Q2TYU - Campbell Biology

Question

Which of the following sequences correctly represents the flow of electrons during photosynthesis?

(A)\({\rm{NADPH}}\, \to {{\rm{O}}_{\rm{2}}} \to {\rm{C}}{{\rm{O}}_{\rm{2}}}\)

(B) \({{\rm{H}}_{\rm{2}}}{\rm{O}}\, \to {\rm{NADPH}}\, \to {\rm{Calvin}}\,{\rm{Cycle}}\)

(C) \({{\rm{H}}_{\rm{2}}}{\rm{O}}\, \to {\rm{photosystem}}\,{\rm{I}}\, \to {\rm{photosystem}}\,{\rm{II}}\)

(D)\({\rm{NADPH}}\, \to {\rm{electron transport chain }} \to {{\rm{O}}_{\rm{2}}}\)

 

Step-by-Step Solution

Answer

1. The option “\({\rm{NADPH}}\, \to {{\rm{O}}_{\rm{2}}} \to {\rm{C}}{{\rm{O}}_{\rm{2}}}\)” is false.
2. The option “\({{\rm{H}}_{\rm{2}}}{\rm{O}}\, \to {\rm{NADPH}}\, \to {\rm{Calvin}}\,{\rm{Cycle}}\)” is true.
3. The option “\({{\rm{H}}_{\rm{2}}}{\rm{O}}\, \to {\rm{photosystem}}\,{\rm{I}}\, \to {\rm{photosystem}}\,{\rm{II}}\)” is false.
4. The option “\({\rm{NADPH}}\, \to {\rm{electron transport chain }} \to {{\rm{O}}_{\rm{2}}}\)” is false.

Step 1: Flow of electrons during photosynthesis

During photosynthesis, solar radiation splits water molecules to release an electron. The released electron flows to PSII, followed by PSI, from where it goes to NADP⁺ to reduce it to NADPH. NADPH then enters the Calvin cycle.

Step 2: Explanation of option (A)

Photolysis occurs in the light reaction of photosynthesis, during which solar radiation splits the water (H₂O) molecule into oxygen molecules, protons, and electrons. The electrons and protons then flow to NADP⁺ and reduce it to NADPH.

 

Thus, the electrons do not flow from NADPH to oxygen. 

Therefore, the given option is false.

 

Step 3: Explanation of option (B)

PSII picks the electrons released during photolysis. From here, it passes through a series of electron carriers, and then PSI picks it up. The electrons then go to NADP⁺, which combines with protons to produce NADPH.

 

NADPH synthesized in light reaction then goes to Calvin cycle, used to synthesize the organic compound. 

 

Thus, the electrons flow from water to NADPH, from where it flows to the Calvin cycle. 

Therefore, the given option is true.

 

Step 4: Explanation of option (C)

Electrons released during photolysis of water flow toward PSII, from where they flow toward PSI through a series of electron carriers. 

 

Thus, electrons flow from PSII to PSI, not from PSI to PSII.

Therefore, the given option is false.

Step 5: Explanation of option (D)

The splitting of water produces oxygen gas, and the electrons are released to pass through the electron transport chain before giving it to NADP+ to form NADPH. 

 

Thus, electrons flow from the electron transport chain to NADPH, not NADPH to the electron transport chain.

Therefore, the given option is false.