State the main function of thylakoids.

Hint: Thylakoids are the membrane-bound sheet structure which is the site of light dependent reactions. They are found inside the chloroplast which is present in plants and some cyanobacteria.

Complete answer: Chloroplast is known as the photosynthetic pigment of the plant cell. The chloroplast consists of small-sized coins called thylakoids. Thylakoids are placed in the stroma of the chloroplast which is the interior portion of the chloroplast. The stroma is the cytoplasm-like for the chloroplast. The stroma has ribosomes, enzymes, and chloroplast DNA. The ribosomes or enzymes present in the stroma are necessary for photosynthesis, electron transport system, etc. The thylakoids have a membrane and enclosed within the thylakoid lumen. The stacks of the thylakoids are known as granum. They are collectively known as grana. They are connected by the lamella which is also called stroma thylakoids. Thus, there are two types of thylakoids: stroma thylakoids and grana thylakoids.
The main functions of thylakoids are in all light dependent reactions like water photolysis, electron transport chain, and ATP synthesis. The initial step of water photolysis takes place in the lumen site of the thylakoid membrane. Water photolysis is breaking the water molecule in presence of light. The electrons produced in the end are important for electron transport chain reactions, the hydrogen protons are pumped into lumen for proton gradient and oxygen is released by the plants. The ATP molecules are produced from both photosystem I and photosystem II and are synthesized in the thylakoids only. They have ATP synthase (enzyme) which produces ATP molecules. This enzyme is present in the thylakoid membrane only. The lumen of the thylakoids also consists of various proteins that help in photosynthesis like cytochrome plastocyanin.

Note: Thylakoids are responsible for all light dependent reactions. It is also responsible for the conversion of $NAD{P^ + }$ to NADPH. It is also responsible for maintaining the acidity required by pumping protons against gradients.