Visualizing structural dynamics of thylakoid membranes
thylakad consists of a granum region (disc) and an intergrana region (stroma lamella). Majority of closed discs would hawe connection with stroma lamellae if the section of the . their advice and criticism in the preparation of the manuscript . Deep inside the chloroplasts of plant cells we find granum, or the stacks tunnels in the lower part of thylakoid stacks called stromal thylakoids. The dynamic aspect of thylakoid membrane structure in relation to the .. P. patens ecotype strain and the extensive technical advice for using the moss; Kaoru . in plants: quasihelical model of the granum-stroma assembly.
However, direct evidence for such protein reorganization has not been visualized in live cells.
How are chloroplasts adapted to their function?
Here we demonstrate structural dynamics of thylakoid membranes by live cell imaging in combination with deconvolution. We observed chlorophyll fluorescence in the antibiotics-induced macrochloroplast in the moss Physcomitrella patens. The three-dimensional reconstruction uncovered the fine thylakoid membrane structure in live cells.
The time-lapse imaging shows that the entire thylakoid membrane network is structurally stable, but the individual thylakoid membrane structure is flexible in vivo. Our observation indicates that grana serve as a framework to maintain structural integrity of the entire thylakoid membrane network.
Both the structural stability and flexibility of thylakoid membranes would be essential for dynamic protein reorganization under fluctuating light environments.
Photosynthetic organisms have developed flexible machinery for effective light energy use 12.
In higher plants, cyclic electron transport is stimulated by the association of chloroplast NADH dehydrogenase-like complex with PSI When PSII is damaged, disassembly of PSII occurs after its migration from the stacked, appressed membranes, or grana, to the single-layer, stroma-exposed membranes, or stroma lamellae, where PSII subunits are replaced 20 Based on these facts, the structure and arrangement of thylakoid membranes have to be flexible for such protein reorganization to be taken place in response to changing light environments.
The structure and arrangement of thylakoid membranes have long been studied since the first observation using light microscopy by Hugo von Mohl in The grana inside chloroplasts are already identified by light microscopy as dense, dot-like structures Introducing electron microscopy has deepened our understanding of the structural complexity of thylakoid membranes, showing the remarkable architecture in which stroma lamellae connect to grana in the helical configuration 232425 Recently, electron tomography has determined the three-dimensional 3D structure of thylakoid membranes in higher plants 2728revealing the distinctive image of the junctional connections between grana and stroma lamellae.
Intriguingly, the junctional slits where stroma lamellae connect grana show significant structural variations 2728reflecting the variability of the membrane structure. Therefore, to demonstrate the dynamic aspect of thylakoid membrane structure in vivo, the visualization by live cell imaging is essential. In this work, we used conventional confocal microscopy to observe chlorophyll Chl fluorescence structures inside chloroplasts.
Previous studies have already shown Chl fluorescence images of chloroplasts in various green algae and higher plants 29 This results in the four major thylakoid protein complexes being encoded in part by the chloroplast genome and in part by the nuclear genome. Plants have developed several mechanisms to co-regulate the expression of the different subunits encoded in the two different organelles to assure the proper stoichiometry and assembly of these protein complexes.
For example, transcription of nuclear genes encoding parts of the photosynthetic apparatus is regulated by light.
botany - Difference between thylakoids and lamellae in a chloroplast? - Biology Stack Exchange
Biogenesis, stability and turnover of thylakoid protein complexes are regulated by phosphorylation via redox-sensitive kinases in the thylakoid membranes. 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. 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 figurefollowed 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 1the Tat-dependent pathway 2or 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.
Some transmembrane proteins may also spontaneously insert into the membrane from the stromal side without energy requirement.
What is the relationship between the granum and the stroma? | Yahoo Answers
These include light-driven water oxidation and oxygen evolutionthe 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. 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.
What is the relationship between the granum and the stroma?
Electron transport chains[ edit ] Two different variations of electron transport are used during photosynthesis: 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.
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.
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 acidicas low as pH 4, compared to pH 8 in the stroma. Source of proton gradient[ edit ] The protons in the lumen come from three primary sources.
Photolysis by photosystem II oxidises water to oxygenprotons 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.