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Difficulty: MediumPlant Nutrition and Photosynthesis

During the light-dependent stage of photosynthesis, non-cyclic electron flow converts solar energy into chemical energy. In which sequential order do the following key physiological and biochemical events occur during this process?

  1. 1Absorption of light photons by Photosystem II, causing photo-excitation and emission of electrons from chlorophyll aa.
  2. 2Photolysis of water molecules into oxygen gas, protons (H+H^+), and electrons to replace those lost by Photosystem II.
  3. 3Passage of excited electrons through an electron transport chain, releasing energy to pump protons and synthesize ATP.
  4. 4Re-excitation of electrons upon light absorption by Photosystem I and their subsequent transfer to ferredoxin.
  5. 5Transfer of high-energy electrons to NADP+NADP^+ reductase to reduce NADP+NADP^+ into NADPHNADPH.

Answer

The correct sequence begins with light absorption by Photosystem II, followed by photolysis of water to replace emitted electrons, passage of electrons through the transport chain to produce ATP, re-excitation of electrons at Photosystem I, and finally the reduction of NADP+NADP^+ to NADPHNADPH.
In non-cyclic photophosphorylation, light absorption by Photosystem II initiates electron emission. Water photolysis immediately supplies replacement electrons while yielding oxygen gas. As these electrons move down the electron transport chain, ATP is generated. The electrons then reach Photosystem I, absorb light energy to become re-excited, and are transferred via ferredoxin to reduce NADP+NADP^+ into NADPHNADPH.

Step-by-Step Solution

1
Identify the initiation event of non-cyclic electron transport.
Photosystem II absorbs light energy, causing chlorophyll electrons to reach an excited state and leave the reaction center.
Light absorption triggers electron emission, starting the primary photochemical reaction.
2
Determine how the electron vacancy in Photosystem II is replenished.
Water molecules undergo photolysis, producing O2O_2, protons, and replacement electrons.
Photolysis must occur right after electron loss to maintain continuous electron flow.
3
Trace the pathway of emitted electrons from Photosystem II.
Electrons travel along an electron transport chain, releasing energy used for ATP synthesis via photophosphorylation.
Energy released as electrons move down carriers generates a proton gradient for ATP production.
4
Identify the secondary photo-excitation event.
Electrons enter Photosystem I, absorb light energy, and are boosted to a higher energy level transferred to ferredoxin.
Photosystem I requires additional light energy input to boost electron energy for reduction reactions.
5
Identify the terminal step of non-cyclic photophosphorylation.
NADP+NADP^+ reductase utilizes electrons and stromal protons to reduce NADP+NADP^+ into NADPHNADPH.
Formation of NADPH terminates the non-cyclic pathway, storing reducing power for the Calvin cycle.

Key Concept

Non-cyclic photophosphorylation electron transport sequence
Estimated Time:1m 30s
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Topics

Plant Nutrition and Photosynthesis

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