`star` Location OF ETS
`star` Cause for Proton Gradient


● 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 the`color{violet}" intermembrane space"` of the mitochondria when electrons move through the `color{violet}"ETS."`


● 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.