`star` Light Reaction
`star` The Electron Transport Chain


● `color{brown}"Light reactions"` or the `color{Brown}"Photochemical phase"` include `color{violet}"light absorption"`, `color{violet}"water splitting"`, `color{violet}"oxygen release"`, and the formation of `color{violet}"high-energy chemical intermediates,"` ATP and NADPH.

● `color{violet}"Several complexes"` are involved in the process.

● The `color{violet}"pigments"` are organised into two discrete `color{violet}"photochemical light"` `color{violet}"harvesting complexes (LHC)"` within the `color{Brown}"Photosystem I (PS I)"` and `color{Brown}"Photosystem II (PS II)"`.

● These are named in the `color{violet}"sequence of their discovery"`, and not in the `color{violet}"sequence in which they function"` during the light reaction.

● The `color{Brown}"LHC"` are made up of `color{violet}"hundreds"` of `color{violet}"pigment molecules"` bound to `color{violet}"proteins"`.

● `color{violet}"Each photosystem"` has all the `color{violet}"pigments"` (except `color{violet}"one molecule of chlorophyll a"`) forming a `color{violet}"light harvesting system"` also called `color{Brown}"antennae"`.

● These pigments help to make `color{violet}"photosynthesis more efficient"` by `color{violet}"absorbing different wavelengths"` of light.

● The `color{violet}"single chlorophyll a"` molecule forms the `color{brown}"reaction centre"`.

● The `color{violet}"reaction centre"` is `color{violet}"different"` in both the photosystems.

● In `color{Brown}"PS I"` the reaction centre chlorophyll a has an `color{violet}"absorption peak at 700 nm"`, hence is called `color{Brown}"P700"`, while in `color{violet}"PS II"` it has `color{violet}"absorption maxima at 680 nm"`, and is called `color{Brown}"P680"`.


● In `color{violet}"photosystem II,"` the reaction centre `color{violet}"chlorophyll a absorbs 680 nm"` wavelength of `color{violet}"red light"` causing `color{violet}"electrons to become excited"` and jump into an orbit farther from the atomic nucleus.

● These electrons are `color{violet}"picked up by an electron acceptor"` which passes them to an `color{Brown}"electrons transport system"` consisting of `color{violet}"cytochromes"`.

● This movement of `color{violet}"electrons is downhill,"` in terms of an `color{violet}"oxidation-reduction"` or `color{violet}"redox potential scale"`.

● The `color{violet}"electrons are not used up"` as they pass through the electron transport chain, but are `color{violet}"passed on to the pigments"` of `color{violet}"photosystem PS I"`.

● Simultaneously, electrons in the `color{violet}"reaction centre of PS I"` are also excited when they receive `color{violet}"red light of wavelength 700 nm"` and are transferred to another `color{violet}"accepter molecule"` that has a `color{violet}"greater redox potential"`.

● These electrons then are `color{violet}"moved downhill again"`, this time to a molecule of `color{violet}"energy-rich"` `NADP^+`.

● The `color{violet}"addition of these"` `color{violet}"electrons reduces"` `NADP^+` to `NADPH` + `H^+`.

● This `color{violet}"whole scheme"` of `color{violet}"transfer of electrons,"`
starting from the PS II, uphill to the acceptor,
down the electron transport chain to PS I,
excitation of electrons, transfer to another accepter, and finally down hill to `NADP^+`
causing it to be reduced to `NADPH` + `H^+`. is called the `color{Brown}"Z scheme"`, due to its characterstic shape.

● This shape is formed when `color{violet}"all the carriers"` are placed in a `color{violet}"sequence"` on a `color{violet}"redox potential scale"`.