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Difficulty: MediumPlant Nutrition and Photosynthesis

In a laboratory investigation, healthy green plant leaves are exposed to continuous light under controlled atmospheric conditions. Which of the following environmental changes would cause an immediate increase in the concentration of ribulose 1,5-bisphosphate (RuBPRuBP) within the chloroplast stroma?

  1. A sudden reduction in carbon dioxide concentrationAnswer
  2. B
    Complete removal of light illumination
  3. C
    Blockage of phloem translocation of organic sugars
  4. D
    A significant increase in water uptake by xylem vessels

Answer

A sudden reduction in carbon dioxide concentration
In the light-independent phase of photosynthesis, ribulose 1,5-bisphosphate (RuBPRuBP) combines with carbon dioxide to form 3-phosphoglycerate (3-PGA3\text{-PGA}). If carbon dioxide concentration is suddenly reduced while light is maintained, RuBPRuBP consumption ceases. However, the light reactions continue providing ATPATP and NADPHNADPH needed to convert remaining triose phosphates into RuBPRuBP. As a consequence, RuBPRuBP accumulates immediately in the stroma.

Step-by-Step Solution

1
Identify the chemical role of ribulose 1,5-bisphosphate (RuBPRuBP) in photosynthesis.
RuBPRuBP acts as the 5-carbon primary carbon dioxide acceptor in the stroma during the light-independent stage (Calvin cycle).
The enzyme RuBisCO catalyzes the carboxylation of RuBPRuBP with CO2CO_2 to produce 3-phosphoglycerate (3-PGA3\text{-PGA}).
2
Analyze the effect of restricting carbon dioxide supply under continuous light.
The consumption of RuBPRuBP stops because there is no CO2CO_2 to fix, but its regeneration from existing triose phosphates continues temporarily using available ATPATP and NADPHNADPH.
This imbalance between continuous regeneration and zero consumption leads to an immediate buildup of RuBPRuBP in the chloroplast stroma.

Key Concept

Factors affecting Calvin cycle intermediates and carbon dioxide fixation
Estimated Time:1m 15s
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Topics

Plant Nutrition and Photosynthesis

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