Analysis of thin-section images of thylakoid grana membranes within intact . (A) Relationship between apparent thickness of appressed membrane zone and. From an estimate of the mean migration time of PSII from grana thylakoids to . Structure-function relationships in photosynthetic membranes: Challenges and. Plant and Cell Physiology, Volume 57, Issue 6, 1 June , Pages Under light stress, stacked grana turn into unstacked thylakoids with bent.
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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.
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.
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.
Thylakoid - Wikipedia
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.
Difference Between Grana and Thylakoid
The reduction of plastoquinone by ferredoxin during cyclic electron transport also transfers two protons from the stroma to the lumen. ATP generation[ edit ] 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. 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. The resulting chemiosmotic potential between the lumen and stroma is high enough to drive ATP synthesis using the ATP synthase. In this manner, the light-dependent reactions are coupled to the synthesis of ATP via the proton gradient. Cyanobacteria have an internal system of thylakoid membranes where the fully functional electron transfer chains of photosynthesis and respiration reside.
The presence of different membrane systems lends these cells a unique complexity among bacteria. Connecting each granum by stromal thylakoids allow the functioning of all grana as a unit during photosynthesis.
The membranes of thylakoid and stromal thylakoid are responsible for the occurrence of light reaction of photosynthesis. The space between grana and inner membrane of the chloroplast is called stroma. Dark reaction of the photosynthesis occurs in the stroma of chloroplast. A single chloroplast contains 10 to grana. Granum inside the chloroplast is shown in figure 1. Granum in Chloroplast What is Thylakoid Thylakoid is the little, round, flat, pillow-shaped things inside the chloroplast.
Thylakoid is a membrane-bound structure. The space between thylakoid membrane is called thylakoid lumen. The functional parts of the chloroplast are its membrane and the lumen. The light-trapping green pigment, chlorophyll is found in the thylakoid membrane, held by the membrane proteins. Chlorophylls are organized into photosystem 1 and photosystem 2 on the thylakoid membrane. The light energy of the sunlight is converted into electrical energy by chlorophyll.
The electrical energy in the form of high energy electrons is passed through membrane proteins from one to another, providing the power to pump protons from stroma into thylakoid lumen. When these pumped proteins are rushed back into the stroma, energy is released, which is readily used by the enzyme, ATP synthase by synthesizing ATP.