Question

Photorespiration reaction catalyzed by RuBisCO is shown below :
\[ \text{RuBP} + \text{O}_2 \rightarrow \text{3-Phosphoglycerate} + \text{X} \] Identify "X" from the given options :

• A. Malate
• B. Phosphoenolpyruvate
• C. 2-Phosphoglycolate
• D. Oxaloacetate

Hint:

Photorespiration is often termed the $C_2$ cycle precisely because the very first stable organic product formed outside the main path contains a chain of 2 carbon atoms (2-Phosphoglycolate). Remembering this carbon-counting trick will help you solve problems involving alternative photosynthetic cycles instantly!

Answer 1

The Correct Option is C

Step 1: Identify the reaction type.
The enzyme RuBisCO is acting here with oxygen, not carbon dioxide, so this is the oxygenase activity that starts photorespiration.
Step 2: Look at the substrate.
RuBP (Ribulose-1,5-bisphosphate) is a 5 carbon sugar, and oxygen adds no carbon, so the total carbon entering the reaction is 5.
Step 3: Note the known product.
One product is 3-Phosphoglycerate, which has 3 carbons.
Step 4: Balance the carbons.
If 5 carbons go in and 3 carbons leave in 3-Phosphoglycerate, then the unknown X must hold $5 - 3 = 2$ carbons.
Step 5: Name the 2 carbon product.
The standard 2 carbon phosphorylated molecule made in this step is 2-Phosphoglycolate, so X is 2-Phosphoglycolate.
Step 6: Reject the other choices.
Malate and Oxaloacetate are 4 carbon acids of the $C_4$ pathway, and Phosphoenolpyruvate is a 3 carbon acceptor in $C_4$ plants, so none of them fit. The answer is the 2 carbon molecule.
\[ \boxed{\text{2-Phosphoglycolate}} \]