The chemiosmotic mechanism of $ATP$ synthesis was proposed by:

During the ionization of water in photosynthesis,the $H^+$ ions released at the end are accepted by which of the following?

Assertion $(A) : H^+$ gradient across the thylakoid membrane is essential for the formation of $\text{ATP}$.
Reason $(R) :$ Breakdown of $H^+$ gradient due to the movement of protons through $F_0$ of $\text{ATP}$ synthase releases enough energy to activate $\text{ATP}$ synthase enzyme that catalyses the formation of $\text{ATP}$.

Cyclic photophosphorylation results in the formation of

The first electron acceptor in Photosystem-$I$ $(PS-I)$ is:

Emerson's enhancement effect and Red drop have been instrumental in the discovery of