrier intermediate in the electron transfer pathway of photosystem II in plants, and the photosynthetic reaction center found in purple bacteria...
, carotenoids), quinones, or iron-sulfur clusters.
Relationship between Photosystems I and II
For oxygenic photosynthesis, both photosystems I and II are required. 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 a single photosystem called BRC, bacterial reaction center.The 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 by a cluster of four Manganese ions in photosystem II and supplied to the chlorophyll via a redox-active tyrosine.
Photoexcited electrons travel through the cytochrome b6f complex
Cytochrome b6f complex
The cytochrome b6f complex is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, catalyzing the transfer of electrons from plastoquinol to plastocyanin...
to photosystem I via an electron transport chain set in the thylakoid membrane. This energy fall is harnessed, (the whole process termed chemiosmosis
Chemiosmosis
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....
), to transport hydrogen (H+) through the membrane, to the lumen, to provide a proton-motive force to generate ATP. The protons are transported by the plastoquinone
Plastoquinone
Plastoquinone is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced , forming plastoquinol...
. 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 that are again trapped in an electron acceptor molecule, this time that of photosystem
ATP is generated when the ATP synthetase 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
Glycerate 3-phosphate
3-Phosphoglyceric acid , or glycerate 3-phosphate , is a biochemically significant 3-carbon molecule that is a metabolic intermediate in both glycolysis and the Calvin cycle. This chemical is often termed PGA when referring to the Calvin cycle...
, along with ATP to form glyceraldehyde 3-phosphate
Glyceraldehyde 3-phosphate
Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GADP, GAP, TP, GALP or PGAL, is a chemical compound that occurs as an intermediate in several central metabolic pathways of all organisms...
, the basic building-block from which plants can make a variety of substances.
See also
- Photosynthetic reaction centrePhotosynthetic reaction centreA photosynthetic reaction center is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis...
- photosynthesisPhotosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
- chlorophyllChlorophyllChlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
- light reaction
- photoinhibitionPhotoinhibitionPhotoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII...
- photosystem IPhotosystem IPhotosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...
- photosystem IIPhotosystem IIPhotosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
External links
- Photosystems I + II: Imperial College, Barber Group
- Photosystem I: Molecule of the Month in the Protein Data Bank
- Photosystem II: Molecule of the Month in the Protein Data Bank
- Photosystem II: ANU - Calculated spatial positions of photosynthetic reaction centers and photosystems in membrane