NCERT Class 11 Science — Chapter 1: Photosynthesis in Higher Plants

The correct option is Negligible or absent.

Explanation

Photorespiration is a physiological process in plants where the enzyme RuBisCO binds with oxygen (O₂) instead of carbon dioxide (CO₂). This process is considered wasteful because it consumes ATP and releases CO₂ without producing glucose, thereby reducing photosynthetic efficiency.

Physiological Distinction between C3 and C4 Plants:

• C3 Plants: In these plants, the primary CO₂ acceptor is RuBP, and the reaction is catalyzed by RuBisCO. Since RuBisCO has an affinity for both CO₂ and O₂, high oxygen concentration or high temperatures lead to the binding of O₂, resulting in significant photorespiration.
• C4 Plants: These plants have evolved a mechanism to concentrate CO₂ at the site of RuBisCO activity. They utilize a specialized leaf structure called Kranz anatomy and a specific biochemical pathway (Hatch-Slack pathway).
  • The initial fixation of CO₂ occurs in mesophyll cells via the enzyme PEP carboxylase, which has no affinity for O₂.
  • The fixed carbon is transported to bundle sheath cells, where CO₂ is released, creating a high-CO₂ environment around RuBisCO.
• Conclusion: This mechanism ensures that RuBisCO functions almost exclusively as a carboxylase. Consequently, photorespiration is negligible or absent in C4 plants.

Key Takeaway: The primary physiological advantage of C4 plants (e.g., Maize, Sorghum) over C3 plants is the absence of photorespiration, which allows for higher productivity and better adaptation to high temperatures and light intensities.