Question

Where does light reaction of photosynthesis take place?
A. Stroma
B. Endoplasmic reticulum
C. Quantasomes or thylakoids
D. Inner membrane of chloroplast

Answer 1

Hint: Some of the reactions in photosynthesis can only be processed when they receive a good amount of light such reactions are known as light reactions. Light energy in these reactions is absorbed by the pigment containing organelles and is converted into ATP and NADPH.

Complete answer:
Photosynthesis is the process by which plants make their own food by using carbon dioxide, water and light and converting these into oxygen and energy. Some reactions need light and are known as photochemical or light reactions whereas some can occur without light by using the products of light reaction (ATP and NADPH) known as carbon or dark reaction. Different pigments are used in the light reactions for the purpose of absorbing light like chlorophyll a, chlorophyll b, xanthophylls and carotenoids. These pigments absorb light at different wavelengths.
Light reaction involves absorption of light which is used to split water and release oxygen along with formation of high energy intermediates like ATP and NADPH. Pigments used in this process are arranged into 2 light harvesting complexes (consisting of many pigment molecules attached to the proteins present in them) in photosystem I and photosystem II. Each photosystem has a reaction centre made of chlorophyll having receptivity of light at different wavelengths as in photosystem I light of absorption peak is 700 nm and in photosystem II the peak is at 680 nm. Photosystem I is also known as PS I or P700 and photosystem II is known as PS II or P680. This reaction takes place in the thylakoids which are membrane bound compartments in chloroplasts.
Stroma is the fluid inside the chloroplast in which thylakoids are present and it is involved in the enzymatic reaction of sugar synthesis which is later on converted into starch. Endoplasmic reticulum in plant cells is involved in cell to cell communication.

Hence, the correct answer is option (C).

Additional information:
-The main purpose of light reaction is the induction of electron transfer across the cytochromes.
-ATP synthesis is dependent on the photon gradient.
-Light reaction involves- transfer of electrons from PSII uphill to the acceptor then down to the electron transport chain to the PS I, by this the electrons get excited and are transferred to another acceptor then reducing NADP downhill into NADPH and proton. This makes a Z scheme. This scheme connects both the photosystems.
Note: Along with this light reaction both photosystems are also involved into the production of ATP from ADP in the presence of light and inorganic phosphates and this process is known as non-cyclic photo-phosphorylation.