9.3 Photorespiration and C4/CAM pathways

Photorespiration can significantly impact photosynthesis efficiency, especially in C3 plants. This process occurs when RuBisCO fixes oxygen instead of carbon dioxide, reducing overall carbon gain. It's particularly problematic in hot, dry environments.

Plants have evolved different photosynthetic pathways to combat photorespiration. C3 is the most common, while C4 and CAM are adaptations for harsh conditions. These pathways help plants thrive in various environments, from grasslands to deserts.

Photorespiration and Its Impact on Photosynthesis

Photorespiration and efficiency impact

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• Photorespiration occurs when RuBisCO enzyme fixes oxygen instead of carbon dioxide competes with carbon fixation in photosynthesis, reducing overall photosynthetic efficiency
• Favored by high temperatures, low CO2 concentrations, and high light intensities (deserts, hot climates)
• Consumes energy and releases previously fixed carbon dioxide, reducing net carbon gain from photosynthesis
• Can decrease photosynthetic efficiency by up to 50% in C3 plants under unfavorable conditions (drought, heat stress)

Comparison of C3, C4, and CAM Photosynthetic Pathways

C3, C4, and CAM pathways

• C3 photosynthesis: most common pathway used by majority of plant species (wheat, rice, soybeans)
  • Carbon fixation in mesophyll cells using RuBisCO enzyme
  • Directly fixes CO2 into 3-carbon compound 3-phosphoglycerate (3-PGA)
  • Vulnerable to photorespiration under high temperatures and low CO2 conditions
• C4 photosynthesis: adaptation to minimize photorespiration and enhance efficiency in hot, dry environments (grasslands, savannas)
  • Carbon fixation in two stages and cell types: mesophyll and bundle sheath cells
  • Mesophyll cells initially fix CO2 into 4-carbon compound using phosphoenolpyruvate carboxylase (PEPC) enzyme
  • 4-carbon compound transported to bundle sheath cells, decarboxylated to release CO2 for RuBisCO
  • High CO2 concentration in bundle sheath cells suppresses photorespiration
• CAM photosynthesis: adaptation to conserve water and minimize photorespiration in arid environments (deserts, rock outcrops)
  • Carbon fixation in same cell but at different times (temporal separation)
  • At night when stomata open, CO2 fixed into 4-carbon compound by PEPC, stored as malic acid in vacuoles
  • During day when stomata closed, stored malic acid decarboxylated, releasing CO2 for RuBisCO in chloroplasts
  • Temporal separation of CO2 fixation and Calvin cycle minimizes water loss and photorespiration

Adaptations for harsh environments

• C4 adaptations:
  1. Spatial separation of initial CO2 fixation (mesophyll) and Calvin cycle (bundle sheath)
  2. High CO2 concentration in bundle sheath suppresses photorespiration by favoring RuBisCO's carboxylase over oxygenase activity
  3. Efficient CO2 fixation even under low atmospheric CO2 and high temperatures
• CAM adaptations:
  1. Temporal separation of CO2 fixation (night) and Calvin cycle (day)
  2. Nocturnal CO2 fixation when stomata open reduces water loss
  3. Fixed CO2 stored as malic acid in vacuoles provides CO2 source for Calvin cycle during day when stomata closed
  4. Minimizes photorespiration by maintaining high CO2 around RuBisCO during day

Ecological roles of C4 and CAM plants

• C4 plants dominate grasslands, savannas, subtropical regions with high temperatures and moderate to low rainfall
  • Important crops like maize, sugarcane, sorghum use C4 pathway
  • Contribute significantly to global primary productivity and carbon fixation
• CAM plants adapted to arid and semi-arid environments like deserts, rock outcrops
  • Examples: cacti, agaves, many succulents
  • Crucial for ecosystem function and biodiversity in water-limited environments
  • Provide food and shelter for various desert animals
• Both C4 and CAM plants contribute to resilience and productivity of ecosystems under changing climatic conditions, especially in regions facing increasing temperatures and water scarcity