Question

During ATP synthesis, movement of electrons occurs from

• A. lumen of thylakoids to membrane of thylakoids
• B. lumen of thylakoids to stroma of thylakoids
• C. membrane of thylakoids to stroma of thylakoids
• D. stroma of thylakoids to lumen of thylakoids

Answer 1

The correct option is B lumen of thylakoids to stroma of thylakoids

Chemiosmotic Hypothesis explains the formation of ATP using the electrochemical proton gradient set up by the passage of the electrons through a series of electron carriers in the electron transport chain.

The gradient is formed in the intermembrane space of chloroplasts. Lumen of thylakoid is enriched with protons due to the photolytic splitting of
$H_{2}O$.

Primary acceptor of electrons (phaeophytin) is located on the outer side of the thylakoid membrane. It transfers its electrons to a proton carrier (PQ) instead of an $e^{-}$ carrier.

The carrier, in doing so, takes a proton from the stroma and transports the electron to the inner side of the membrane. The proton is released into the lumen while the electron is passed onto the next carrier.

NADP reductase is situated on the outer side of the thylakoid membrane. It obtains electrons from PS I and protons from the matrix to reduce ${\mathrm{NADP}}^{+}$ to NADPH.

This leads to the increase in the concentration of protons in the lumen of thylakoids and decrease in the stroma. This develops a proton gradient across the membrane of thylakoid.

This established proton gradient can be broken down by the passage of protons through the transmembrane channels, ${\mathrm{CF}}_0$ of ATPase. It provides the facilitated diffusion of protons. The other portion is called ${\mathrm{CF}}_1$ and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.

As the protons move from lumen to stroma, they bring about a conformational change in the ${\mathrm{CF}}_1$ particle of ATPase which helps in the formation of ATP from ADP and Pi.