Biology CHEMIOSMOTIC HYPOTHESIS

KEY TOPICS

star Location OF ETS
star Cause for Proton Gradient

LOCATION OF ETS

● The color{Brown}"chemiosmotic hypothesis" has been put forward to explain the mechanism of how actually color{violet}"ATP is synthesised" in the color{violet}"chloroplast."

● Like in color{violet}"respiration", in photosynthesis too, color{violet}"ATP synthesis" is linked to development of a color{violet}"proton gradient" across a membrane.

● This time these are color{violet}"membranes of the thylakoid."

● There is one difference though, here the color{violet}"proton accumulation" is towards the inside of the membrane, i.e., color{violet}"in the lumen".

● In respiration, color{violet}"protons accumulate" in thecolor{violet}" intermembrane space" of the mitochondria when electrons move through the color{violet}"ETS."

CAUSE OF PROTON GRADIENT

● We need to consider again the color{violet}"processes" that take place during the color{violet}"activation of electrons" and their transport to determine the steps that cause a color{violet}"proton gradient to develop".

● Since color{Brown}"splitting of the water molecule" takes place on the color{violet}"inner side" of the membrane, the color{violet}"protons or hydrogen ions" that are produced by the splitting of water accumulate within the color{violet}"lumen of the thylakoids".

● As color{violet}"electrons move" through the photosystems, color{violet}"protons are transported" across the membrane.

star This happens because the color{violet}"primary accepter of electron" which is located towards the color{violet}"outer side of the membrane" transfers its electron not to an color{violet}"electron carrier" but to an color{violet}"H carrier".

star Hence, this molecule removes a color{violet}"proton from the stroma" while transporting an color{violet}"electron".

star When this molecule color{violet}"passes on its electron" to the electron carrier on the color{violet}"inner side of the membrane," the proton is released into the inner side or the color{violet}"lumen side of the membrane".

● The color{violet}"NADP reductase enzyme" is located on the color{violet}"stroma side" of the membrane.

● Along with electrons that come from the color{violet}"accepter of electrons of PS I", protons are necessary for the color{violet}"reduction" of NADP^+ to NADPH^+ H^+.

● These protons are also color{violet}"removed from the stroma".

● Hence, within the color{violet}"chloroplast, protons" in the stroma decrease in number, while in the color{violet}"lumen" there is color{violet}"accumulation of protons".

● This creates a color{Brown}"proton gradient" across the color{violet}"thylakoid membrane" as well as a color{violet}"measurable decrease in pH" in the lumen.

● This proton gradient is important because it is the color{violet}"breakdown of this gradient" that leads to color{violet}"release of energy".

● The gradient is broken down due to the color{violet}"movement of protons" across the color{violet}"membrane to the stroma" through the color{violet}"transmembrane channel" of the F_0 of the color{violet}"ATPase".

● The color{Brown}"ATPase enzyme" consists of color{violet}"two parts": one called the F_0 is embedded in the color{violet}"membrane" and forms a color{violet}"transmembrane channel" that carries out color{violet}"facilitated diffusion" of protons across the membrane.

● The other portion is called F_1 and color{violet}"protrudes on the outer surface" of the thylakoid membrane on the side that color{violet}"faces the stroma."

● The color{violet}"break down of the gradient" provides enough energy to cause a color{violet}"conformational change" in the F_1 particle of the color{violet}"ATPase", which makes the enzyme synthesise color{violet}"several molecules" of color{Brown}"energy-packed ATP."

● color{Brown}"Chemiosmosis" requires a membrane, a color{violet}"proton pump", a color{violet}"proton gradient" and color{violet}"ATPase".

● color{violet}"Energy is used" to pump protons across a membrane, to create a color{violet}"gradient or a high concentration" of protons within the thylakoid lumen.

● color{violet}"ATPase has a channel" that allows diffusion of protons back across the membrane; this color{violet}"releases enough energy" to activate ATPase enzyme that catalyses the color{violet}"formation of ATP."

● Along with the color{violet}"NADPH produced" by the movement of electrons, the color{violet}"ATP will be used immediately" in the biosynthetic reaction taking place in the color{violet}"stroma", responsible for color{violet}"fixing" CO_2, and synthesis of sugars.