● Although the `color{violet}"aerobic process of respiration"` takes place only in the `color{violet}"presence of oxygen"`, the role of oxygen is limited to the `color{violet}"terminal stage"` of the process.

● Yet, the `color{violet}"presence of oxygen is vital"`, since it drives the whole process by `color{violet}"removing hydrogen"` from the system.

● Oxygen acts as the `color{violet}"final hydrogen acceptor."`

● Unlike `color{violet}"photophosphorylation"` where it is the light energy that is utilised for the `color{violet}"production of proton gradient"` required for `color{violet}"phosphorylation"`, in respiration it is the energy of `color{violet}"oxidation-reduction"` utilised for the same process.

● It is `color{violet}"for this reason"` that the process is called `color{Brown}"oxidative phosphorylation"`.

● The `color{violet}"energy released"` during the `color{violet}"electron transport system"` is utilised in `color{violet}"synthesising ATP"` with the help of `color{violet}"ATP synthase"` (complex V).

● This complex consists of `color{violet}"two major components"`, `F_1` and `F_0`.

● The `F_1` `color{violet}"headpiece"` is a `color{violet}"peripheral membrane protein"` complex and contains the site for `color{violet}"synthesis of ATP"` from ADP and inorganic phosphate.

● `F_0` is an `color{violet}"integral membrane protein"` complex that forms the `color{violet}"channel through which protons"` cross the inner membrane.

● The `color{violet}"passage of protons"` through the channel is coupled to the `color{violet}"catalytic site"` of the `F_1` `color{violet}"component"` for the production of ATP.

● For each `color{violet}"ATP produced"`, `2H^+` passes through `F_0` from the `color{violet}"intermembrane space"` to the matrix down the `color{violet}"electrochemical proton gradient."`

● It is possible to `color{violet}"make calculations"` of the `color{violet}"net gain of ATP"` for `color{violet}"every glucose molecule"` oxidised; but in reality this can remain only a `color{violet}"theoretical exercise."`

● These `color{violet}"calculations"` can be made only on `color{violet}"certain assumptions"` that:

`star` There is a `color{violet}"sequential, orderly pathway"` functioning, with `color{violet}"one substrate"` forming the next and with `color{violet}"glycolysis, TCA cycle and ETS"` pathway following one after another.

`star` The `color{violet}"NADH synthesised in glycolysis"` is transferred into the mitochondria and undergoes `color{violet}"oxidative phosphorylation"`.

`star` `color{violet}"None of the intermediates"` in the pathway are `color{violet}"utilised to synthesise"` any other compound.

`star` `color{violet}"Only glucose is being respired"` – `color{violet}"no other alternative substrates"` are entering in the pathway at any of the `color{violet}"intermediary stages."`

● But this kind of assumptions are `color{violet}"not really valid in a living system"`; all pathways `color{violet}"work simultaneously"` and do not take place `color{violet}"one after another"`; substrates enter the pathways and are `color{violet}"withdrawn"` from it as and when necessary; `color{violet}"ATP is utilised"` as and when needed; `color{violet}"enzymatic rates"` are controlled by multiple means.

● Yet, it is `color{violet}"useful to do this exercise"` to appreciate the beauty and efficiency of the living system in `color{violet}"extraction
and storing energy"`.

● Hence, there can be a `color{violet}"net gain of 36 ATP molecules"` during `color{Brown}"aerobic respiration"` of `color{violet}"one molecule of glucose"`.

● Now let us compare `color{violet}"fermentation and aerobic respiration"`:

`star` `color{brown}"Fermentation"` accounts for only a `color{violet}"partial breakdown of glucose"` whereas in `color{Brown}"aerobic respiration"` it is `color{violet}"completely degraded"` to `CO_2` and `H_2O`.

`star` In fermentation there is a `color{violet}"net gain of only two molecules"` of `color{violet}"ATP"` for each molecule of `color{violet}"glucose"` degraded to `color{violet}"pyruvic acid"` whereas many more `color{violet}"molecules of ATP"` are generated under aerobic conditions.

`star` `NADH` is `color{violet}"oxidised"` to `NAD^+` rather slowly in `color{violet}"fermentation"`, however the reaction is very `color{violet}"vigorous"` in case of `color{violet}"aerobic respiration"`.