Question

Z-scheme shows the non-cyclic electron flow of light reaction in oxygenic photosynthetic organisms. According to the Z-scheme, choose the correct statement(s) :
I. $P_{680}^{+}$ is the $P_{680}$ molecule that lost its electron. It has the maximum reduction potential.
II. $P_{680}^{\ast }$ is the excited $P_{680}$ molecule after absorption of the light energy. It has the lowest reduction potential
III. Final electron acceptor of PS I is Fe-S centre
IV. Flow of high energy electron in the electron transport chain creates a pH difference between the chloroplast stroma and thylakoid lumen

• A. Only I
• B. I and IV
• C. III and IV
• D. II, III and IV

Answer 1

The correct option is B I and IV

In Z-scheme, the outlines of pathway of electron flow between photosystems and the energy relationships in the light reaction is represented.

Reduction means the gain of electrons and reduction potential means the tendency to gain electrons. Higher the reduction potential of an entity, higher the tendency to gain electrons.

When sunlight excites $P_{680}$ in PS II, the electrons get excited and $P_{680}^{\ast }$ is formed. $P_{680}^{\ast }$ gives its electron to pheophytin which has a higher reduction potential than $P_{680}^{\ast }$.

$P_{680}^{+}$ is formed when $P_{680}^{\ast }$ looses its electron. It has maximum reduction potential in the Z-scheme and hence, it gains electron from the splitting of a water molecule.

Flow of the high energy electrons pumps protons from the stroma to the thylakoid lumen creates a pH difference between stroma and lumen. This pH difference is responsible for ATP synthesis in light reaction.

Similarly, electron in $P_{700}$ also get excited and it becomes $P_{700}^{\ast }$. It has the lowest reduction potential.

The final electron acceptor of PS I is ferrodoxin. Ferrodoxin transfers its electron to NADPH and this reaction is carried out in the presence of FNR enzyme.