Explain how during the light reaction of photosynthesis,$ATP$ synthesis is a chemiosmotic phenomenon.

(N/A) To describe the process of synthesis of $ATP$ in the chloroplast,the chemiosmotic hypothesis has been put forward to explain the mechanism.
Like in respiration,in photosynthesis too,$ATP$ synthesis is linked to the development of a proton gradient across a membrane. These are the membranes of the thylakoid.
Main difference: In respiration,protons $(H^{+})$ accumulate in the intermembrane space of the mitochondria when electrons move through the $ETS$.
Formation of proton gradient:
$(a)$ Since the splitting of the water molecule takes place on the inner side of the membrane,the protons or hydrogen ions $(H^{+})$ that are produced by the splitting of water accumulate within the lumen of the thylakoids.
$(b)$ As electrons move through the photosystems,protons are transported across the membrane. This happens because the primary acceptor of electrons,which is located towards the outer side of the membrane,transfers its electron not to an electron carrier but to an $H$ carrier. Hence,this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron to the electron carrier on the inner side of the membrane,the proton is released into the inner side or the lumen side of the membrane.
Reduction of $NADP^{+}$:
$(c)$ The $NADP$ reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the acceptor of electrons of $PS I$,protons are necessary for the reduction of $NADP^{+}$ to $NADPH + H^{+}$. These protons are also removed from the stroma.
$NADP^{+} + 2H^{+} \longrightarrow NADPH + H^{+}$
Hence,within the chloroplast,protons in the stroma decrease in number,while in the lumen there is an accumulation of protons. This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in $pH$ in the lumen.
Importance: This proton gradient is important because it is the breakdown of this gradient that leads to the release of energy. The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel.
Structure of $ATPase$ enzyme:
- $F_{0}$: The $ATPase$ enzyme consists of two parts: one called the $F_{0}$ is embedded in the membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane.
- $F_{1}$: The other portion is called $F_{1}$ and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.
- The breakdown of the gradient provides enough energy to cause a conformational change in the $F_{1}$ particle of the $ATPase$,which makes the enzyme synthesize several molecules of energy-packed $ATP$.
Necessary elements of Chemiosmosis:
$(a)$ Chemiosmosis requires a membrane,a proton pump,a proton gradient,and $ATPase$.
$(b)$ Energy is used to pump protons across a membrane to create a gradient or a high concentration of protons within the thylakoid lumen.
$(c)$ $ATPase$ has a channel that allows the diffusion of protons back across the membrane; this releases enough energy to activate the $ATPase$ enzyme that catalyses the formation of $ATP$.