Question

PS-II occurs only in the:
A. Stroma
B. Granal thylakoids
C. Stromal lamellae
D. Matric

Answer 1

Hint: Thylakoids are an exclusive site for light reactions. These are membrane-bound organelles found inside the chloroplast, which frequently form stacks of disks referred to as grana, which are further connected by stroma thylakoids.

Complete answer: Thylakoids are membrane-bound organelles, found inside chloroplast and cyanobacteria, which are the site of light-dependent reactions of photosynthesis. It is the site for two systems of photosynthetic pigments, namely, photosystem I and II. A chlorophyll ‘a’ molecule, P680, processes light of 680 nm wavelength in photosystem II, while, in photosystem I, the light of wavelength 700 nm is absorbed by the chlorophyll ‘a’ molecule. While photosystem I occurs mostly in stroma thylakoids and outer layers of grana, photosystem II occurs in the appressed regions of grana of thylakoids. The intactness of thylakoids is crucial for the occurrence of light reactions and the formation of ATP. It requires the presence of the complex structural organisation of lamella for the proper functioning of the thylakoid. Fragmented thylakoids can no longer form ATP, however, such fragments are capable of carrying out “Hill reaction”. Photosystem II plays an important role in the transfer of electrons generated from the water. It undergoes oxidation-reduction reactions and the electrons are being passed to NADPH molecules.
Hence, the correct answer is option B.

Note: The grana and stroma thylakoids can be differentiated by their respective protein compositions. The main function of stroma thylakoid is to provide support, it also contains some enzymes for light-independent reactions.