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Difficulty: Very hardPlant Nutrition and Photosynthesis

Arrange the following physiological steps of non-cyclic photophosphorylation during the light-dependent phase of photosynthesis in their correct chronological sequence from first to last.

  1. 1Photon absorption by Photosystem II causing photo-excitation and release of high-energy electrons.
  2. 2Photolysis of water molecules yielding oxygen gas, protons (H+H^+), and replacement electrons.
  3. 3Transfer of electrons along a series of electron carriers, pumping protons into the thylakoid lumen to synthesize ATP.
  4. 4Re-excitation of electrons at Photosystem I upon absorbing additional photon energy.
  5. 5Reduction of NADP+NADP^+ to NADPHNADPH in the stroma by NADP+NADP^+ reductase using high-energy electrons.

Answer

The correct sequence begins with the photo-excitation of Photosystem II, followed by photolysis of water, electron transport coupled to ATP synthesis, re-excitation of electrons at Photosystem I, and finally the reduction of NADP+ to NADPH.
Non-cyclic photophosphorylation begins when photons excite Photosystem II (P680). The oxidized P680 pulls electrons from water via photolysis, producing oxygen and protons. Emitted electrons travel down an electron transport chain, generating a proton gradient that synthesizes ATP via chemiosmosis. These electrons then fill the electron gap in Photosystem I (P700), where a second photon absorption re-energizes them. Finally, NADP+NADP^+ reductase transfers these high-energy electrons to NADP+NADP^+, forming NADPHNADPH in the stroma.

Step-by-Step Solution

1
Identify the primary initiation step of the light reactions.
Photons strike Photosystem II (P680), exciting electrons which escape the chlorophyll a molecule.
Absorption of light energy is required to kickstart electron transport.
2
Determine how the electron deficit in Photosystem II is replenished.
Water undergoes photolysis (2H2O→4H++4e−+O22H_2O \rightarrow 4H^+ + 4e^- + O_2), providing replacement electrons to P680.
Photosystem II must recover lost electrons to remain functionally receptive to further light.
3
Trace the movement of excited electrons from Photosystem II.
Electrons travel along cytochrome carriers, generating a proton gradient across the thylakoid membrane to drive ATP photophosphorylation.
The step-down energy release of electrons is coupled to proton translocation.
4
Locate the second light absorption event.
Electrons enter Photosystem I (P700) and absorb secondary photon energy.
Electrons lose energy during passage through the transport chain and need re-excitation at Photosystem I.
5
Identify the final electron sink of the light-dependent phase.
Electrons pass to ferredoxin and NADP+NADP^+ reductase to produce NADPHNADPH in the stroma.
NADPH, alongside ATP, provides reducing power for carbon fixation in the light-independent phase.

Key Concept

Non-cyclic Photophosphorylation and Electron Transport in Photosynthesis
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Topics

Plant Nutrition and Photosynthesis

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