The Question is from neet-botany photosynthesis-in-higher-plants , and it was created by the author Mr. NARESH KUMAR K

Question

In a hypothetical experiment, a scientist genetically modifies a C4 plant to lack PEP carboxylase. Which of the following would be the MOST likely outcome under normal atmospheric conditions?

Mr. NARESH KUMAR K · Accepted Answer

## CORE INFORMATION
✓ **Answer**: B
## CONCEPT FRAMEWORK
 **Primary Concept**: C4 Photosynthesis and the role of PEP carboxylase
 **Related Topics**: Photorespiration, CAM photosynthesis, CO2 fixation
 **Prerequisites**: Understanding of the C4 photosynthetic pathway and the role of key enzymes.
## SOLUTION PATHWAY
**Given Information**:
A C4 plant is genetically modified to lack PEP carboxylase.
**Method & Steps**:
1. Understand the role of PEP carboxylase in C4 photosynthesis.
  Reasoning: PEP carboxylase is a crucial enzyme in C4 plants. It fixes CO2 in mesophyll cells, forming oxaloacetate, which is then converted to malate. Malate is transported to bundle sheath cells, where CO2 is released and used in the Calvin cycle. This spatial separation of CO2 fixation and the Calvin cycle minimizes photorespiration.
  Formula/Rule: No specific formula is required.
2. Consider the consequences of removing PEP carboxylase.
  Working: Without PEP carboxylase, the initial CO2 fixation step in mesophyll cells is eliminated.  This means CO2 will not be efficiently concentrated in the bundle sheath cells, leading to a reduction in CO2 fixation in the Calvin cycle. Consequently, RuBisCo will be more likely to bind to O2 instead of CO2, resulting in increased photorespiration.
  Key Point:  The absence of PEP carboxylase disrupts the entire C4 mechanism designed to suppress photorespiration.
3. Analyze the options and choose the most likely outcome.
  Result: The plant will exhibit significantly reduced CO2 fixation and increased photorespiration.
  Verification: The other options are less likely because C4 plants don't switch to CAM under these conditions, and removing a crucial step wouldn't increase efficiency.  The water requirement is not directly affected by PEP carboxylase deficiency in this manner.
 ## OPTION ANALYSIS
A :x:  Switching to CAM photosynthesis requires a completely different anatomical and physiological adaptation, not just the absence of one enzyme. 
B ✓: This is the direct consequence of losing the initial CO2 fixation step, leading to less CO2 for the Calvin cycle and more photorespiration.
C :x: CO2 fixation would definitely be reduced, not remain unchanged.
D :x: Removing a key enzyme would not improve efficiency; it would severely impair the plant's ability to photosynthesize.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing C4 and CAM photosynthesis: Understanding the distinct mechanisms of these pathways is crucial.
  2.Overlooking the role of RuBisCo: Remember that RuBisCo's affinity for O2 increases when CO2 concentration is low, increasing photorespiration.
 **Quick Tips**:
  1.Focus on the specific role of PEP carboxylase in the C4 pathway.
  2.Remember that C4 photosynthesis is an adaptation to reduce photorespiration.

Mr. NARESH KUMAR K · Answer

The plant would switch to CAM photosynthesis.

Mr. NARESH KUMAR K · Answer

The correct Option is "The plant would exhibit significantly reduced CO2 fixation and increased photorespiration."

Mr. NARESH KUMAR K · Answer

CO2 fixation would remain unchanged, but the plant would require more water.

Mr. NARESH KUMAR K · Answer

The plant would grow more efficiently due to reduced energy expenditure in the C4 pathway.

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