Thylakoids,Grana and Lumen - BS 14 Cell Biology
Explain the relationship between granum and stroma Grana are stacks of thylakoids inside a chloroplast; the stroma is the solution that surrounds the. The chloroplasts of most higher plants have regions, called grana, in which the connected to other stacks by simple membranes (lamellae) within the stroma. Photosystem II is located on the grana lamellae. Stroma lamella are the folds of the thylakoid that are between the grana - this is where NADPH.
Granum and stroma lamellae In higher plants thylakoids are organized into a granum-stroma membrane assembly. Agranum plural grana is a stack of thylakoid discs.
Chloroplasts can have from 10 to 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.
Difference Between Grana and Thylakoid
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.
Grana comprise the energy-producing structures in the chloroplasts of plants and algae, and are necessary for photosynthesis. Grana plural of "granum" are stacks of structures called thylakoids, which are little disks of membrane on which the light-dependent reactions of photosynthesis take place.
Stacked into grana, the shape of the thylakoids allow for optimum surface area, maximizing the amount of photosynthesis that can take place. 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.
what is the relationship between Grana and thylakoid? | Yahoo Answers
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.
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.
Electron transport chains[ edit ] Two different variations of electron transport are used during photosynthesis: Cyclic electron transport or Cyclic photophosphorylation produces only ATP.
- Difference Between Grana and Stroma
- What is the relationship between Grana and thylakoid?
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.
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.