Question

Which of the following cell organelles is concerned with photophosphorylation?
a. Mitochondria
b. Plastochondria
c. Chloroplast
d. More than one option is concerned

Answer 1

Hint: As the term itself suggests, photophosphorylation must involve phosphorylation of a species using light energy. It is thus one of the steps of photosynthesis wherein, phosphorylation of ADP to ATP is accomplished using the energy from sunlight.

Complete answer:
So as said above also, it is the process by which ATP is produced by phosphorylation of ADP using sunlight. It takes place in the chloroplast of plants. Electrons spontaneously move from high-energy electron donors to low-energy electron acceptors through the electron transport chain. There are primarily two types of photophosphorylation reactions based on the path taken by electrons in the electron- transport systems of the primary photochemical reaction. They are-

Cyclic photophosphorylation- This involves the synthesis of ATP coupled to electron transport and is activated and thus involved with only one Photosystem, PS- I (P700). The site of occurrence is stroma lamellae. This is initiated by PS-I and the electrons flow from primary acceptor to ferredoxin, to be followed to be transported to plastoquinone and then to cytochrome b6f and finally to plastocyanin before returning back to PS-I. Thus, high energy electrons releases from P700 of PS-I flow down in a cyclic manner. This electron transport creates a proton-motive force producing concentration gradient which powers ATP synthase.

Non-cyclic photophosphorylation- In this, two photosystems, PS-I and PS- II are involved. Electrons flow in a non-cyclic manner to synthesize ATP molecules. It occurs in the thylakoid membrane. In this, electrons are removed from water which then pass through both the photosystems (PS-II and then PS-I) before they end up in NADPH. It is a light-dependent reaction.

Hence, the correct answer is option (C).

Note: Neither $O_2$ nor NADPH are produced in cyclic photophosphorylation. Only ATP is produced. A single photosystem is used in bacterial photosynthesis and therefore cyclic photophosphorylation is involved.