`star` Splitting of Water
`star` Cyclic and Non-cyclic Photo-phosphorylation


● The `color{violet}"electrons"` that were moved from `color{violet}"photosystem II"` must be replaced.

● This is `color{violet}"achieved by electrons"` available due to `color{violet}"splitting of water"`.

● The `color{violet}"splitting of water"` is associated with the `color{violet}"PS II;"` `color{violet}"water is split"` into `H^+`, [O] and `color{violet}"electrons"`.

● This `color{violet}"creates oxygen"`, one of the `color{violet}"net products of photosynthesis"`.

● The `color{violet}"electrons needed to replace"` those removed from `color{violet}"photosystem I"` are provided by `color{violet}"photosystem II"`.

`2H_2O` ⎯⎯→`4H^+` +`O_2` + `4e^−`

● We need to emphasise here that the `color{violet}"water splitting complex"` is associated with the `color{violet}"PS II,"` which itself is `color{violet}"physically located"` on the inner side of the `color{violet}"membrane of the thylakoid"`.


● Living organisms have the `color{violet}"capability of extracting energy"` from oxidisable substances and store this in the `color{violet}"form of bond energy"`.

● Special substances like `color{violet}"ATP, carry this energy"` in their `color{violet}"chemical bonds"`.

● The process of which `color{violet}"ATP is synthesised by cells"` (in mitochondria and chloroplasts) is named `color{Brown}"phosphorylation"`.

● `color{violet}"Photophosphorylation"` is the `color{violet}"synthesis of ATP"` from `color{violet}"ADP and inorganic phosphate"` in the presence of light.

● When the `color{violet}"two photosystems"` work in a series, first `color{violet}"PS II"` and then the `color{violet}"PS I,"` a process called `color{Brown}"non-cyclic photo-phosphorylation"` occurs.

● The `color{violet}"two photosystems"` are connected through an `color{violet}"electron transport chain"`, as seen earlier – in the `color{violet}"Z scheme."`

● Both `color{violet}"ATP"` and `NADPH` + `H_+` are synthesised by this kind of `color{violet}"electron flow"`.

● When only `color{violet}"PS I is functional,"` the electron is circulated within the `color{violet}"photosystem"` and the phosphorylation occurs due to `color{violet}"cyclic flow of electrons"`.

● A `color{violet}"possible location"` where this could be happening is in the `color{violet}"stroma lamellae"`.

● While the `color{violet}"membrane or lamellae"` of the grana have `color{violet}"both PS I and PS II"` the stroma lamellae membranes lack PS II as well as `color{violet}"NADP reductase enzyme"`.

● The excited electron `color{violet}"does not pass on"` to `NADP^`+ but is cycled back to the `color{violet}"PS I complex"` through the electron transport chain

● The `color{violet}"cyclic flow"` hence, results only in the `color{violet}"synthesis of ATP, "`but not of `NADPH` + `H^+`.

● `color{Brown}"Cyclic photophosphorylation"` also occurs when only `color{violet}"light of wavelengths"` beyond `color{violet}"680 nm"` are available for excitation.