Explain: Electron Transport.

(N/A) In photosystem $II$ $(PS-II)$,the reaction centre chlorophyll-$a$ absorbs $680 \ nm$ wavelength of red light,causing electrons to become excited and jump into an orbit farther from the atomic nucleus.
These electrons are picked up by an electron acceptor,which passes them to an electron transport system.
This movement of electrons is downhill in terms of an oxidation-reduction or redox potential scale.
The electrons are not used up as they pass through the electron transport chain but are passed on to the pigments of photosystem $I$ $(PS-I)$.
Simultaneously,electrons in the reaction centre of $PS-I$ are also excited when they receive red light of wavelength $700 \ nm$ and are transferred to another acceptor molecule that has a greater redox potential.
These electrons are then moved downhill again,this time to a molecule of energy-rich $NADP^+$. The addition of these electrons reduces $NADP^+$ to $NADPH + H^+$.
This is a whole scheme of 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 acceptor,and finally downhill to $NADP^+$ causing it to be reduced to $NADPH + H^+$.
This is called the $Z$-scheme due to its characteristic shape,which is formed when all the carriers are placed in a sequence on a redox potential scale.