Question

Choose the correct sequence of no. of molecules formed when 6 mol of CO\textsubscript{2 enters into calvin cycle pathway}

• A. 4 xylulose 5P, 6 Ribulose 5P
• B. 4 xylulose 5P, 2 Ribose 5P, 6 Ribulose 5P
• C. 2 Ribose 5P, 4 xylulose 5P, 6 Ribulose 5P
• D. 4 xylulose 5P, 2 Ribulose 5P, 2 Ribose 5P

Hint:

To remember the stoichiometry of the Calvin cycle for 6 CO\textsubscript{2} inputs, focus on the carbon flow: \begin{itemize} \item 6 CO\textsubscript{2} + 6 RuBP (6x5C=30C) $\rightarrow$ 12 PGA (12x3C=36C) \item 12 PGA $\rightarrow$ 12 G3P (12x3C=36C) \item 2 G3P (2x3C=6C) leave for glucose synthesis. \item 10 G3P (10x3C=30C) regenerate 6 RuBP (6x5C=30C) via intermediates including 4 Xylulose-5P, 2 Ribose-5P, and finally 6 Ribulose-5P. \end{itemize}

Answer 1

The Correct Option is B

Step 1: Understand Calvin Cycle Stoichiometry for 6 CO\textsubscript{2}
The Calvin cycle must run 6 times to produce one molecule of glucose. Each turn fixes one CO\textsubscript{2}.
For 6 CO\textsubscript{2} molecules to enter the cycle:
1. Carboxylation: 6 molecules of Ribulose-1,5-bisphosphate (RuBP, a 5-carbon sugar) combine with 6 molecules of CO\textsubscript{2} to form 12 molecules of 3-Phosphoglycerate (PGA, a 3-carbon compound). 2. Reduction: The 12 molecules of PGA are then reduced to 12 molecules of Glyceraldehyde-3-phosphate (G3P or triose phosphate, a 3-carbon sugar).
3. Regeneration: Out of the 12 G3P molecules, 2 G3P molecules leave the cycle to synthesize glucose or other carbohydrates. The remaining 10 G3P molecules (10 x 3 carbons = 30 carbons) are used to regenerate 6 molecules of RuBP (6 x 5 carbons = 30 carbons) to continue the cycle. Step 2: Focus on the Regeneration Phase Intermediates
The regeneration phase involves a complex series of reactions that interconvert various sugar phosphates. For the regeneration of 6 RuBP from 10 G3P, the key 5-carbon (pentose) sugar phosphate intermediates that are formed and then ultimately converted to Ribulose-5-phosphate (which is then phosphorylated to RuBP) are:
\begin{itemize} \item Xylulose-5-phosphate (Xylulose 5P): Approximately 4 molecules are formed as intermediates. \item Ribose-5-phosphate (Ribose 5P): Approximately 2 molecules are formed as intermediates. \item Ribulose-5-phosphate (Ribulose 5P): All 6 molecules of RuBP are regenerated via the phosphorylation of 6 molecules of Ribulose-5-phosphate. This is the final 5-carbon sugar before it becomes RuBP. \end{itemize} So, for 6 turns of the Calvin cycle, the number of molecules of these key 5-carbon intermediates formed during regeneration are: 4 Xylulose 5P, 2 Ribose 5P, and 6 Ribulose 5P. Step 3: Compare with Options
Let's check the given options against our derived sequence (4 Xylulose 5P, 2 Ribose 5P, 6 Ribulose 5P):
% Option (1) 4 xylulose 5P, 6 Ribulose 5P (Missing Ribose 5P)
% Option (2) 4 xylulose 5P, 2 Ribose 5P, 6 Ribulose 5P (Matches our sequence and numbers)
% Option (3) 2 Ribose 5P, 4 xylulose 5P, 6 Ribulose 5P (Numbers are correct but the order is different, though the question asks for a "sequence" implying the constituents)
% Option (4) 4 xylulose 5P, 2 Ribulose 5P, 2 Ribose 5P (Incorrect numbers for Ribulose 5P and Ribose 5P)
Option (2) provides the correct types and quantities of the 5-carbon molecules involved in the regeneration phase for 6 CO\textsubscript{2} inputs.