● The `color{Brown}"TCA cycle"` starts with the `color{violet}"condensation of acetyl group"` with `color{violet}"oxaloacetic acid (OAA)"` and water to yield `color{violet}"citric acid"`.

● The reaction is catalysed by the `color{violet}"enzyme citrate synthase"` and a `color{violet}"molecule of CoA"` is released.

● `color{violet}"Citrate"` is then isomerised to `color{violet}"isocitrate"`.

● It is followed by `color{violet}"two successive steps"` of `color{violet}"decarboxylation"`, leading to the formation of `color{violet}"α-ketoglutaric acid"` and then `color{violet}"succinyl-CoA"`.

● In the `color{violet}"remaining steps"` of citric acid cycle, `color{violet}"succinyl-CoA"` is oxidised to `color{violet}"OAA"` allowing the cycle to continue.

● During the conversion of `color{violet}"succinyl-CoA to succinic acid"` a molecule of `color{violet}"GTP"` is synthesised.

● This is a `color{Brown}"substrate level phosphorylation"`.

● In a `color{violet}"coupled reaction GTP"` is converted to `color{violet}"GDP"` with the simultaneous synthesis of `color{violet}"ATP from ADP."`

● Also there are `color{violet}"three points"` in the cycle where `NAD_+` is `color{violet}"reduced"` to `NADH` + `H^+` and one point where `FAD^+` is reduced to `FADH_2`.

● The continued `color{violet}"oxidation of acetic acid"` via the TCA cycle requires the continued replenishment of `color{violet}"oxaloacetic acid"`, the first member of the cycle.

● In addition it also `color{violet}"requires regeneration"` of `NAD^+` and `FAD^+` from `NADH` and `FADH_2` respectively.

● The `color{Brown}"summary equation"` for this phase of respiration may be written as follows:



● We have till now seen that `color{violet}"glucose has been broken down"` to release `CO_2` and `color{violet}"eight molecules"` of `NADH` + `H_+`; `color{violet}"two"` of `FADH_2` have been synthesised besides just `color{violet}"two molecules of ATP."`

● The following steps in the `color{violet}"respiratory process"` are to `color{violet}"release and utilize"` the energy stored in `NADH^+` + `H^+` and `FADH_2`.

● This is accomplished when they are `color{violet}"oxidised through the"` `color{violet}"electron transport system"` and the electrons are passed on to `O_2` resulting in the `color{violet}"formation"` of `H_2O`.

● The `color{violet}"metabolic pathway"` through which the `color{violet}"electron passes"` from one carrier to another, is called the `color{violet}"electron transport system"` (ETS) and it is present in the `color{violet}"inner mitochondrial membrane"`.

● `color{violet}"Electrons from NADH"` produced in the `color{violet}"mitochondrial matrix"` during `color{violet}"citric acid cycle"` are oxidised by an `color{violet}"NADH dehydrogenase"` (complex I), and electrons are then transferred to `color{violet}"ubiquinone"` located within the inner membrane.

● `color{violet}"Ubiquinone"` also receives `color{violet}"reducing equivalents"` via `FADH_2` (complex II) that is generated during
`color{violet}"oxidation of succinate"` in the citric acid cycle.

● The `color{violet}"reduced ubiquinone"` (ubiquinol) is then oxidised with the `color{violet}"transfer of electrons"` to `color{violet}"cytochrome c"` via c`color{violet}"ytochrome bc1 complex"` (complex III).

● `color{violet}"Cytochrome c"` is a small protein attached to the `color{violet}"outer surface of the inner membrane"` and acts as a `color{violet}"mobile carrier"` for transfer of electrons between `color{violet}"complex III and IV"`.

● `color{violet}"Complex IV"` refers to `color{violet}"cytochrome c oxidase complex"` containing cytochromes `a` and `a_3`, and two `color{violet}"copper centres"`.

● When the `color{violet}"electrons pass"` from `color{violet}"one carrier to another"` via `color{violet}"complex I to IV"` in the electron transport chain, they are `color{violet}"coupled to ATP synthase"` (complex V) for the production of `color{violet}"ATP from ADP"` and `color{violet}"inorganic phosphate"`.

● The `color{violet}"number of ATP molecules"` synthesized depends on the `color{violet}"nature of the electron donor"`.

● `color{violet}"Oxidation of one molecule"` of NADH gives rise to `color{violet}"3 molecules of ATP,"` while that of `color{violet}"one molecule"` of `FADH_2` produces `color{violet}"2 molecules of ATP."`