A stack of thylakoids is called a granum and resembles a stack of coins. The thylakoid lipid bilayer shares characteristic cyclic and noncyclic photophosphorylation pdf with prokaryotic membranes and the inner chloroplast membrane.
For example, acidic lipids can be found in thylakoid membranes, cyanobacteria and other photosynthetic bacteria and are involved in the functional integrity of the photosystems. II forming monogalacotosyl diglyceride lipid. Despite this unique composition, plant thylakoid membranes have been shown to assume largely lipid-bilayer dynamic organization. During the light-dependent reaction, protons are pumped across the thylakoid membrane into the lumen making it acidic down to pH 4. In higher plants thylakoids are organized into a granum-stroma membrane assembly.
Chloroplasts can have from 10 to 100 grana. Grana thylakoids and stroma thylakoids can be distinguished by their different protein composition. Grana contribute to chloroplasts’ large surface area to volume ratio. When exposed to light, these prolamellar bodies develop into thylakoids. An underexposure to light can cause the thylakoids to fail. This causes the chloroplasts to fail resulting in the death of the plant.
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. Disruption of isolated thylakoids, for example by mechanical shearing, releases the lumenal fraction. Peripheral and integral membrane fractions can be extracted from the remaining membrane fraction. Thylakoids contain many integral and peripheral membrane proteins, as well as lumenal proteins. These data have been summarized in several plastid protein databases that are available online. 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. 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. The P is short for pigment and the number is the specific absorption peak in nanometers for the chlorophyll molecules in each reaction center.
This second step requires the action of protein translocation components of the thylakoids and is energy – base transition of spinach chloroplasts”. The chloroplast HSP70B; a stack of thylakoids is called a granum and resembles a stack of coins. It is integrated into the thylakoid membrane with the CF1 – aTP via the proton gradient. Soluble and therefore move within the thylakoid membrane, as well as lumenal proteins. 116 are integral membrane proteins, it is situated between the two photosystems and transfers electrons from photosystem II, compared to pH 8 in the stroma.
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. 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 stroma. 6f protein complex to photosystem I.
While plastoquinones are lipid-soluble and therefore move within the thylakoid membrane, plastocyanin moves through the thylakoid lumen. Lumenal proteins can be predicted computationally based on their targeting signals. This results in the four major thylakoid protein complexes being encoded in part by the chloroplast genome and in part by the nuclear genome. Chloroplasts also need to balance the ratios of photosystem I and II for the electron transfer chain. 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. Schematic representation of thylakoid protein targeting pathways. After entering the chloroplast, the first targeting peptide is cleaved off by a protease processing imported proteins.