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Difficulty: MediumPlant Nutrition and Photosynthesis

Arrange the following sequential biochemical events of the light-independent stage (Calvin cycle) of photosynthesis in the correct chronological order from beginning to end.

  1. 1Carbon dioxide combines with ribulose 1,5-bisphosphate (RuBP) catalyzed by the enzyme RuBisCO.
  2. 2An unstable 6-carbon intermediate rapidly splits to yield two molecules of 3-phosphoglyceric acid (PGA).
  3. 3ATP and NADPH from the light-dependent reactions convert 3-phosphoglyceric acid into glyceraldehyde-3-phosphate (G3P).
  4. 4Glyceraldehyde-3-phosphate molecules are utilized to synthesize hexose sugars and regenerate RuBP.

Answer

The correct chronological sequence of the Calvin cycle is: Carbon dioxide fixation with RuBP by RuBisCO →\rightarrow Formation of 3-phosphoglyceric acid (PGA) →\rightarrow Reduction of PGA to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH →\rightarrow Synthesis of hexose sugars and regeneration of RuBP.
The Calvin cycle progresses through three main stages: Carbon Fixation, Reduction, and Regeneration. First, carbon dioxide combines with RuBP to produce an unstable 6-carbon intermediate. Second, this intermediate breaks down into two molecules of 3-phosphoglycerate (PGA). Third, PGA is phosphorylated and reduced by ATP and NADPH to form glyceraldehyde-3-phosphate (G3P). Finally, G3P is used to produce carbohydrates such as glucose and to regenerate RuBP so the cycle can continue.

Step-by-Step Solution

1
Identify the initial carboxylation step
Carbon dioxide binds with ribulose 1,5-bisphosphate (RuBP) via RuBisCO enzyme.
Photosynthetic carbon fixation begins when atmospheric CO2CO_2 enters the chloroplast stroma and reacts with the 5-carbon sugar RuBP.
2
Track the immediate cleavage of the 6-carbon intermediate
Two molecules of 3-phosphoglycerate (PGA) are formed.
The 6-carbon compound produced by carboxylation is chemically unstable and instantly hydrolyzes into two stable 3-carbon PGA molecules.
3
Identify the reduction phase requiring light reaction products
PGA is phosphorylated by ATP and reduced by NADPH to form G3P (triose phosphate).
Chemical energy (ATPATP) and reducing power (NADPHNADPH) from the light-dependent stage are consumed to reduce the carboxyl group of PGA into an aldehyde group in G3P.
4
Determine the output and regeneration phase
G3P is converted into hexose sugars (glucose) and RuBP is regenerated.
For every six molecules of G3P produced, one net G3P is allocated to carbohydrate synthesis, while the remaining five undergo enzymatic conversion using ATP to regenerate RuBP.

Key Concept

Calvin cycle sequence: Carbon fixation, cleavage to PGA, reduction to G3P (triose phosphate), and sugar synthesis/RuBP regeneration.
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Topics

Plant Nutrition and Photosynthesis

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