Chloroplast membrane: Difference between revisions
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[[Chloroplast]]s contain several important [[biological membrane|membranes]], vital for their function. Like [[mitochondria]], chloroplasts have a double-membrane envelope, called the '''chloroplast envelope''', but unlike [[mitochondria]], chloroplasts also have internal membrane structures called '''[[thylakoid]]s'''. Furthermore, one or two additional membranes may enclose chloroplasts in organisms that underwent [[secondary endosymbiosis]], such as the [[euglenid]]s and [[chlorarachniophyte]]s.<ref>Kim, E., and Archibald, J. M. (2009) "Diversity and Evolution of Plastids and Their Genomes". In ''The Chloroplast'', Anna Stina Sandelius and Henrik Aronsson (eds.), 1–39. Plant Cell Monographs 13. Springer Berlin Heidelberg. {{doi|10.1007/978-3-540-68696-5_1}} {{ISBN|978-3-540-68696-5}}</ref> |
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The chloroplasts come via [[endosymbiosis]] by engulfment of a photosynthetic cyanobacterium by the eukaryotic, already mitochondriate cell.<ref>{{cite journal|last1=Ochoa de Alda|first1=Jesús A. G.|last2=Esteban|first2=Rocío|last3=Diago|first3=María Luz|last4=Houmard|first4=Jean|title=The plastid ancestor originated among one of the major cyanobacterial lineages|journal=Nature Communications|date=15 September 2014|volume=5|pages=4937|doi=10.1038/ncomms5937|pmid=25222494|bibcode=2014NatCo...5.4937O |doi-access=free}}</ref> Over millions of years the endosymbiotic cyanobacterium evolved structurally and functionally, retaining its own DNA and the ability to divide by binary fission (not mitotically) but giving up its autonomy by the transfer of some of its genes to the nuclear genome. |
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[[File:Chloroplast diagram.svg|thumb|300px|The simplified internal structure of a chloroplast]] |
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'''[[Chloroplast]]s''' contain several important [[biological membrane|membranes]], vital for their function. Like [[mitochondria]], chloroplasts have a double-membrane envelope, called the '''chloroplast envelope'''. Each membrane is a [[phospholipid bilayer]], between 6 and 8 [[nanometre|nm]] thick, and the two are separated by a gap of 10-20 nm, called the [[intermembrane space]]. The outer membrane is permeable to most [[ions]] and [[metabolite]]s, but the [[inner membrane]] is highly specialised with [[transport protein]]s. Carbohydrates are transported across the outer membrane by a [[triose phosphate translocator]]. One or two additional membranes may enclose chloroplasts in [[algae]]. |
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==Envelope membranes== |
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The origin of chloroplasts is now largely accepted by the botany community as occurring via [[endosymbiosis]] on an ancestral basis with the engulfment of photosynthetic bacterium within the eukaryotic cell. Over millions of years the endosymbiotic cyanobacterium evolved structurally and functionally, retaining its own DNA and the ability to divide by binary fission (not mitotically) but giving up its autonomy by the transfer of some of its genes to the nuclear genome. |
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Each of the envelope membranes is a [[lipid bilayer]] that is between 6 and 8 [[nanometre|nm]] thick. The lipid composition of the outer membrane has been found to be 48% [[phospholipid]]s, 46% [[galactolipid]]s and 7% [[sulfolipid]]s, while the inner membrane has been found to contain 16% [[phospholipid]]s, 79% [[galactolipid]]s and 5% [[sulfolipid]]s in spinach chloroplasts.<ref name=Block1983>{{cite journal|last=Block|first=MA|author2=Dorne, AJ |author3=Joyard, J |author4= Douce, R |title=Preparation and characterization of membrane fractions enriched in outer and inner envelope membranes from spinach chloroplasts. II. Biochemical characterization|journal=The Journal of Biological Chemistry|date=Nov 10, 1983|volume=258|issue=21|pages=13281–6|doi=10.1016/S0021-9258(17)44113-5 |pmid=6630230|doi-access=free }}</ref> |
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The outer membrane is permeable to most [[ions]] and [[metabolite]]s, but the inner membrane of the chloroplast is highly specialised with [[transport protein]]s.<ref>{{cite journal|last=Heldt|first=HW|author2=Sauer, F |title=The inner membrane of the chloroplast envelope as the site of specific metabolite transport.|journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics|date=Apr 6, 1971|volume=234|issue=1|pages=83–91|pmid=5560365|doi=10.1016/0005-2728(71)90133-2}}</ref><ref>{{cite journal|last=Inoue|first=Kentaro|title=The Chloroplast Outer Envelope Membrane: The Edge of Light and Excitement|journal=Journal of Integrative Plant Biology|date=1 August 2007|volume=49|issue=8|pages=1100–1111|doi=10.1111/j.1672-9072.2007.00543.x|doi-access=free}}</ref> For example, carbohydrates are transported across the inner envelope membrane by a [[triose phosphate translocator]].<ref>{{cite journal|last1=Walters|first1=R. G.|title=A Mutant of Arabidopsis Lacking the Triose-Phosphate/Phosphate Translocator Reveals Metabolic Regulation of Starch Breakdown in the Light|journal=Plant Physiology|year=2004|volume=135|issue=2|pages=891–906|doi=10.1104/pp.104.040469|pmid=15173568|first2=DG|last3=Horton|first3=P|last4=Kruger|first4=NJ|pmc=514124|last2=Ibrahim}}</ref> The two envelope membranes are separated by a gap of 10–20 nm, called the [[intermembrane space]]. |
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==Internal parts== |
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==Thylakoid membrane== |
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===Functions of thylakoids=== |
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Within the envelope membranes, in the region called the [[stroma (fluid)|stroma]], there is a system of interconnecting flattened membrane compartments, called the '''[[thylakoid]]s'''. The '''thylakoid membrane''' is quite similar in lipid composition to the inner envelope membrane, containing 78% [[galactolipid]]s, 15.5% [[phospholipid]]s and 6.5% [[sulfolipid]]s in spinach chloroplasts.<ref name=Block1983 /> The thylakoid membrane encloses a single, continuous aqueous compartment called the '''[[thylakoid lumen]]'''.<ref>{{cite journal|last=Mustárdy|first=L|author2=Garab, G |title=Granum revisited. A three-dimensional model—where things fall into place|journal=Trends in Plant Science|date=March 2003|volume=8|issue=3|pages=117–22|pmid=12663221|doi=10.1016/S1360-1385(03)00015-3}}</ref> |
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| ⚫ | The membranes of the thylakoid contain photosystems I and II which harvest solar energy to excite electrons which travel down the [[electron transport chain]]. This exergonic fall in potential energy along the way is used to draw (not pump!) H+ ions from the lumen of the thylakoid into the cytosol of a [[cyanobacterium]] or the stroma of a chloroplast. A steep H+ gradient is formed, which allows [[chemiosmosis]] to occur, where the thylakoid, |
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| ⚫ | These are the sites of light absorption and [[Adenosine triphosphate|ATP]] synthesis, and contain many proteins, including those involved in the [[electron transport chain]]. Photosynthetic pigments such as chlorophylls a,b,c and some others, e.g., xanthophylls, carotenoids, phycobilins are also embedded within the granum membrane. With exception of chlorophyll a, all the other associated pigments are "accessory" and transfer energy to the reaction centers of Photosytems I and II. |
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| ⚫ | Experiments have shown that the pH within the stroma is about 7.8, while that of the lumen of the thylakoid is 5. This corresponds to a six-hundredfold difference in concentration of H+ ions. The H+ ions pass down through the ATP-synthase catalytic gate. This chemiosmotic phenomenon occurs in mitochondria. |
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| ⚫ | The membranes of the thylakoid contain photosystems I and II which harvest solar energy to excite electrons which travel down the [[electron transport chain]]. This exergonic fall in potential energy along the way is used to draw (not pump!) H<sup>+</sup> ions from the lumen of the thylakoid into the cytosol of a [[cyanobacterium]] or the stroma of a chloroplast. A steep H<sup>+</sup> gradient is formed, which allows [[chemiosmosis]] to occur, where the thylakoid, transmembrane ATP-synthase serves a dual function as a "gate" or channel for H<sup>+</sup> ions and a catalytic site for the formation of ATP from ADP + a PO<sub>4</sub><sup>3−</sup> ion. |
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| ⚫ | Experiments have shown that the pH within the stroma is about 7.8, while that of the lumen of the thylakoid is 5. This corresponds to a six-hundredfold difference in concentration of H<sup>+</sup> ions. The H<sup>+</sup> ions pass down through the ATP-synthase catalytic gate. This chemiosmotic phenomenon also occurs in mitochondria. |
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==References== |
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{{Reflist}} |
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==See also== |
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*[[TIC/TOC complex]] |
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{{DEFAULTSORT:Chloroplast Membrane}} |
{{DEFAULTSORT:Chloroplast Membrane}} |
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[[Category:Membrane biology]] |
[[Category:Membrane biology]] |
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{{Botany-stub}} |
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Latest revision as of 03:07, 28 November 2023
| Cell biology | |
|---|---|
| Chloroplast | |
 Components of a typical chloroplast
1 Granum
4 Stromal thylakoid |
Chloroplasts contain several important membranes, vital for their function. Like mitochondria, chloroplasts have a double-membrane envelope, called the chloroplast envelope, but unlike mitochondria, chloroplasts also have internal membrane structures called thylakoids. Furthermore, one or two additional membranes may enclose chloroplasts in organisms that underwent secondary endosymbiosis, such as the euglenids and chlorarachniophytes.[1]
The chloroplasts come via endosymbiosis by engulfment of a photosynthetic cyanobacterium by the eukaryotic, already mitochondriate cell.[2] Over millions of years the endosymbiotic cyanobacterium evolved structurally and functionally, retaining its own DNA and the ability to divide by binary fission (not mitotically) but giving up its autonomy by the transfer of some of its genes to the nuclear genome.
Envelope membranes
[edit]Each of the envelope membranes is a lipid bilayer that is between 6 and 8 nm thick. The lipid composition of the outer membrane has been found to be 48% phospholipids, 46% galactolipids and 7% sulfolipids, while the inner membrane has been found to contain 16% phospholipids, 79% galactolipids and 5% sulfolipids in spinach chloroplasts.[3]
The outer membrane is permeable to most ions and metabolites, but the inner membrane of the chloroplast is highly specialised with transport proteins.[4][5] For example, carbohydrates are transported across the inner envelope membrane by a triose phosphate translocator.[6] The two envelope membranes are separated by a gap of 10–20 nm, called the intermembrane space.
Thylakoid membrane
[edit]Within the envelope membranes, in the region called the stroma, there is a system of interconnecting flattened membrane compartments, called the thylakoids. The thylakoid membrane is quite similar in lipid composition to the inner envelope membrane, containing 78% galactolipids, 15.5% phospholipids and 6.5% sulfolipids in spinach chloroplasts.[3] The thylakoid membrane encloses a single, continuous aqueous compartment called the thylakoid lumen.[7]
These are the sites of light absorption and ATP synthesis, and contain many proteins, including those involved in the electron transport chain. Photosynthetic pigments such as chlorophylls a,b,c and some others, e.g., xanthophylls, carotenoids, phycobilins are also embedded within the granum membrane. With exception of chlorophyll a, all the other associated pigments are "accessory" and transfer energy to the reaction centers of Photosytems I and II.
The membranes of the thylakoid contain photosystems I and II which harvest solar energy to excite electrons which travel down the electron transport chain. This exergonic fall in potential energy along the way is used to draw (not pump!) H+ ions from the lumen of the thylakoid into the cytosol of a cyanobacterium or the stroma of a chloroplast. A steep H+ gradient is formed, which allows chemiosmosis to occur, where the thylakoid, transmembrane ATP-synthase serves a dual function as a "gate" or channel for H+ ions and a catalytic site for the formation of ATP from ADP + a PO43− ion.
Experiments have shown that the pH within the stroma is about 7.8, while that of the lumen of the thylakoid is 5. This corresponds to a six-hundredfold difference in concentration of H+ ions. The H+ ions pass down through the ATP-synthase catalytic gate. This chemiosmotic phenomenon also occurs in mitochondria.
References
[edit]- ^ Kim, E., and Archibald, J. M. (2009) "Diversity and Evolution of Plastids and Their Genomes". In The Chloroplast, Anna Stina Sandelius and Henrik Aronsson (eds.), 1–39. Plant Cell Monographs 13. Springer Berlin Heidelberg. doi:10.1007/978-3-540-68696-5_1 ISBN 978-3-540-68696-5
- ^ Ochoa de Alda, Jesús A. G.; Esteban, Rocío; Diago, María Luz; Houmard, Jean (15 September 2014). "The plastid ancestor originated among one of the major cyanobacterial lineages". Nature Communications. 5: 4937. Bibcode:2014NatCo...5.4937O. doi:10.1038/ncomms5937. PMID 25222494.
- ^ a b Block, MA; Dorne, AJ; Joyard, J; Douce, R (Nov 10, 1983). "Preparation and characterization of membrane fractions enriched in outer and inner envelope membranes from spinach chloroplasts. II. Biochemical characterization". The Journal of Biological Chemistry. 258 (21): 13281–6. doi:10.1016/S0021-9258(17)44113-5. PMID 6630230.
- ^ Heldt, HW; Sauer, F (Apr 6, 1971). "The inner membrane of the chloroplast envelope as the site of specific metabolite transport". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 234 (1): 83–91. doi:10.1016/0005-2728(71)90133-2. PMID 5560365.
- ^ Inoue, Kentaro (1 August 2007). "The Chloroplast Outer Envelope Membrane: The Edge of Light and Excitement". Journal of Integrative Plant Biology. 49 (8): 1100–1111. doi:10.1111/j.1672-9072.2007.00543.x.
- ^ Walters, R. G.; Ibrahim, DG; Horton, P; Kruger, NJ (2004). "A Mutant of Arabidopsis Lacking the Triose-Phosphate/Phosphate Translocator Reveals Metabolic Regulation of Starch Breakdown in the Light". Plant Physiology. 135 (2): 891–906. doi:10.1104/pp.104.040469. PMC 514124. PMID 15173568.
- ^ Mustárdy, L; Garab, G (March 2003). "Granum revisited. A three-dimensional model—where things fall into place". Trends in Plant Science. 8 (3): 117–22. doi:10.1016/S1360-1385(03)00015-3. PMID 12663221.