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Thylakoid

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Thylakoid

Cell biology
The chloroplast
Components of a typical chloroplast

1 Granum
2 Chloroplast envelope

2.1 Outer membrane
2.2 Intermembrane space
2.3 Inner membrane

3 Thylakoid   ◄ You are here

3.1 Thylakoid space (lumen)
3.2 Thylakoid membrane

4 Stromal thylakoid
5 Stroma
6 Nucleoid (DNA ring)
7 Ribosome
8 Plastoglobulus
9 Starch granule


Thylakoids (dark green) inside a chloroplast

A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana (singular: granum). Grana are connected by intergranal or stroma thylakoids, which join granum stacks together as a single functional compartment.

Contents

  • Etymology 1
  • Thylakoid structure 2
    • Membrane 2.1
    • Lumen 2.2
    • Granum and stroma lamellae 2.3
  • Thylakoid formation 3
  • Thylakoid isolation and fractionation 4
  • Thylakoid proteins 5
    • Integral membrane proteins 5.1
      • Photosystems 5.1.1
      • Cytochrome b6f complex 5.1.2
      • ATP synthase 5.1.3
    • Thylakoid lumen proteins 5.2
    • Thylakoid protein expression 5.3
    • Protein targeting to the thylakoids 5.4
  • Thylakoid function 6
    • Water photolysis 6.1
    • Electron transport chains 6.2
    • Chemiosmosis 6.3
      • Source of proton gradient 6.3.1
      • ATP generation 6.3.2
  • Thylakoid membranes in cyanobacteria 7
  • See also 8
  • References 9
    • Textbook sources 9.1

Etymology

The word thylakoid come via Latin from Greek thylakos meaning "sac" or "pouch".[1] Thus, thylakoid means "sac-like" or "pouch-like".

Thylakoid structure

Thylakoid structures
TEM image of grana

Thylakoids are membrane-bound structures embedded into the chloroplast stroma. A stack of thylakoids in a granum resembles a stack of coins.

Membrane

The thylakoid membrane is the site of the

  • Heller, H. Craig; Orians, Gordan H.; Purves, William K.; & Sadava, David (2004). LIFE: The Science of Biology (7th ed.). Sinauer Associates, Inc.  
  • Raven, Peter H.; Ray F. Evert; Susan E. Eichhorn (2005). Biology of Plants (7th ed.). New York: W.H. Freeman and Company Publishers. pp. 115–127.  
  • Herrero A and Flores E (editors). (2008). The Cyanobacteria: Molecular Biology, Genomics and Evolution (1st ed.). Caister Academic Press.  

Textbook sources

  1. ^ θύλακος. Scott, Robert; A Greek–English Lexicon at the Perseus Project
  2. ^ "Photosynthesis" McGraw Hill Encyclopedia of Science and Technology, 10th ed. 2007. Vol. 13 p. 469
  3. ^ Sato N (2004). "Roles of the acidic lipids sulfoquinovosyl diacylglycerol and phosphatidylglycerol in photosynthesis: their specificity and evolution". J Plant Res 117 (6): 495–505.  
  4. ^ "photosynthesis."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 9 Apr. 2008
  5. ^ Spraque SG (1987). "Structural and functional organization of galactolipids on thylakoid membrane organization". J Bioenerg Biomembr 19 (6): 691–703.  
  6. ^ YashRoy, R.C. (1990). "Magnetic resonance studies of dynamic organization of lipids in chloroplast membranes" (PDF). Journal of Biosciences 15 (4): 281–288. 
  7. ^ YashRoy, R.C. (1987). "13C NMR studies of lipid fatty-acyl chains of chloroplast membranes". Indian Journal of Biochemistry and Biophysics 24: 177–178. 
  8. ^ Benning C, Xu C, Awai K (2006). "Non-vesicular and vesicular lipid trafficking involving plastids". Curr Opin Plant Biol 9 (3): 241–7.  
  9. ^ Mustárdy, L.; Buttle, K.; Steinbach, G.; Garab, G. (2008). "The Three-Dimensional Network of the Thylakoid Membranes in Plants: Quasihelical Model of the Granum-Stroma Assembly". Plant Cell 20 (10): 2552–2557.  
  10. ^ Elena Aseeva; Friederich Ossenbühl; Claudia Sippel; Won K. Cho; Bernhard Stein; Lutz A. Eichacker; Jörg Meurer; Gerhard Wanner; Peter Westhoff; Jürgen Soll; Ute C. Vothknecht (2007). "Vipp1 is required for basic thylakoid membrane formation but not for the assembly of thylakoid protein complexes". Plant Physiol Biochem 45 (2): 119–28.  
  11. ^ Westphal S, Heins L, Soll J, Vothknecht U (2001). "Vipp1 deletion mutant of Synechocystis: A connection between bacterial phage shock and thylakoid biogenesis?". Proc Natl Acad Sci USA 98 (7): 4243–8.  
  12. ^ Liu C, Willmund F, Golecki J, Cacace S, Markert C, Heß B, Schroda M, Schroda M (2007). "The chloroplast HSP70B-CDJ2-CGE1 chaperones catalyse assembly and disassembly of VIPP1 oligomers in Chlamydomonas". Plant J 50 (2): 265–77.  
  13. ^ Kroll D, Meierhoff K, Bechtold N, Kinoshita M, Westphal S, Vothknecht U, Soll J, Westhoff P (2001). "VIPP1, a nuclear gene of Arabidopsis thaliana essential for thylakoid membrane formation". Proc Natl Acad Sci USA 98 (7): 4238–42.  
  14. ^ a b c Peltier J, Emanuelsson O, Kalume D, Ytterberg J, Friso G, Rudella A, Liberles D, Söderberg L, Roepstorff P, von Heijne G, van Wijk KJ (2002). "Central Functions of the Lumenal and Peripheral Thylakoid Proteome of Arabidopsis Determined by Experimentation and Genome-Wide Prediction". Plant Cell 14 (1): 211–36.  
  15. ^ van Wijk K (2004). "Plastid proteomics". Plant Physiol Biochem 42 (12): 963–77.  
  16. ^ a b Friso G, Giacomelli L, Ytterberg A, Peltier J, Rudella A, Sun Q, Wijk K (2004). "In-Depth Analysis of the Thylakoid Membrane Proteome of Arabidopsis thaliana Chloroplasts: New Proteins, New Functions, and a Plastid Proteome Database". Plant Cell 16 (2): 478–99.  - The Plastid Proteome Database
  17. ^ Kleffmann T, Hirsch-Hoffmann M, Gruissem W, Baginsky S (2006). "plprot: a comprehensive proteome database for different plastid types". Plant Cell Physiol 47 (3): 432–6.   – Plastid Protein Database
  18. ^ Peltier J, Friso G, Kalume D, Roepstorff P, Nilsson F, Adamska I, van Wijk K (2000). "Proteomics of the Chloroplast: Systematic Identification and Targeting Analysis of Lumenal and Peripheral Thylakoid Proteins". Plant Cell 12 (3): 319–41.  
  19. ^ Vener AV, Ohad I, Andersson B (1998). "Protein phosphorylation and redox sensing in chloroplast thylakoids". Curr Opin Plant Biol 1 (3): 217–23.  
  20. ^ Choquet Y, Wostrikoff K, Rimbault B, Zito F, Girard-Bascou J, Drapier D, Wollman F (2001). "Assembly-controlled regulation of chloroplast gene translation". Biochem Soc Trans 29 (Pt 4): 421–6.  
  21. ^ Minai L, Wostrikoff K, Wollman F, Choquet Y (2006). "Chloroplast Biogenesis of Photosystem II Cores Involves a Series of Assembly-Controlled Steps That Regulate Translation". Plant Cell 18 (1): 159–75.  
  22. ^ Allen J, Pfannschmidt T (2000). "Balancing the two photosystems: photosynthetic electron transfer governs transcription of reaction centre genes in chloroplasts". Philos Trans R Soc Lond B Biol Sci 355 (1402): 1351–9.  
  23. ^ Gutensohn M, Fan E, Frielingsdorf S, Hanner P, Hou B, Hust B, Klösgen R (2006). "Toc, Tic, Tat et al.: structure and function of protein transport machineries in chloroplasts". J. Plant Physiol. 163 (3): 333–47.  
  24. ^ Herrero A and Flores E (editor). (2008). The Cyanobacteria: Molecular Biology, Genomics and Evolution (1st ed.). Caister Academic Press.  
  25. ^ van de Meene, AM; Hohmann-Marriott, MF; Vermaas, WF; Roberson, RW (January 2006). "The three-dimensional structure of the cyanobacterium Synechocystis sp. PCC 6803.". Archives of microbiology 184 (5): 259–70.  
  26. ^ Olive, J; Ajlani, G; Astier, C; Recouvreur, M; Vernotte, C. "Ultrastructure and light adaptation of phycobilisome mutants of Synechocystis PCC 6803". Biochimica et Biophysica Acta (BBA) - Bioenergetics 1319 (2-3): 275–282.  
  27. ^ Nagy, G; Posselt, D; Kovács, L; Holm, JK; Szabó, M; Ughy, B; Rosta, L; Peters, J; Timmins, P; Garab, G (Jun 1, 2011). "Reversible membrane reorganizations during photosynthesis in vivo: revealed by small-angle neutron scattering.". The Biochemical journal 436 (2): 225–30.  

References

See also

The thylakoid membranes of the cyanobacteria are not differentiated into granum and stroma regions as observed in plants. They form stacks of parallel sheets close to the cytoplasmic membrane with a low packing density.[25] The relatively large distance between the thylakoids provides space for the external light harvesting antennae, the phycobilisomes.[26] This macrostructure, as in the case of higher plants, shows some flexibility during changes in the physicochemical environment.[27]

[24]

Thylakoids (green) inside a cyanobacterium (Synechocystis)

Thylakoid membranes in cyanobacteria

The molecular mechanism of ATP (Adenosine triphosphate) generation in chloroplasts is similar to that in mitochondria and takes the required energy from the proton motive force (PMF). However, chloroplasts rely more on the chemical potential of the PMF to generate the potential energy required for ATP synthesis. The PMF is the sum of a proton chemical potential (given by the proton concentration gradient) and a transmembrane electrical potential (given by charge separation across the membrane). Compared to the inner membranes of mitochondria, which have a significantly higher membrane potential due to charge separation, thylakoid membranes lack a charge gradient. To compensate for this, the 10,000 fold proton concentration gradient across the thylakoid membrane is much higher compared to a 10 fold gradient across the inner membrane of mitochondria. The resulting chemiosmotic potential between the lumen and stroma is high enough to drive ATP synthesis using the ATP synthase. As the protons travel back down the gradient through channels in ATP synthase, ADP + Pi are combined into ATP. In this manner, the light-dependent reactions are coupled to the synthesis of ATP via the proton gradient.

ATP generation

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

  • Photolysis by photosystem II oxidises 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. These are 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, electrons pass through the cytochrome b6f complex. This integral membrane assembly resembles cytochrome bc1.
  • The reduction of plastoquinone by ferredoxin during cyclic electron transport also transfers two protons from the stroma to the lumen.

The protons in the lumen come from three primary sources.

Source of proton gradient

A major function of the thylakoid membrane and its integral photosystems is the establishment of chemiosmotic potential. The carriers in the electron transport chain use some of the electron's 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.

Chemiosmosis

  • Photosystem I uses light energy to reduce NADP+ to NADPH + H+, and is active in both noncyclic and cyclic electron transport. In cyclic mode, the energized electron is passed down a chain that ultimately returns it (in its base state) to the chlorophyll that energized it.
  • Photosystem II uses light energy to oxidize water molecules, producing electrons (e), protons (H+), and molecular oxygen (O2), and is only active in noncyclic transport. Electrons in this system are not conserved, but are rather continually entering from oxidized 2H2O (O2 + 4 H+ + 4 e) and exiting with NADP+ when it is finally reduced to NADPH.

The noncyclic variety involves the participation of both photosystems, while the cyclic electron flow is dependent on only photosystem I.

  • Noncyclic electron transport or Non-cyclic photophosphorylation produces NADPH + H+ and ATP.
  • Cyclic electron transport or Cyclic photophosphorylation produces only ATP.

Two different variations of electron transport are used during photosynthesis:

Electron transport chains

The first step in photosynthesis is the light-driven reduction (splitting) of water to provide the electrons for the photosynthetic electron transport chains as well as protons for the establishment of a proton gradient. The water-splitting reaction occurs on the lumenal side of the thylakoid membrane and is driven by the light energy captured by the photosystems. It is interesting to note that this reduction of water conveniently produces the waste product O2 that is vital for cellular respiration. The molecular oxygen formed by the reaction is released into the atmosphere.

Water photolysis

The thylakoids are the site of the light-dependent reactions of photosynthesis. These include light-driven water oxidation and oxygen evolution, the pumping of protons across the thylakoid membranes coupled with the electron transport chain of the photosystems and cytochrome b6f complex, and ATP synthesis by the ATP synthase utilizing the generated proton gradient.

Light-dependent reactions of photosynthesis at the thylakoid membrane

Thylakoid function

Thylakoid proteins are targeted to their destination via signal peptides and prokaryotic-type secretory pathways inside the chloroplast. Most thylakoid proteins encoded by a plant's nuclear genome need two targeting signals for proper localization: An N-terminal chloroplast targeting peptide (shown in yellow in the figure), followed by a thylakoid targeting peptide (shown in blue). Proteins are imported through the translocon of outer and inner membrane (Toc and Tic) complexes. After entering the chloroplast, the first targeting peptide is cleaved off by a protease processing imported proteins. This unmasks the second targeting signal and the protein is exported from the stroma into the thylakoid in a second targeting step. This second step requires the action of protein translocation components of the thylakoids and is energy-dependent. Proteins are inserted into the membrane via the SRP-dependent pathway (1), the Tat-dependent pathway (2), or spontaneously via their transmembrane domains (not shown in figure). Lumenal proteins are exported across the thylakoid membrane into the lumen by either the Tat-dependent pathway (2) or the Sec-dependent pathway (3) and released by cleavage from the thylakoid targeting signal. The different pathways utilize different signals and energy sources. The Sec (secretory) pathway requires ATP as energy source and consists of SecA, which binds to the imported protein and a Sec membrane complex to shuttle the protein across. Proteins with a twin arginine motif in their thylakoid signal peptide are shuttled through the Tat (twin arginine translocation) pathway, which requires a membrane-bound Tat complex and the pH gradient as an energy source. Some other proteins are inserted into the membrane via the SRP (signal recognition particle) pathway. The chloroplast SRP can interact with its target proteins either post-translationally or co-translationally, thus transporting imported proteins as well as those that are translated inside the chloroplast. The SRP pathway requires GTP and the pH gradient as energy sources. Some transmembrane proteins may also spontaneously insert into the membrane from the stromal side without energy requirement.[23]

Schematic representation of thylakoid protein targeting pathways.

Protein targeting to the thylakoids

Chloroplasts have their own stoichiometry and assembly of these protein complexes. For example, transcription of nuclear genes encoding parts of the photosynthetic apparatus is regulated by light. Biogenesis, stability and turnover of thylakoid protein complexes are regulated by phosphorylation via redox-sensitive kinases in the thylakoid membranes.[19] The translation rate of chloroplast-encoded proteins is controlled by the presence or absence of assembly partners (control by epistasy of synthesis).[20] This mechanism involves negative feedback through binding of excess protein to the 5' untranslated region of the chloroplast mRNA.[21] Chloroplasts also need to balance the ratios of photosystem I and II for the electron transfer chain. The redox state of the electron carrier plastoquinone in the thylakoid membrane directly affects the transcription of chloroplast genes encoding proteins of the reaction centers of the photosystems, thus counteracting imbalances in the electron transfer chain.[22]

Thylakoid protein expression

Lumenal proteins can be predicted computationally based on their targeting signals. In Arabidopsis, out of the predicted lumenal proteins possessing the Tat signal, the largest groups with known functions are 19% involved in protein processing (proteolysis and folding), 18% in photosynthesis, 11% in metabolism, and 7% redox carriers and defense.[14]

The lumen of the thylakoids is also the site of water oxidation by the oxygen evolving complex associated with the lumenal side of photosystem II.

The electron transport protein plastocyanin is present in the lumen and shuttles electrons from the cytochrome b6f protein complex to photosystem I. While plastoquinones are lipid-soluble and therefore move within the thylakoid membrane, plastocyanin moves through the thylakoid lumen.

Thylakoid lumen proteins

The thylakoid ATP synthase is a CF1FO-ATP synthase similar to the mitochondrial ATPase. It is integrated into the thylakoid membrane with the CF1-part sticking into stroma. Thus, ATP synthesis occurs on the stromal side of the thylakoids where the ATP is needed for the light-independent reactions of photosynthesis.

ATP synthase

The cytochrome b6f complex is part of the thylakoid electron transport chain and couples electron transfer to the pumping of protons into the thylakoid lumen. Energetically, it is situated between the two photosystems and transfers electrons from photosystem II-plastoquinone to plastocyanin-photosystem I.

Cytochrome b6f complex

These photosystems are light-driven redox centers, each consisting of an antenna complex that uses chlorophylls and accessory photosynthetic pigments such as carotenoids and phycobiliproteins to harvest light at a variety of wavelengths. Each antenna complex has between 250 and 400 pigment molecules and the energy they absorb is shuttled by resonance energy transfer to a specialized chlorophyll a at the reaction center of each photosystem. When either of the two chlorophyll a molecules at the reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule. Photosystem I contains a pair of chlorophyll a molecules, designated P700, at its reaction center that maximally absorbs 700 nm light. Photosystem II contains P680 chlorophyll that absorbs 680 nm light best (note that these wavelengths correspond to deep red - see the visible spectrum). The P is short for pigment and the number is the specific absorption peak in nanometers for the chlorophyll molecules in each reaction center.

Photosystems

Together, these proteins make use of light energy to drive electron transport chains that generate a chemiosmotic potential across the thylakoid membrane and NADPH, a product of the terminal redox reaction. The ATP synthase uses the chemiosmotic potential to make ATP during photophosphorylation.

Photosystem II is located mostly 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 complex is distributed evenly throughout thylakoid membranes. Due to the separate location of the two photosystems in the thylakoid membrane system, mobile electron carriers are required to shuttle electrons between them. These carriers are plastoquinone and plastocyanin. Plastoquinone shuttles electrons from photosystem II to the cytochrome b6f complex, whereas plastocyanin carries electrons from the cytochrome b6f complex to photosystem I.

Thylakoid membranes contain integral membrane proteins which play an important role in light harvesting and the light-dependent reactions of photosynthesis. There are four major protein complexes in the thylakoid membrane:

Integral membrane proteins

According to these studies, the thylakoid proteome consists of at least 335 different proteins. Out of these, 89 are in the lumen, 116 are integral membrane proteins, 62 are peripheral proteins on the stroma side, and 68 peripheral proteins on the lumenal side. Additional low-abundance lumenal proteins can be predicted through computational methods.[14][18] Of the thylakoid proteins with known functions, 42% are involved in photosynthesis. The next largest functional groups include proteins involved in protein targeting, processing and folding with 11%, oxidative stress response (9%) and translation (8%).[16]

Thylakoids contain many integral and peripheral membrane proteins, as well as lumenal proteins. Recent proteomics studies of thylakoid fractions have provided further details on the protein composition of the thylakoids.[15] These data have been summarized in several plastid protein databases that are available online.[16][17]

Thylakoid disc with embedded and associated proteins

Thylakoid proteins

Thylakoids can be purified from plant cells using a combination of differential and gradient integral membrane proteins.

Thylakoid isolation and fractionation

Thylakoid formation requires the action of vesicle-inducing protein in plastids 1 (VIPP1). Plants cannot survive without this protein, and reduced VIPP1 levels lead to slower growth and paler plants with reduced ability to photosynthesize. VIPP1 appears to be required for basic thylakoid membrane formation, but not for the assembly of protein complexes of the thylakoid membrane.[10] It is conserved in all organisms containing thylakoids, including cyanobacteria,[11] green algae, such as Chlamydomonas,[12] and higher plants, such as Arabidopsis thaliana.[13]

Chloroplasts develop from proplastids when seedlings emerge from the ground. Thylakoid formation requires light. In the plant embryo and in the absence of light, proplastids develop into etioplasts that contain semicrystalline membrane structures called prolamellar bodies. When exposed to light, these prolamellar bodies develop into thylakoids. This does not happen in seedlings grown in the dark, which undergo etiolation. An underexposure to light can cause the thylakoids to fail. This causes the chloroplasts to fail resulting in the death of the plant.

Thylakoid formation

In higher plants thylakoids are organized into a granum-stroma membrane assembly. A granum (plural grana) is a stack of thylakoid discs. Chloroplasts can have from 10 to 100 grana. Grana are connected by stroma thylakoids, also called intergrana thylakoids or lamellae. Grana thylakoids and stroma thylakoids can be distinguished by their different protein composition. Grana contribute to chloroplasts' large surface area to volume ratio. Different interpretations of electron tomography imaging of thylakoid membranes has resulted in two models for grana structure. Both posit that lamellae intersect grana stacks in parallel sheets, though whether these sheets intersect in planes perpendicular to the grana stack axis, or are arranged in a right-handed helix is debated.[9]

Granum and stroma lamellae

The thylakoid lumen is a continuous aqueous phase enclosed by the thylakoid membrane. It plays an important role for photophosphorylation during photosynthesis. During the light-dependent reaction, protons are pumped across the thylakoid membrane into the lumen making it acidic down to pH 4.

Lumen

[8] and inner membrane of the plastid envelope and transported from the inner membrane to the thylakoids via vesicles.endoplasmic reticulum are synthesized in a complex pathway involving exchange of lipid precursors between the [7] Lipids forming the thylakoid membranes, richest in high-fluidity linolenic acid [6]

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