Assertion (A): Movement of electrons in ETS is coupled to pumping of protons in to lumen of thylakoid

Reason (R): ${\mathrm{F}}_{\mathrm{o}}$ part of ATP synthase is associated with breakdown of proton gradient in chloroplast

Assertion is correct. During the light-dependent reactions of photosynthesis in the thylakoid membrane, the movement of electrons through the Electron Transport System (ETS) is coupled to the pumping of protons (H⁺ ions) into the thylakoid lumen. This process occurs in both Photosystem II (PS II) and Photosystem I (PS I) of the thylakoid membrane.

Reason is also correct. ATP synthase is an enzyme complex responsible for ATP synthesis during the light-dependent reactions of photosynthesis. It consists of two main parts: the F₁ part, which protrudes into the stroma of the chloroplast, and the F_o part, which spans the thylakoid membrane. The Fo part of ATP synthase forms a channel for protons (H⁺ ions) to move from the thylakoid lumen back into the stroma.

As the electrons flow through the ETS in the thylakoid membrane, protons are pumped from the stroma into the thylakoid lumen. This creates a proton gradient, with a higher concentration of protons in the lumen and a lower concentration in the stroma. The protons then flow back into the stroma through the F_o part of ATP synthase, utilizing the energy released in this process to drive the synthesis of ATP from ADP and inorganic phosphate (P_i). This coupling of electron flow in the ETS with proton pumping and the subsequent breakdown of the proton gradient by ATP synthase is crucial for the production of ATP during photosynthesis.

Both A and R are correct but R is not the correct explanation of A.