Page 10 - LN-PHOTOSYNTHESIS IN PLANTS

This hypothesis was given by Peter Mitchell (1961) in order to explain the ATP
synthesis in photosynthesis (also in respiration).
The synthesis of ATP is directly linked to the developinent of a proton gradient
across the thylakoid membranes of a chloroplast.
The main difference that lies between the photosynthesis and respiration is the
location where the accumulation of protons takes place. In chloroplast
(photosynthesis), it occurs in thylakoid lumen while in mitochondria (respiration), it
occurs in intermembrane space.
Now, the point arises that what causes the proton gradient across the membrane?
The development of proton gradient results due to the reasons given below
(i) As the water molecule splits into the inner side of the membrane the protons or
hydrogen ions that are produced by the water splitting gets accumulate within the
thylakoids lumen.
(ii) Transportation of protons takes place across the membrane when the electron
moves through the photosystems. The primary acceptor of electron is located
towards the outer side of the membrane, which transfers electron to the proton (H )
+
carrier and not to the electron carrier.
So, this molecule, while transporting an electron removes a proton from the stroma,
thus, release of proton takes place into the inner side, i.e., on the lumen of the
membrane.

(iii) The enzyme NADP reductase is present on the stromal side of the membrane.
Thus, alongwith the electrons that comes from the acceptor of electrons of PS-I,
protons are also necessary to reduce NADP to NADPH + H
+
+
Hence, protons in the stroma within the chloroplasts decreases in number, while
accumulation of protons takes place in the lumen. Due to which the proton gradient