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Difficulty: HardPlant Nutrition and Photosynthesis

A plant physiologist selectively inhibits the photolysis of water in isolated chloroplasts while artificially maintaining a constant supply of ATP, NADPH, and carbon dioxide within the stroma under continuous light. Which of the following correctly predicts the immediate effect on oxygen evolution and carbon assimilation?

  1. Oxygen evolution ceases completely, but carbon assimilation continues using the supplied ATP and NADPH.Answer
  2. B
    Oxygen evolution continues by splitting carbon dioxide, while carbon assimilation stops due to lack of water splitting.
  3. C
    Both oxygen evolution and carbon assimilation stop immediately because water translocates organic nutrients through xylem.
  4. D
    Oxygen evolution increases to compensate for missing photolysis products, while carbon assimilation decreases.

Answer

Oxygen evolution ceases completely, but carbon assimilation continues using the supplied ATP and NADPH.
Molecular oxygen evolved during photosynthesis originates strictly from the photolysis of water in the thylakoid lumen. Inhibiting photolysis halts oxygen release. However, the light-independent Calvin cycle takes place in the stroma and depends only on CO2CO_2, ATP, and NADPH. Since ATP and NADPH are artificially supplied, carbon fixation and reduction proceed normally.

Step-by-Step Solution

1
Identify the primary source of oxygen evolution in photosynthesis.
Photolysis of water (2H2O→O2+4H++4e−2H_2O \rightarrow O_2 + 4H^+ + 4e^-) during light-dependent reactions in the thylakoid membrane is responsible for releasing oxygen gas.
Oxygen does not come from carbon dioxide; it comes strictly from the splitting of water molecules.
2
Determine the impact of inhibiting photolysis on oxygen production.
Inhibiting photolysis eliminates oxygen evolution entirely.
No water molecules are being split to generate molecular oxygen.
3
Analyze the requirements for carbon assimilation in the light-independent reactions (Calvin cycle).
The Calvin cycle requires carbon dioxide, ATP (energy), and NADPH (reducing power) within the stroma to produce triose phosphate sugars.
Although ATP and NADPH are normally generated by light-dependent photophosphorylation, providing them artificially allows the Calvin cycle to continue functioning.

Key Concept

Decoupling of Photolysis and Calvin Cycle Reactions
Estimated Time:2m 0s
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Topics

Plant Nutrition and Photosynthesis

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