Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 10.5 Alternative Mechanisms of Carbon Fixation Have Evolved in Hot, Arid Climates
Plants have evolved adaptations to minimize water loss and reduce photorespiration under hot, dry conditions. Two major strategies are C₄ photosynthesis and CAM.
Photorespiration in C₃ Plants
C₃ plants use rubisco to fix CO₂, forming a 3-carbon compound
Under hot, dry conditions, stomata close → CO₂ drops, O₂ increases
Rubisco binds O₂ instead of CO₂, initiating photorespiration
Photorespiration consumes ATP and releases CO₂ without producing sugar
Can reduce carbon fixation efficiency by up to 50% in some crop species
C₄ Photosynthesis
Found in plants like corn and sugarcane
Separates initial carbon fixation and the Calvin cycle into different cell types
CO₂ is first fixed in mesophyll cells by PEP carboxylase (not rubisco) → forms 4-carbon compound
Compound moves to bundle-sheath cells → CO₂ is released for rubisco and the Calvin cycle
Requires ATP to regenerate PEP, but minimizes photorespiration
Effective in high light, high temperature, and dry environments
CAM Photosynthesis
Found in succulents like pineapples, cacti, and aloe
Separates steps temporally:
Stomata open at night → CO₂ is fixed into organic acids
Stomata close during the day → CO₂ is released from acids and enters Calvin cycle
Reduces water loss while maintaining carbon fixation
CAM and C₄ both use preliminary CO₂ fixation, but differ in cell vs. time separation
Climate and Crop Implications
Rising CO₂ may favor C₃ plants by reducing photorespiration
Rising temperatures may increase photorespiration, disadvantaging C₃ plants
C₄ plants are less affected by rising CO₂ but perform better in heat
Genetic engineering aims to introduce C₄ traits into C₃ crops (e.g., rice) to boost yield
Climate change may shift balance between C₃ and C₄ plant populations regionally
In a Nutshell
To adapt to hot, dry climates, some plants evolved C₄ and CAM pathways that reduce photorespiration and water loss. These modifications increase efficiency of carbon fixation under stress conditions and have major implications for agriculture and climate resilience.