Examkin

Soru

Zorluk: ZorPlant 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.Cevap
  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.

Cevap

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.

Adım Adım Çözüm

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.

Anahtar Kavram

Interdependence of Photosynthetic Light and Dark Reactions
Bu soruyu puanla

Konular

Plant Nutrition and Photosynthesis

Bu konudan daha fazla soru