Question

Where does non-cyclic photophosphorylation take place ? Describe the process. Why is process referred to as non-cyclic ?

Answer 1

Non-cyclic photophosphorylation takes place in the granal thylakoids of chloroplasts. Non-cyclic photophosphorylation involves both Photosystem I and Photosystem II. These two photosystems work in series, first PS II and the PS I. The two photosystems are connected through an electron transport chain. Both ATP and $NADPH+H^{+}$ are synthesised by this kind of electron flow. First, in PS II, the $P_{680}$ molecule absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit which is farther from the atomic nucleus.

These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. This movement of electrons is downhill on redox potential scale. The electrons are then passed onto the pigments of PS I, without being used as they pass through the electron transport chain. Simultaneously, electrons in the reaction center of PS I ($P_{700}$) are excited when they receive light of wavelength 700 nm and these electrons are transferred to another acceptor molecule that has a greater redox potential. These electrons are then moved downhill again to a molecule of $NAD{P}^{+}$. The addition of these electrons reduces the $NAD{P}^{+}$ to $NADPH+H^{+}$. It is also known as Z-scheme due to the zig-zag shape formed by the carriers.

This process is referred to as non-cyclic because the flow of electrons is non-cyclic. Electrons do not return back to PS I, they move to a molecule of $NAD{P}^{+}$.