Function of Thylakoid

In chloroplasts and cyanobacteria, a thylakoid is a sheet-like membrane-bound structure that hosts light-dependent photosynthetic reactions. It is the location where chlorophyll is found, which is used to absorb light and use it in metabolic operations. The term “thylakoid” comes from the Greek word “thylakoids,” which meaning “pocket” or “sack.” “Thylakoid” signifies “pouch-like” because of the -oid suffix.

Although this term can be used to refer to the section of a thylakoid that joins grana, thylakoids are also known as lamellae.Although the photosynthetic apparatus and light-driven electron transport have been widely investigated, there is still much to understand about the processes that control PET in response to metabolic pathways’ energy requirements and environmental cues.

Thylakoid Structure and Function

All plants and blue-green algae have thylakoids in their chloroplasts. Thylakoids are interconnected disc-like sacs that make up the chloroplast’s internal membrane system. They’re observed in the stroma, suspended.

Thylakoids are organized in grana, which is a stack of them. The photosynthetic pigment chlorophyll, which absorbs sunlight during photosynthesis, is found in the thylakoid membrane.

Thylakoids have the following main characteristics:

• Each thylakoid is a membrane-bound capsule encased in chloroplast stroma.

• Grana is a thylakoid stack that resembles a stack of coins. They are where photosynthesis’ light response takes place.

• Stroma lamellae join the thylakoids of two distinct grana.

• The thylakoid membrane and thylakoid lumen make up each thylakoid.

Role of the Thylakoid in Photosynthesis

Water photolysis, the electron transport chain, and ATP synthesis are all reactions that take place in the thylakoid.

The thylakoid membrane is embedded with photosynthetic pigments (e.g., chlorophyll), making it the site of light-dependent photosynthesis reactions. The grana’s stacked coil form affords the chloroplast a large surface area to volume ratio, which aids photosynthesis efficiency.

During photosynthesis, the thylakoid lumen is used for photophosphorylation. The membrane’s light-dependent processes push protons into the lumen, reducing the pH to 4. The stroma, on the other hand, has a pH of 8.

Water Photolysis

The first stage is water photolysis, which takes place on the thylakoid membrane’s lumen site. Water is reduced or split using light energy. This mechanism creates electron transport chains as well as protons that are pushed into the lumen to create a proton gradient and oxygen. The gas created by this reaction is discharged into the atmosphere, despite the fact that oxygen is necessary for cellular respiration.

Electron Transport Chain

The electrons produced by photolysis are directed to the electron transport chains’ photosystems. An antenna complex in photosystems collects light at multiple wavelengths using chlorophyll and associated pigments. Light is used by Photosystem I to decrease NADP + and generate NADPH and H+. Photosystem II produces molecular oxygen (O2), electrons (e-), and protons (H+) by oxidising water with light. In both systems, electrons convert NADP+ to NADPH.

ATP Synthesis

Both Photosystem I and Photosystem II generate ATP. Thylakoids use an ATP synthase enzyme that is comparable to mitochondrial ATPase to produce ATP. The thylakoid membrane incorporates the enzyme. The CF1 region of the synthase molecule expanded into the stroma, where ATP fuels photosynthetic reactions that are not light dependent.

Proteins involved in protein processing, photosynthesis, metabolism, redox reactions, and defence are found in the thylakoid lumen. The electron transport protein plastocyanin delivers electrons from the cytochrome proteins to Photosystem I. The cytochrome b6f complex is a part of the electron transport chain that connects proton pumping with electron transfer in the thylakoid lumen. Between Photosystem I and Photosystem II is the cytochrome complex.

Thylakoids in Algae and Cyanobacteria

Although thylakoids in plant cells form grana stacks in plants, they may be unstacked in algae.

Cyanobacteria are photosynthetic prokaryotes, unlike algae and plants, which are eukaryotes. They are devoid of chloroplasts. Rather, the entire cell functions as a thylakoid. The cyanobacterium has a cell membrane, an exterior cell wall, and a thylakoid membrane. Bacterial DNA, cytoplasm, and carboxysomes are all contained within this membrane. Photosynthesis and cellular respiration are supported by functioning electron transport chains in the thylakoid membrane. The thylakoid membranes of Cyanobacteria do not generate grana and stroma. Instead, near the cytoplasmic membrane, the membrane creates parallel sheets with enough space between each sheet for phycobilisomes, light-harvesting structures.

Thylakoid membrane

The innermost compartment, or thylakoid lumen, is enclosed by the thylakoid membrane. The chloroplast inner membrane is sometimes joined to the thylakoid membrane. Phospholipids and galactolipids make up the thylakoid membrane. It resembles the inner membrane of chloroplasts and bears some similarities with prokaryotic membranes such as those found in cyanobacteria. Chlorophyll and other photosynthetic pigments are found in the thylakoid membrane.

Many membrane proteins are found in the thylakoid membrane:

. Photosystem I – is primarily

found in grana’s stroma lamellae and outer thylakoids. The reaction centre (chlorophyll a) of the light-harvesting complex exhibits a maximum absorption at 700 nm (P700). Both cyclic and non-cyclic 

Photophosphorylation involves it.

. Photosystem II –  is primarily found in grana thylakoids. The reaction centre (chlorophyll a) of the light-harvesting complex has a maximum absorption wavelength of 680 nm (P680). It is involved in noncyclic photophosphorylation. PS II is linked to a water splitting complex.

. The electron transport chain is made up of the cytochrome B6f complex, which is uniformly distributed.

. ATP Synthase is found predominantly in grana’s stroma lamellae and outer thylakoids. The CF0 subunit is embedded in the thylakoid membrane and forms the transmembrane channel. The CF1 component is found in the stroma and catalyses the synthesis of ATP.

Thylakoid Lumen

The thylakoid lumen is the chloroplast’s innermost watery compartment. A thylakoid membrane surrounds it. It’s involved in ATP generation and phosphorylation, both of which are aided by chemiosmosis. The thylakoid membrane causes a concentration gradient as protons are pushed across the membrane into the lumen.

The water-splitting complex is found on the inner side of PS II’s thylakoid membrane, and it causes the release of proton and oxygen into the thylakoid lumen by splitting water.

Plastocyanin, an electron transport protein that shuttles electrons from the cytochrome B6f complex to PS I, is also found in the thylakoid lumen.

CONCLUSION

The thylakoid lumen is known to provide an environment for oxygen evolution, plastocyanin-mediated electron transport, and photoprotection over a long time. Lumenal proteins have recently been discovered to play a function in a variety of processes, most notably controlling thylakoid formation and the activity and turnover of photosynthetic protein complexes, including photosystem II and NAD(P)H dehydrogenase-like complexes.The bulk of lumenal proteins in Arabidopsis thaliana have yet to be discovered. While the thylakoid lumen proteome contains at least 80 proteins and numerous big protein families, individual members of many of these protein families have quite different roles. This is evidence of evolutionary pressure leading to luminal protein neofunctionalization, stressing the vital significance of the thylakoid lumen in photosynthetic electron transport and, ultimately, plant fitness.