Question

Which of the following statements is correct as regard to $C_2$ photorespiratory cycle?

• A. The $O_2$ is consumed in the chloroplasts while the $CO_2$ is released in the mitochondria
• B. The $O_2$ is consumed in the chloroplasts while the $CO_2$ is released in the peroxisomes
• C. The $O_2$ is consumed in the peroxisomes while the $CO_2$ is released in the mitochondria
• D. The term photorespiration is a misnomer as there is no release of the $CO_2$ in it.

Hint:

Organelle sequence: Chloroplast $\rightarrow$ Peroxisome $\rightarrow$ Mitochondria.
Mnemonic: CPM (like "Counts Per Minute").
Oxygen is taken in "C" and "P".
Carbon dioxide is lost in "M".

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The $C_2$ cycle, or photorespiration, is a process in plants where Rubisco fixes oxygen instead of carbon dioxide. The question seeks to identify the correct physiological locations for the gas exchange events in this pathway.

Step 2: Detailed Explanation:

• Initiation in Chloroplasts: Photorespiration begins when the enzyme Rubisco acts as an oxygenase. It adds $O_2$ (instead of $CO_2$) to Ribulose-1,5-bisphosphate (RuBP). This oxygen consumption occurs strictly in the chloroplast.

• Primary Products: This reaction produces one molecule of 3-PGA (3-phosphoglycerate) and one molecule of 2-phosphoglycolate (a 2-carbon compound, hence the name $C_2$ cycle).

• Transition to Peroxisomes: The phosphoglycolate is converted to glycolate, which moves into the peroxisome. Here, more $O_2$ is consumed by glycolate oxidase to produce glyoxylate and $H_2O_2$. Glyoxylate is then transaminated to Glycine.

• Decarboxylation in Mitochondria: Two molecules of Glycine enter the mitochondria. The enzyme Glycine Decarboxylase Complex (GDC) converts them into one molecule of Serine, while releasing one molecule of $CO_2$ and one molecule of $NH_3$.

• Net Result: Photorespiration results in a loss of fixed carbon (as $CO_2$) and energy (ATP/NADPH), making it an inherently wasteful process for the plant.

• Analysis of Option A: This option correctly captures the start ($O_2$ use in chloroplasts) and the most significant end-point ($CO_2$ loss in mitochondria) of the cycle.

Step 3: Final Answer:
In the $C_2$ cycle, oxygen uptake begins in the chloroplasts through the activity of Rubisco, and the characteristic release of carbon dioxide occurs in the mitochondria during glycine metabolism.