How is triose phosphate used to regenerate RuBP and produce carbohydrates?

Sourav PanNovember 9, 2024

Answered

Triose phosphate, specifically glyceraldehyde 3-phosphate (G3P), plays a critical role in both the regeneration of ribulose bisphosphate (RuBP) and the production of carbohydrates in the Calvin cycle. Here’s how this process unfolds:

Regeneration of RuBP

1. Production of G3P:
   • During the reduction phase of the Calvin cycle, 3-phosphoglycerate (3-PGA) is converted into G3P using ATP and NADPH. For every three molecules of CO₂ that enter the cycle, six molecules of G3P are produced, but only one G3P exits the cycle to contribute to carbohydrate synthesis.
2. Recycling G3P to Regenerate RuBP:
   • The remaining five G3P molecules are utilized to regenerate three molecules of RuBP. This regeneration process requires ATP and involves several enzymatic steps:
     • Conversion to Ribulose-5-Phosphate: The five G3P molecules undergo a series of transformations involving enzymes such as transketolase and aldolase, ultimately leading to the production of ribulose-5-phosphate (Ru5P).
     • Phosphorylation to RuBP: Finally, Ru5P is phosphorylated by phosphoribulokinase using ATP to regenerate RuBP. This step is crucial for allowing the Calvin cycle to continue by providing the substrate for CO₂ fixation .

Production of Carbohydrates

1. Utilization of G3P for Carbohydrate Synthesis:
   • The G3P molecule that exits the Calvin cycle can be used as a building block for synthesizing various carbohydrates. Specifically, two G3P molecules can combine to form one glucose molecule (C₆H₁₂O₆), which is a fundamental energy source for plants.
   • Additionally, G3P can be converted into other carbohydrates such as fructose and sucrose or stored as starch, depending on the plant’s metabolic needs .
2. Net Gain from the Calvin Cycle:
   • For every three turns of the Calvin cycle (fixing three CO₂ molecules), one net G3P molecule is produced that can be used for carbohydrate synthesis. This requires a total input of nine ATP and six NADPH molecules per three CO₂ fixed .