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Difficulty: HardPlant Nutrition and Photosynthesis

An illuminated suspension of green algae undergoing steady-state photosynthesis was supplied with radioactively labeled carbon dioxide (14CO2^{14}CO_2). When the light source was abruptly turned off while maintaining the supply of 14CO2^{14}CO_2, analysis revealed an immediate accumulation of glycerate-3-phosphate (PGA) alongside a rapid decline in ribulose 1,5-bisphosphate (RuBP). Which of the following physiological mechanisms explains why glycerate-3-phosphate accumulates in the absence of light?

  1. The reduction of glycerate-3-phosphate to glyceraldehyde-3-phosphate requires ATP and NADPH from the light-dependent stage, whereas carbon dioxide fixation into glycerate-3-phosphate proceeds temporarily using remaining ribulose 1,5-bisphosphate.Answer
  2. B
    Oxygen gas produced during the photolysis of water accumulates in the dark and directly oxidizes ribulose 1,5-bisphosphate into glycerate-3-phosphate.
  3. C
    The enzyme RuBisCO becomes permanently inactivated in the dark, causing ribulose 1,5-bisphosphate to break down directly into stored organic starch within the phloem.
  4. D
    Photolysis of water ceases, eliminating the essential organic carbon source required for the regeneration of ribulose 1,5-bisphosphate.

Answer

The reduction of glycerate-3-phosphate to glyceraldehyde-3-phosphate requires ATP and NADPH synthesized during the light-dependent phase, while carbon dioxide fixation onto ribulose 1,5-bisphosphate continues until the pool of ribulose 1,5-bisphosphate is depleted.
In photosynthesis, the light-dependent reactions in the thylakoids yield ATP and NADPH. These assimilation products drive the reduction of glycerate-3-phosphate (PGA) to glyceraldehyde-3-phosphate (G3P/GALP) in the stroma. When light is removed, ATP and NADPH levels drop immediately. Carbon dioxide fixation onto ribulose 1,5-bisphosphate (RuBP) continues briefly using residual RuBP, forming PGA. However, because PGA cannot be reduced without ATP and NADPH, PGA accumulates while RuBP is consumed and cannot be regenerated.

Step-by-Step Solution

1
Identify the chemical requirements of the light-independent stage (Calvin cycle).
Carbon fixation converts carbon dioxide (CO2CO_2) and ribulose 1,5-bisphosphate (RuBP) into glycerate-3-phosphate (PGA) via RuBisCO.
Carbon fixation itself does not directly consume ATP or NADPH.
2
Analyze the effect of eliminating the light source.
Light-dependent reactions stop, cutting off the immediate production of ATP and reduced NADP (NADPH).
ATP and NADPH are photochemically generated in the thylakoid membranes.
3
Evaluate the metabolic bottleneck caused by the absence of light energy products.
The reduction step converting PGA to glyceraldehyde-3-phosphate (GALP/G3P) halts due to lack of ATP and NADPH, leading to PGA accumulation and RuBP depletion.
RuBP regeneration also requires ATP; hence, existing RuBP is converted to PGA but cannot be regenerated.

Key Concept

Interdependence of Photosynthetic Light and Dark Reactions
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Topics

Plant Nutrition and Photosynthesis

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