Thylakoid

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A thylakoid is a membrane-bound compartment in chloroplasts and cyanobacteria. They are the sites of photosynthetic reactions that depend on light. Thylakoids consist of thylakoid membranes that surround tylakoid lumen . Chloroplast thylakoids often form a disk heap called grana (single: granum ). Granules are associated with an intergranal or stroma thylakoids, which join the shared granulation pile as a functional compartment.

Video Thylakoid

Etimologi

The word thylakoid comes from the Greek thylakos meaning "sac" or "pouch". So, thylakoid means "like a bag" or "like a bag".

Maps Thylakoid

Structure

Thylakoids are structures bound to a membrane embedded in the chloroplast stroma. The thylakoid stack is called granum and resembles a stack of coins.

Membrane

The thylakoid membrane is where the light-dependent reaction of photosynthesis with photosynthetic pigments is embedded directly in the membrane. This is an alternating pattern of dark and bright bands that measure every 1 nanometer. Thylakoid lipid bilayers share characteristic features with prokaryotic membranes and inner chloroplast membranes. For example, acid lipids can be found in the thylakoid membranes, cyanobacteria and other photosynthetic bacteria and are involved in the functional integrity of the photosystem. The thylakoid membranes of higher plants consist mainly of phospholipids and galactolipids that are arranged asymmetrically along and across the membrane. The thylakoid membrane is richer in galactolipids than in phospholipids; also they are mainly composed of a hexagonal phase II which forms a monogalacchilel diglyceride lipid. Despite this unique composition, plant thylakoid membranes have been shown to assume dynamic lipid-bilayer organizations. Lipids form the thylakoid membranes, richest in high fluidity linolenic acids synthesized in complex pathways involving the exchange of lipid precursors between the endoplasmic reticulum and the inner membrane of the plastid envelope and transported from the inner membrane to the thylakoids via the vesicles.

Lumen

The tylakoid lumen is a continuous aqueous phase enclosed by a thylakoid membrane. This plays an important role for photophosphorylation during photosynthesis. During a light-dependent reaction, the proton is pumped across the thylakoid membrane to the lumen so that the acid becomes pH 4.

Granium and lamellae stroma

In high plants, the tilakoid is organized into the granule-stromal membrane. A granum (plural grana ) is a stack of thylakoid discs. Chloroplasts can have 10 to 100 granes. Granules are linked by stroma thylakoids, also called intergranal thylakoids or lamellae . Granyl thylakoids and stroma thylakoids can be distinguished on the basis of their different protein compositions. Granules contribute to the large surface area of ​​the chloroplast to the volume ratio. Different interpretations of electron tomography imaging of the thylakoid membrane have resulted in two models for the granular structure. Both assume that lamellae cuts the grane pile in parallel sheets, although these sheets intersect in a plane perpendicular to the axis of the granary pile, or arranged in a right-handed helix of debate.

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Formation

Chloroplasts develop from proplastides when seedlings emerge from the soil. Thylakoid formation requires light. In plant embryos and without light, proplastids develop into etioplasts containing a semicrystalline membrane structure called the prolamellar body. When exposed to light, this prolamaral body develops into thylakoid. This does not happen to seeds planted in the dark, which are etiolated. Less light to light can cause the thylakoid to fail. This causes the chloroplast to fail to produce the death of the plant.

The formation of thylakoid requires the action of protein-stimulating vesicles in plastida 1 (VIPP1). Plants can not survive without this protein, and reducing VIPP1 levels leads to slower growth and more pale plants with reduced ability to photosynthesise. VIPP1 appears to be necessary for the formation of basic thylakoid membranes, but not for the assembly of protein complexes from the thylakoid membrane. It is preserved in all thylakoid-containing organisms, including cyanobacteria, green algae, such as Chlamydomonas, and higher plants, such as Arabidopsis thaliana .

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Isolation and fractionation

Thylakoids can be purified from plant cells using a combination of differential centrifugation and gradients. Disturbance of isolated thylakoids, for example by mechanical shear, releases the lumenal fraction. The peripheral and integral membrane fractions can be extracted from the remaining fraction of the membrane. Treatment with sodium carbonate (Na ₂ CO ₃ ) releases peripheral membrane proteins, while treatment with detergents and organic solvents dissolves integral membrane proteins.

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Protein

Thylakoids contain many integral and peripheral membrane proteins, as well as lumenal proteins. Recent proteomic research of the thylakoid fraction has provided more details about the protein composition of thylakoids. These data have been summarized in several plastid protein databases available online.

According to this study, thylakoid proteoma consists of at least 335 different proteins. Of these, 89 are in lumen, 116 are integral membrane proteins, 62 are peripheral proteins on the stromal side, and 68 are peripheral proteins on the lumenal side. Additional low abundance lumenal proteins can be predicted through computational methods. Of the known thylakoid proteins with a known function, 42% are involved in photosynthesis. The next largest functional group includes proteins involved in protein targeting, processing and folding by 11%, oxidative stress response (9%) and translation (8%).

Integral membrane proteins

The thylakoid membrane contains an integral membrane protein that plays an important role in harvesting light and photosynthetic reactions depending on light. There are four major protein complexes in the thylakoid membrane:

• Photosystem I and II
• Cytochrome b6f complex
• ATP synthase

Photosystem III is mostly located in the grana thylakoids, whereas photosystem I and ATP synthase are mostly located in the stroma thylakoids and the outer layers of grana. The cytochrome b6f is distributed evenly throughout the thylakoid membrane. Due to the separate location of the two photo systems in the thylakoid membrane system, cellular electron carriers are required to resend the electrons between them. These carriers are plastoquinone and plastocyanin. Plastoquinone transports electrons from Photosystem II to the cytochrome b6f complex, whereas plastocyanin carries electrons from the cytochrome b6f complex to photosystem I.

Together, these proteins utilize light energy to drive the electron transport chain that produces a potency of kemiosmotik across the thylakoid membrane and NADPH, the product of the terminal redox reaction. ATP synthase uses a potency of chemothermotic to make ATP during photophosphorylation.

Photo System

This shooting system is a light-driven redox center, each comprising an antenna complex that uses chlorophyll and photosynthetic pigment accessories such as carotenoids and phycobiliproteins to harvest light at various wavelengths. Each antenna complex has between 250 and 400 pigment molecules and its absorbed energy is diverted by the transfer of resonance energy to a special chlorophyll a at the reaction center of each photo system. When one of the two chlorophylls of molecules at the reaction center absorbs energy, the electrons are excited and transferred to the electron acceptor molecule. Photosystem I contains a pair of chlophophyll molecules a , designated P700, at its reaction center which absorbs 700 nm light maximally. Photosystem II contains the best 680 nm of P680 chlorophyll (note that this wavelength corresponds to dark red - see the visible spectrum). P is short for pigment and the sum is the specific absorption peak in nanometers for chlorophyll molecules in each reaction center.

Cytochrome b6f complex

The cytochrome b6f complex is part of the thylakoid electron transport chain and electron transfer pairs to the pumping of protons into the thylakoid lumen. It is energetically located between two photo systems and transferring electrons from the II-plastoquinone photosystem to plastocyanin-photosystem I.

ATP synthase

The tilakoid ATP synthase is a CF1FO-ATP synthase similar to mitochondrial ATPase. It is integrated into the thylakoid membrane with the CF1 section attached to the stroma. Thus, ATP synthesis occurs on the stromal side of the thylakoids where ATP is required for light-free photosynthetic reactions.

Lumen Protein

Plastocyanin electron transport proteins are present in the lumen and electron shuttles of the b6f cytochrome protein complex to photosystem I. While the plastoquinones are fat soluble and therefore move in the thylakoid membrane, plastocyanine travels through the thylakoid lumen.

The thylakoid lumen is also a site of water oxidation by an evolving oxygen complex associated with the lumenal side of Photosystem II.

Lumenal protein can be predicted computationally based on its targeting signal. In Arabidopsis, of the lumenal protein predicted to have the Tat signal, the largest group with known function is 19% involved in protein processing (proteolysis and folding), 18% in photosynthesis, 11% in metabolism, and 7% redox and defense carriers..

protein expression

Chloroplasts have their own genome, which encodes a number of thylakoid proteins. However, during the plastid evolution of their cyanobacterial endosymbiosis ancestors, extensive gene transfer from the chloroplast genome to the cell nucleus occurs. This results in four major protein thylakoid complexes that are partially encoded by the chloroplast genome and partly by the nuclear genome. Plants have developed several mechanisms to co-manage the expression of various subunits that are encoded in two different organelles to ensure proper stoichiometry and assembly of this protein complex. For example, the transcription of the core genes that encode the parts of the photosynthesis apparatus is governed by light. Biogenesis, stability and replacement of thylakoid protein complexes is governed by phosphorylation through a redox-sensitive kinase in the thylakoid membrane. The degree of protein translation encoded by chloroplasts is controlled by the presence or absence of assembly partners (control by epistasy synthesis). This mechanism involves negative feedback through the binding of excess protein to the 5 'untranslated region of chloroplast mRNA. Chloroplasts also need to balance the ratios of photosystem I and II for electron transfer chains. The redox state of the electron carrier plastoquinone in the thylakoid membrane directly affects the transcription of the chloroplast gene that encodes the protein from the reaction center of the photosystem, thus counteracting the imbalance in the electron transfer chain.

Preding protein to thylakoids

Thylakooid proteins are targeted to their destination through signal peptides and prokaryotic type secretion pathways within the chloroplast. Most thylakoid proteins encoded by plant nuclear genomes require two targeting signals for proper localization: An N-terminal chloroplast targeting peptides (shown in yellow in the figure), followed by a targeted peptide (indicated by blue) tilakoid. Proteins are imported via translocon from the outer and inner membrane (Toc and Tic) complexes. After entering the chloroplast, the first targeting peptide is cleaved by protease protease import proteins. This unmasks the second targeting signal and the protein is exported from the stroma to the thylakoid in the second targeting step. This second step requires action of the protein translocation component of the thylakoids and is dependent on energy. Proteins are fed into the membrane via an SRP-dependent path (1), a Tat-dependent path (2), or spontaneously via their transmembrane domain (not shown in the figure). Lumenal proteins are exported across the thylakoid membrane to the lumen by a Tat-dependent (2) or Sec-dependent (3) pathway and are released by the division of the thylakoid targeting signal. Different paths utilize different signals and energy sources. The Sec (secretory) pathways require ATP as an energy source and consist of SecA, which binds imported proteins and Sec membrane complexes to transport proteins throughout. Proteins with twin arginine motifs in their thylakoid signal peptides are flowed through the Tat (translocated twin arginine translocation) path, which requires a membrane-bound Tat compound and a pH gradient as an energy source. Several other proteins are fed into the membrane through the SRP (signal recognition particle) pathway. The SRP chloroplasts can interact with the target protein either post-translated or co-translated, thereby transporting imported proteins as well as those translated in chloroplasts. SRP path requires GTP and pH gradient as energy source. Some transmembrane proteins can also be spontaneously inserted into the membrane from the stromal side without any energy requirement.

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Function

Tylakoids are the sites of photosynthetic reactions that depend on light. These include the oxidation of light-driven water and the evolution of oxygen, the pumping of protons across the thylakoid membrane combined with the electron transport chain of the photosystem and cytochrome complex, and the synthesis of ATP by ATP synthase that utilizes the resulting proton gradient.

Photolysis water

The first step in photosynthesis is the reduction caused by light (separation) of water to provide electrons for the electron transport chain of photosynthesis as well as protons for the formation of proton gradients. The water-solving reaction occurs on the lumenal side of the thylakoid membrane and is driven by the light energy captured by the photo system. It is interesting to note that this water oxidation easily produces an O ₂ waste product which is essential for cellular respiration. The molecular oxygen formed by the reaction is released into the atmosphere.

Electronic transport chain

Two different variations of electron transport are used during photosynthesis:

• Non-cyclic electron transport or Non-cyclic phosphorylation photos produces NADPH H and ATP.
• Cyclic electron transport or Phytochemical phosphorylation only generates ATP.

Non-cyclical varieties involve the participation of both photo systems, whereas cyclic electron currents depend on only photosystem I.

• Photosystem I uses light energy to reduce NADP to NADPH H , and is active in non-cyclic and cyclic electron transport. In cyclic mode, the energy electrons are passed to the chain that eventually returns it (in the ground state) to the chlorophyll that gives it energy.
• Photosystem II uses light energy to oxidize water molecules, produces electrons (e ^- ), protons (H ), and molecular oxygen ( O ₂ ), and is only active in non-local transport. The electrons in this system are not preserved, but rather continue to enter from 2 H oxidized ₂ O (O ₂ 4 4 e ^{- ) and came out with NADP when it was finally reduced to NADPH.}

Chemiosmosis

The main function of the thylakoid membrane and its integral photo system is the formation of the potency of the chemiosmotic. Carriers in the electron transport chain use a portion of the electron energy to actively transport protons from the stroma to the lumen. During photosynthesis, the lumen becomes acidic, as low as pH 4, compared to pH 8 in the stroma. This represents a 10,000-fold concentration gradient for protons across the thylakoid membrane.

Source of proton gradient

The protons in lumen come from three main sources.

• Photolisis by Photosystem II oxidizes water to oxygen, protons and electrons in the lumen.
• The transfer of electrons from photosystem II to plastoquinone during non-cyclic electron transport consumes two protons from the stroma. It is released in the lumen when the reduced plastoquinol is oxidized by the cytochrome b6f protein complex on the lumen side of the thylakoid membrane. From the plastoquinone pool, the electrons pass through the cytochrome b6f complex. This integral membrane assembly resembles the cytochrome bc1.
• Reduction of plastoquinone by ferredoxin during cyclic electron transport also transfers two protons from the stroma to the lumen.

The proton gradient is also caused by the consumption of protons in the stroma to make NADPH from NADP in the NADP reductase.

ATP generation

The molecular mechanism of ATP (Adenosine triphosphate) generation in chloroplasts is similar to that in mitochondria and takes the required energy from proton motive forces (PMF). However, chloroplasts rely more heavily on the chemical potential of PMF to generate the potential energy required for ATP synthesis. The PMF is the sum of the proton chemical potentials (given by the proton concentration gradient) and the transmembrane power potential (given by the separation of charge across the membrane). Compared with inner mitochondrial membranes, which have significantly higher membrane potentials due to charge separation, the thylakoid membranes do not have a gradient charge. To compensate for this, the 10,000-fold proton gradient across the thylakoid membrane is much higher than the 10-fold gradient across the inner membrane of the mitochondria. The resulting kemiosmotic potential between lumen and stroma is high enough to induce ATP synthesis using ATP synthase. When the proton goes back down the gradient through the channel in the ATP synthase, ADP P _i is combined into ATP. In this way, light dependent reactions are coupled to ATP synthesis via a proton gradient.

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The thylakoid membrane in cyanobacteria

Cyanobacteria are the prokaryotes of photosynthesis with very different membrane systems. Cyanobacteria have an internal system of thylakoid membranes in which the electron transfer chain is fully functioning photosynthesis and respiration is present. The presence of different membrane systems lends these cells a unique complexity among bacteria. Cyanobacteria must be able to rearrange the membrane, synthesize new membrane membranes, and target the protein correctly to the correct membrane system. The outer membranes, plasma membranes, and thylakoid membranes each have a special role in cyanobacterial cells. Understanding the organization, functionality, protein composition and dynamics of membrane systems remains a major challenge in the biology of cyanobacterial cells.

The thylakoid membrane of the cyanobacteria is not distinguished into granum and stromal regions as observed in plants. They form a pile of parallel sheets close to the cytoplasmic membrane with low packing density. The relatively large distance between the thylakoids provides a space for external light-eating antennas, phycobilisomes. These macro structures, as in the case of higher plants, exhibit some flexibility during changes in the physicochemical environment.

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See also

• Arthur Meyer (botanist)
• AndrÃÆ' Â © Jagendorf
• Chemiosmosis
• Electrochemical gradient
• Endosymbiosis
• Evolution of oxygen
• Photosynthesis

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References

Textbook sources

• Heller, H. Craig; Orians, Gordan H.; Purves, William K. & amp; Sadava, David (2004). LIFE: The Science of Biology (7th ed.). Sinauer Associates, Inc. ISBNÃ, 0-7167-9856-5.
• Raven, Peter H.; Ray F. Evert; Susan E. Eichhorn (2005). Plant Biology (7th ed.). New York: W.H. Freeman and Corporate Publisher. pp. 115-127. ISBN 0-7167-1007-2.
• Herrero A and Flores E (editor). (2008). The Cyanobacteria: Molecular Biology, Genomics and Evolution (1st ed.). Caister Academic Press. ISBN 978-1-904455-15-8.

Source of the article : Wikipedia