Thylakoid
Cell biology | |
---|---|
Chloroplast | |
Thylakoids are membrane-bound compartments inside
In thylakoid membranes, chlorophyll pigments are found in packets called quantasomes. Each quantasome contains 230 to 250 chlorophyll molecules.
Etymology
The word Thylakoid comes from the Greek word thylakos or θύλακος, meaning "sac" or "pouch".[1] Thus, thylakoid means "sac-like" or "pouch-like".
Structure
Thylakoids are membrane-bound structures embedded in the chloroplast stroma. A stack of thylakoids is called a granum and resembles a stack of coins.
Membrane
The thylakoid membrane is the site of the
Lumen
The thylakoid
Granum and stroma lamellae
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 intergranal 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. A recent electron tomography study of the thylakoid membranes has shown that the stroma lamellae are organized in wide sheets perpendicular to the grana stack axis and form multiple right-handed helical surfaces at the granal interface.[2] Left-handed helical surfaces consolidate between the right-handed helices and sheets. This complex network of alternating helical membrane surfaces of different radii and pitch was shown to minimize the surface and bending energies of the membranes.[2] This new model, the most extensive one generated to date, revealed that features from two, seemingly contradictory, older models[10][11] coexist in the structure. Notably, similar arrangements of helical elements of alternating handedness, often referred to as "parking garage" structures, were proposed to be present in the endoplasmic reticulum[12] and in ultradense nuclear matter.[13][14][15] This structural organization may constitute a fundamental geometry for connecting between densely packed layers or sheets.[2]
Formation
Chloroplasts develop from
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.[16] It is conserved in all organisms containing thylakoids, including cyanobacteria,[17] green algae, such as Chlamydomonas,[18] and higher plants, such as Arabidopsis thaliana.[19]
Isolation and fractionation
Thylakoids can be purified from plant cells using a combination of differential and gradient
Proteins
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.[21] These data have been summarized in several plastid protein databases that are available online.[22][23]
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.[20][24] 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%).[22]
Integral membrane proteins
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:
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.
Together, these proteins make use of light energy to drive
Photosystems
These photosystems are light-driven redox centers, each consisting of an
Cytochrome b6f complex
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.
ATP synthase
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 the stroma. Thus, ATP synthesis occurs on the stromal side of the thylakoids where the ATP is needed for the
Lumen proteins
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.
The lumen of the thylakoids is also the site of water oxidation by the
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.[20]
Protein expression
Chloroplasts have their own
Protein targeting to the thylakoids
Thylakoid proteins are targeted to their destination via
Function
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 complex, and ATP synthesis by the ATP synthase utilizing the generated proton gradient.
Water photolysis
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. This oxidation 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.
Electron transport chains
Two different variations of electron transport are used during photosynthesis:
- Noncyclic electron transport or non-cyclic photophosphorylation produces NADPH + H+ and ATP.
- Cyclic electron transport or cyclic photophosphorylation produces only ATP.
The noncyclic variety involves the participation of both photosystems, while the cyclic electron flow is dependent on only photosystem I.
- 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.
Chemiosmosis
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
Source of proton gradient
The protons in the lumen come from three primary sources.
- 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 proton gradient is also caused by the consumption of protons in the stroma to make NADPH from NADP+ at the NADP reductase.
ATP generation
The molecular mechanism of ATP (Adenosine triphosphate) generation in chloroplasts is similar to that in
Thylakoid membranes in cyanobacteria
In contrast to the thylakoid network of higher plants, which is differentiated into grana and stroma lamellae, the thylakoids in cyanobacteria are organized into multiple concentric shells that split and fuse to parallel layers forming a highly connected network. This results in a continuous network that encloses a single lumen (as in higher‐plant chloroplasts) and allows water‐soluble and lipid‐soluble molecules to diffuse through the entire membrane network. Moreover, perforations are often observed within the parallel thylakoid sheets. These gaps in the membrane allow for the traffic of particles of different sizes throughout the cell, including ribosomes, glycogen granules, and lipid bodies.
See also
- Arthur Meyer (botanist)
- André Jagendorf
- Chemiosmosis
- Electrochemical gradient
- Endosymbiosis
- Oxygen evolution
- Photosynthesis
References
- Perseus Project
- ^ PMID 31611387.
- ^ "Photosynthesis" McGraw Hill Encyclopedia of Science and Technology, 10th ed. 2007. Vol. 13 p. 469
- S2CID 27225926.
- Encyclopædia Britannica 2006 Ultimate Reference Suite DVD9 Apr. 2008
- S2CID 6076741.
- S2CID 360223.
- PMID 3428918.
- PMID 16603410.
- PMID 16055630.
- PMID 18952780.
- PMID 23870120.
- S2CID 36462725.
- S2CID 12021024.
- S2CID 28272522.
- PMID 17346982.
- PMID 11274448.
- PMID 17355436.
- PMID 11274447.
- ^ PMID 11826309.
- PMID 15707834.
- ^
- PMID 16418230. – Plastid Protein Database
- PMID 10715320.
- PMID 10066592.
- PMID 11498001.
- PMID 16339851.
- PMID 11127990.
- ^ PMID 16386331.
- PMID 5220864.
- ISBN 978-1-904455-15-8.
- PMID 17304210.
- .
- PMID 21473741.
Textbook sources
- Heller, H. Craig; Orians, Gordan H.; Purves, William K. & Sadava, David (2004). LIFE: The Science of Biology (7th ed.). Sinauer Associates, Inc. ISBN 978-0-7167-9856-9.
- 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. ISBN 978-0-7167-1007-3.
- Herrero, Antonia; Flores, Enrique, eds. (2008). The Cyanobacteria: Molecular Biology, Genomics and Evolution (1st ed.). Caister Academic Press. ISBN 978-1-904455-15-8.