The Question is from neet-botany photosynthesis-in-higher-plants , and it was created by the author NISHANTH

Question

Under conditions of high temperature and light intensity, why do C4 plants show higher photosynthetic rates compared to C3 plants?

NISHANTH · Accepted Answer

## CORE INFORMATION
✓ **Answer**: D
## CONCEPT FRAMEWORK
 **Primary Concept**: Photosynthesis in C3 and C4 plants
 **Related Topics**: Photorespiration, Carbon fixation, Enzyme kinetics, Stomatal conductance
 **Prerequisites**: Understanding of the Calvin cycle, the C3 and C4 pathways
## SOLUTION PATHWAY
**Given Information**:
  High temperature and light intensity conditions.
  Comparison between C3 and C4 plants' photosynthetic rates.
**Method & Steps**:
 1. Understanding C3 and C4 photosynthesis:
  Reasoning: C3 plants use RuBisCO for initial CO2 fixation, which can also bind O2 leading to photorespiration, reducing efficiency.
  Formula/Rule:  None directly applicable, but understanding of the biochemical pathways is crucial.
 2. Analyzing the effect of high temperature and light intensity:
  Working: High temperatures and light intensify photorespiration in C3 plants because RuBisCO's oxygenase activity increases relative to its carboxylase activity.  C4 plants, however, spatially separate initial CO2 fixation (in mesophyll cells using PEP carboxylase) from the Calvin cycle (in bundle sheath cells). This concentrates CO2 around RuBisCO in bundle sheath cells, minimizing oxygenase activity and photorespiration.
  Key Point: PEP carboxylase has a much higher affinity for CO2 than RuBisCO, allowing efficient CO2 uptake even when stomata are partially closed to reduce water loss.
 3. Comparing photosynthetic rates:
  Result: Under high temperature and light, C4 plants maintain higher photosynthetic rates due to the suppression of photorespiration.
  Verification:  The spatial separation of CO2 fixation in C4 plants is a key adaptation to hot, dry environments where photorespiration would severely limit photosynthetic efficiency.
 ## OPTION ANALYSIS
A :x: While C4 plants do have a more efficient CO2 concentrating mechanism, it's not the primary reason for higher rates under high temperature and light. The key is minimizing photorespiration.
B :x:  C4 plants may sometimes open their stomata less frequently than C3 plants under water stress conditions, to reduce water loss. This is not the main reason for higher photosynthetic rates.
C :x: Both C3 and C4 plants utilize a similar range of light wavelengths for photosynthesis.
D ✓: This accurately describes the key advantage of C4 photosynthesis under conditions of high temperature and light: the spatial separation of CO2 fixation minimizes photorespiration, allowing for higher photosynthetic rates.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing the roles of RuBisCO and PEP carboxylase: Remember RuBisCO is the enzyme responsible for the Calvin cycle in both C3 and C4 plants, but PEP carboxylase is crucial for the initial CO2 fixation in C4 plants.
  2.Focusing solely on CO2 affinity: While CO2 affinity is important, the spatial separation and the resulting reduction in photorespiration are the main reasons for the higher photosynthetic rates in C4 plants under stress.
 **Quick Tips**:
  1.Remember the key difference: Spatial separation of initial CO2 fixation and the Calvin cycle.
  2.Think about the effect of high temperatures on RuBisCO's oxygenase activity.

NISHANTH · Answer

C4 plants have a higher affinity for CO2 than C3 plants.

NISHANTH · Answer

C4 plants open their stomata more frequently than C3 plants.

NISHANTH · Answer

C4 plants can utilize a wider range of light wavelengths compared to C3 plants.

NISHANTH · Answer

The correct Option is "Spatial separation of initial CO2 fixation and the Calvin cycle minimizes photorespiration."

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