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.
Role of the Thylakoid in Photosynthesis
Reactions performed in the thylakoid include water photolysis, the electron transport chain, and ATP synthesis.
Photosynthetic pigments (e.g., chlorophyll) are embedded into the thylakoid membrane, making it the site of the light-dependent reactions in photosynthesis. The stacked coil shape of the grana gives the chloroplast a high surface area-to-volume ratio, aiding the efficiency of photosynthesis.
The thylakoid lumen is used for photophosphorylation during photosynthesis. The light-dependent reactions in the membrane pump protons into the lumen, lowering its pH to 4. In contrast, the pH of the stroma is 8.
Water Photolysis
The first step is water photolysis, which occurs on the lumen site of the thylakoid membrane. Energy from light is used to reduce or split water. This reaction produces electrons that are needed for the electron transport chains, protons that are pumped into the lumen to produce a proton gradient, and oxygen. Although oxygen is needed for cellular respiration, the gas produced by this reaction is returned to the atmosphere.
Electron Transport Chain
The electrons from photolysis go to the photosystems of the electron transport chains. The photosystems contain an antenna complex that uses chlorophyll and related pigments to collect light at various wavelengths.
Photosystem I uses light to reduce NADP + to produce NADPH and H+. Photosystem II uses light to oxidize water to produce molecular oxygen (O2), electrons (e–), and protons (H+). The electrons reduce NADP+ to NADPH in both systems.
ATP Synthesis
ATP is produced from both Photosystem I and Photosystem II. Thylakoids synthesize ATP using an ATP synthase enzyme that is similar to mitochondrial ATPase. The enzyme is integrated into the thylakoid membrane. The CF1-portion of the synthase molecule extended into the stroma, where ATP supports the light-independent photosynthesis reactions.
The lumen of the thylakoid contains proteins used for protein processing, photosynthesis, metabolism, redox reactions, and defense. The protein plastocyanin is an electron transport protein that transports electrons from the cytochrome proteins to Photosystem I.
Cytochrome b6f complex is a portion of the electron transport chain that couples proton pumping into the thylakoid lumen with electron transfer. The cytochrome complex is located between Photosystem I and Photosystem II.
