Thylakoid is the site of photochemical or light-dependent reactions of photosynthesis. Chlorophyll present in the thylakoid membrane absorbs energy from the sunlight and is involved in the formation of ATP and NADPH in the light reaction of photosynthesis through electron transport chains. Thus, light energy is converted into chemical energy.
It is also involved in the water oxidation or photolysis of water resulting in the release of oxygen during photosynthesis.
Thylakoids are membranous sacs present in the chloroplast. Chlorophyll is present in the thylakoid membrane. Multiple thylakoids are attached together to form a stack of discs known as grana.
Thylakoids are present in the chloroplasts of all the plants and blue-green algae. Thylakoids are interconnected disc-like sacs of the internal membrane system of the chloroplast. They are found suspended in the stroma.
Thylakoids are arranged in a stack, which is called grana. The thylakoid membrane contains the photosynthetic pigment chlorophyll, which absorbs the sunlight during photosynthesis.
The main characteristics of thylakoids are:
- Each thylakoid is a membrane-bound sac embedded in the stroma of chloroplasts.
- Grana is a stack of thylakoids, which looks like a stack of coins. They are the site of light reaction of photosynthesis.
- Thylakoids of two different grana are connected by stroma lamellae.
- Each thylakoid is made up of thylakoid membrane and thylakoid lumen.
Thylakoid membrane encloses the innermost compartment or thylakoid lumen. The inner membrane of chloroplast is sometimes continuous with the thylakoid membrane. Thylakoid membrane is composed of phospholipids and galactolipids. It is similar to the inner membrane of chloroplast and also shares some characteristics with the prokaryotic membranes such as cyanobacteria. The thylakoid membrane contains chlorophyll and other photosynthetic pigments.
Thylakoid membrane contains many integral membrane proteins:
- Photosystem I – mostly present in the stroma lamellae and outer thylakoids of grana. The light-harvesting complex has the reaction centre (chlorophyll a) with maximum absorption at 700 nm (P700). It takes part in both cyclic and non-cyclic photophosphorylation.
- Photosystem II – mostly present in the grana thylakoids. The light-harvesting complex has the reaction centre (chlorophyll a) with maximum absorption at 680 nm (P680). It takes part in the non-cyclic photophosphorylation. Water splitting complex is associated with PS II.
- Cytochrome B6f complex – evenly distributed and is a part of the electron transport chain.
- ATP Synthase – mostly present in the stroma lamellae and outer thylakoids of grana. The CF0 subunit forms the transmembrane channel and is embedded in the thylakoid membrane. The CF1 subunit is present towards the stroma and catalyses ATP synthesis.
Thylakoid lumen is the innermost aqueous compartment of the chloroplast. It is enclosed by a thylakoid membrane. It plays an important role in the ATP synthesis or phosphorylation driven by chemiosmosis. Protons are pumped across the membrane into the lumen, which generates concentration gradient across the thylakoid membrane.
The water-splitting complex is present at the inner side of the thylakoid membrane of PS II and results in the release of proton and oxygen produced by splitting of water into the thylakoid lumen.
Thylakoid lumen also contains plastocyanin, which is an electron transport protein and shuttles electrons from cytochrome B6f complex to PS I.
This was in brief about Thylakoid. Test your understanding with MCQs on Chloroplasts, only at BYJU’S.