Book : Cell Biology
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Posted by: CHELSEA
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Writer CHELSEA

Thylakoid

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A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Thylakoids are tiny compartments found inside of chloroplasts. Their role is to help absorb sunlight in order for photosynthesis to occur. They contain all of the chlorophyll that the plant has, which in turn allows for the absorption of sunlight. This is why the thylakoid is the site of the light dependent portion of photosynthesis, which is the portion that requires sunlight. Thylakoids are membrane-bound structures embedded in the chloroplast stroma. A stack of thylakoids is called a granum and resembles a stack of coins.

Thylakoid membranes are laterally differentiated into appressed and non-appressed regions called grana and stroma lamellae. Pure stroma lamellae isolated from wild type maize and barley leaves contain photosystem I and its light-harvesting antennae, the cytochromeb6/f complex and coupling factor. Maize stroma lamellae contained only 2% of the total photosystem II polypeptides found in whole thylakoids, and most of the small amount of the photosystem II light-harvesting complex (LHCII) was associated with photosystem I. These results were consistent with the low rates of photosystem II electron transport and low levels of the high potential form of cytochromeb-559. Immune blot assays indicated that about half of the low potential form of cytochromeb-559 in stroma lamellae was antigenically distinct from that derived from the high potential form. The amount of LHCII in stroma lamellae could be increased by exposing leaves to bright white light (state 2) prior to the isolation of stromal lamellae. This LHCII caused a 15% increase in photosystem I antenna size and was different from the LHCII found in grana lamellae, since it lacked a 26 kD polypeptide possibly involved in thylakoid appression. These results demonstrate that the migration of LHCII from appressed to non-appressed lamellae as a result of changes in the relative amounts of energy absorbed by the 2 different photosystems, also occurs in vivo.

The reaction centre core of photosystem I was isolated from barley thylakoids and its molecular weight determined to be 650 kD. Attack by various proteases cleaved it into fragments of less than 5 kD, although the complex was still photoactive. However, the kinetics of photo-oxidation of P700 under light-limiting conditions were slower after proteolysis, indicating less efficient energy transfer. In a barley mutant lacking photosystem I, a chlorophylla species absorbing at 689 nm was lacking, accounting for about 30 molecules out of every 500 in wild type thylakoids.

The thylakoids themselves contain the chlorophyll, but the thylakoid membrane, which is the layer that surrounds the thylakoid, is where the light reactions take place. Embedded in the thylakoid membrane are two photosystems, named photosystem I and photosystem II. In each photosystem, there are different proteins and slightly different chlorophyll pigments that allow for different kinds of light absorption.

Finally, a mutant completely lacking photosystem II activity, viridis−115, has been examined. It contained only 4% of the photosystem II-containing EFS particles found in wild type thylakoids, and lacked a chlorophylla species absorbing at 683 nm. Immune electron microscopy revealed that the α-subunit of cytochromeb-559 and the 33 kD polypeptide of the oxygen evolving complex were correctly located in appressed thylakoids, in spite of the lack of the major photosystem II core polypeptides. A double mutant, lacking both photosystem II and LHCII was found to contain grana, even though its thylakoids lacked the 2 complexes normally associated with maintenance of membrane appression in vivo.



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