Question

The Z scheme of photophosphorylation follows the following sequence:
$PSII\underset{A}{\to }{e}^{-}acceptor\underset{B}{\to }ETS\underset{B}{\to }PSI\underset{C}{\to }{e}^{-}acceptor\underset{D}{\to }NAD{P}^{+}$
Which of the following options for A, B, C, and D transfer of electrons.
A. A-Uphill; B-Downhill; C-Uphill; D-Downhill
B. A-Downhill; B-Uphill; C-Downhill; D-Uphill
C. A-Downhill; B-Uphill; C-Uphill; D-Downhill
D. A-Uphill; B-Downhill; C-Downhill; D-Uphill

Answer 1

Hint: Since the given sequence involves both photosystems I and photosystem II, it is non-cyclic phosphorylation which is also known as the Z scheme.

Complete answer:
The noncyclic phosphorylation begins with photosystem II in which the reaction center chlorophyll 'a' absorbs radiation of wavelength 680nm due to which the electrons of the reaction center get excited which are then accepted by different electron acceptors.
The transport of electrons from photosystem II to electron acceptor is uphill in terms of redox potential scale.
Then the electrons are passed on to the electron transport chain which contains a series of cytochromes. The movement of electrons from the electron acceptor to the electron transport system (ETS) is downhill in terms of redox potential scale.
The electrons are not used by an electron transport system and it is further passed to the photosystem I and this movement is also downhill in terms of redox potential scale.
The photosystem I absorb light of wavelength 700 nm and the electrons get excited which are again accepted by the electron acceptors, this movement of electrons is uphill transport in terms of redox potential scale.
Then finally the electrons are passed to the NADP+ molecule which leads to a reduction of NADP+ to form the NADPH molecule, the movement of electrons from the electron acceptor to the NADP+ molecule is downhill in terms of redox potential scale.

Option A: From the above points, we can label the given diagram as A-Uphill; B-Downhill; C-Uphill; D-Downhill. Therefore, this is the correct option.

All other options are irrelevant.

Hence, the correct answer is option (A)

Note: The electrons that are removed from photosystem II at the beginning of noncyclic phosphorylation are replaced by a water-splitting reaction. Water gets split to form hydrogen ion, nascent oxygen, and electrons and thus the electrons obtained from this reaction are supplied to the photosystem II.