Question:
The mitochondria play a direct role in which of the following subtype(s) of $\text{C}_4$ photosynthesis?}
The mitochondria play a direct role in which of the following subtype(s) of $\text{C}_4$ photosynthesis?}
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To remember where decarboxylation occurs in $\text{C}_4$ subtypes:
NADP-ME subtype $\rightarrow$ Occurs in chloroplast stroma.
NAD-ME subtype $\rightarrow$ Occurs in mitochondria.
PCK subtype $\rightarrow$ Occurs in cytosol, but requires mitochondrial ATP.
NADP-ME subtype $\rightarrow$ Occurs in chloroplast stroma.
NAD-ME subtype $\rightarrow$ Occurs in mitochondria.
PCK subtype $\rightarrow$ Occurs in cytosol, but requires mitochondrial ATP.
Updated On: Aug 7, 2026
- NADP-dependent malic enzyme (NADP-ME) subtype
- NADP-ME and NAD-dependent malic enzyme (NAD-ME) subtypes
- PEP-carboxykinase (PCK) subtype
- NAD-ME and PCK subtypes
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The Correct Option is D
Solution and Explanation
Step 1: Understanding the Concept:
$\text{C}_4$ plants have evolved metabolic adaptations to reduce photorespiration by concentrating carbon dioxide ($\text{CO}_2$) around Rubisco in bundle sheath cells.
This concentration mechanism is divided into three subtypes based on the primary enzyme used to decarboxylate $\text{C}_4$ acids and the subcellular compartment where decarboxylation occurs.
Step 2: Detailed Explanation:
The three biochemical subtypes of $\text{C}_4$ photosynthesis are:
1. NADP-ME Subtype (e.g., maize, sugarcane): Malate is transported to bundle sheath cells, where decarboxylation is catalyzed by NADP-malic enzyme inside the chloroplasts. Mitochondria are not directly involved.
2. NAD-ME Subtype (e.g., millet, amaranth): Aspartate is transported to bundle sheath cells and converted to malate.
This malate enters the mitochondria, where it is decarboxylated by NAD-malic enzyme to release $\text{CO}_2$ and produce pyruvate. Thus, mitochondria play a direct role.
3. PCK Subtype (e.g., guinea grass): Aspartate is converted to oxaloacetate (OAA) in the cytosol.
This OAA is decarboxylated in the cytosol by PEP-carboxykinase.
However, this pathway depends on the mitochondria to generate the ATP needed by the PCK enzyme, which is produced through the oxidation of malate inside the mitochondria.
Therefore, mitochondria are directly involved in both the NAD-ME and PCK subtypes of $\text{C}_4$ photosynthesis.
Step 3: Final Answer:
The mitochondria play a direct role in the NAD-ME and PCK subtypes of $\text{C}_4$ photosynthesis, which corresponds to Option (D).
$\text{C}_4$ plants have evolved metabolic adaptations to reduce photorespiration by concentrating carbon dioxide ($\text{CO}_2$) around Rubisco in bundle sheath cells.
This concentration mechanism is divided into three subtypes based on the primary enzyme used to decarboxylate $\text{C}_4$ acids and the subcellular compartment where decarboxylation occurs.
Step 2: Detailed Explanation:
The three biochemical subtypes of $\text{C}_4$ photosynthesis are:
1. NADP-ME Subtype (e.g., maize, sugarcane): Malate is transported to bundle sheath cells, where decarboxylation is catalyzed by NADP-malic enzyme inside the chloroplasts. Mitochondria are not directly involved.
2. NAD-ME Subtype (e.g., millet, amaranth): Aspartate is transported to bundle sheath cells and converted to malate.
This malate enters the mitochondria, where it is decarboxylated by NAD-malic enzyme to release $\text{CO}_2$ and produce pyruvate. Thus, mitochondria play a direct role.
3. PCK Subtype (e.g., guinea grass): Aspartate is converted to oxaloacetate (OAA) in the cytosol.
This OAA is decarboxylated in the cytosol by PEP-carboxykinase.
However, this pathway depends on the mitochondria to generate the ATP needed by the PCK enzyme, which is produced through the oxidation of malate inside the mitochondria.
Therefore, mitochondria are directly involved in both the NAD-ME and PCK subtypes of $\text{C}_4$ photosynthesis.
Step 3: Final Answer:
The mitochondria play a direct role in the NAD-ME and PCK subtypes of $\text{C}_4$ photosynthesis, which corresponds to Option (D).
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