Photosystems (ancient
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
:
phos = light and
systema = assembly) are
protein complexA multiprotein complex is a group of two or more proteins. Protein complexes are a form of quaternary structure. Proteins in a protein complex are linked by non-covalent protein-protein interactions, and different protein complexes have different degrees of stability over time...
es involved in
photosynthesisPhotosynthesis is a 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...
. They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the
chloroplastChloroplasts 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.The word chloroplast is...
s), or in the cytoplasmic membrane of photosynthetic bacteria. A photosystem (or
Reaction CenterA photosynthetic reaction center is a complex of three types of protein that is the site where molecular excitations originating from sunlight are transformed into a series of electron-transfer reactions. The reaction center proteins bind functional co-factors, chromophores or pigments such as...
) is an
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
which uses light to
reduceRedox describes all chemical reactions in which atoms have their oxidation number changed....
molecules. This
membrane proteinA 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....
complex is made of several subunits and contains numerous
cofactorsA 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/ions" that assist in biochemical transformations...
.
Photosystems (ancient
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
:
phos = light and
systema = assembly) are
protein complexA multiprotein complex is a group of two or more proteins. Protein complexes are a form of quaternary structure. Proteins in a protein complex are linked by non-covalent protein-protein interactions, and different protein complexes have different degrees of stability over time...
es involved in
photosynthesisPhotosynthesis is a 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...
. They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the
chloroplastChloroplasts 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.The word chloroplast is...
s), or in the cytoplasmic membrane of photosynthetic bacteria. A photosystem (or
Reaction CenterA photosynthetic reaction center is a complex of three types of protein that is the site where molecular excitations originating from sunlight are transformed into a series of electron-transfer reactions. The reaction center proteins bind functional co-factors, chromophores or pigments such as...
) is an
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
which uses light to
reduceRedox describes all chemical reactions in which atoms have their oxidation number changed....
molecules. This
membrane proteinA 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....
complex is made of several subunits and contains numerous
cofactorsA 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/ions" that assist in biochemical transformations...
. In the photosynthetic membranes, reaction centers provide the driving force for the bioenergetic electron and proton transfer chain. When
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
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 photosystems exist: type I reaction centers (like photosystem I (
P700P700, 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. The resulting P700 with excited...
) in chloroplasts and in green-sulphur bacteria) and type II reaction centers (like
photosystem 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 which are then transferred through a variety of coenzymes and cofactors to reduce...
(
P680P680, or Photosystem II primary donor, is a group of pigments associated with photosystem II and consists of 4 chlorophyll a molecules. These four molecules are excitonically coupled which means that they effectively act 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
wavelengthIn 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), and the type of terminal electron acceptor. Type I photosystems use
ferredoxinFerredoxins 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
quinoneQuinones are "compounds having a fully conjugated cyclic dione structure, such as that of benzoquinones, derived from aromatic compounds by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds ."Benzoquinone, sometimes referred to simply as...
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 is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
,
pheophytinPheophytin or Pheo is a chlorophyll molecule lacking a central Mg2+ ion which serves as the first electron carrier intermediate in the electron transfer pathway of photosystem II in plants and the photosynthetic reaction center RC P870 found in purple bacteria...
, carotenoids), quinones or iron-sulfur clusters.
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 through the
cytochrome b6f complexThe cytochrome b6f complex of chloroplasts and cyanobacteria transfers electrons between the two reaction center complexes of oxygenic photosynthetic membranes, photosystem I and photosystem II, and participates in formation of the transmembrane electrochemical proton gradient by also...
to photosystem I via an electron transport chain set in the thylakoid membrane. This energy fall is harnessed, (the whole process termed
chemiosmosisChemiosmosis is the diffusion of ions across a selectively-permeable membrane. 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
plastoquinonePlastoquinone 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 which are again trapped in an electron acceptor molecule, this time that of photosystem I.
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-phosphate3-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. 3-Phosphoglycerate is the resultant of the split of 6...
, along with ATP to form
glyceraldehyde 3-phosphateGlyceraldehyde 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 centre
A photosynthetic reaction center is a complex of three types of protein that is the site where molecular excitations originating from sunlight are transformed into a series of electron-transfer reactions. The reaction center proteins bind functional co-factors, chromophores or pigments such as...
- photosynthesis
Photosynthesis is a 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...
- chlorophyll
Chlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
- light reaction
- photoinhibition
Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga or a 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 Photosystem II...
- photosystem I
Photosystem 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 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 which are then transferred through a variety of coenzymes and cofactors to reduce...
External links