15.3 Photorespiration and C4/CAM Pathways

Photorespiration is a process that reduces photosynthetic efficiency in C3 plants. It occurs when RuBisCO fixes oxygen instead of carbon dioxide, wasting energy. C3 plants struggle in hot, dry environments due to increased photorespiration.

C4 and CAM plants evolved to minimize photorespiration and improve water use efficiency. C4 plants spatially separate carbon fixation steps, while CAM plants temporally separate them. Both adaptations allow for better survival in challenging environments.

Photorespiration and C3 Plants

Understanding Photorespiration

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Top images from around the web for Understanding Photorespiration

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  RuBisCO - wikidoc View original

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• Photorespiration occurs when RuBisCO fixes oxygen instead of carbon dioxide
• Process wastes energy and reduces photosynthetic efficiency in C3 plants
• Triggered by high temperatures, low CO2 concentrations, or high O2 levels
• Results in the production of 3-phosphoglycerate and 2-phosphoglycolate
• 2-phosphoglycolate must be converted back to 3-phosphoglycerate through a series of reactions
  • Involves peroxisomes, mitochondria, and chloroplasts
  • Consumes ATP and releases previously fixed CO2

C3 Plant Characteristics

• C3 plants use the Calvin cycle as their primary carbon fixation method
• Named for the 3-carbon compound (3-phosphoglycerate) produced during carbon fixation
• RuBisCO serves as the primary enzyme for carbon fixation in C3 plants
• Most common type of plants, including rice, wheat, and soybeans
• Adapted to moderate climates with adequate water availability
• Struggle in hot, dry environments due to increased photorespiration
• Stomata remain open during the day, allowing for continuous gas exchange

C4 Plants

C4 Carbon Fixation Pathway

• C4 plants evolved to minimize photorespiration and improve water use efficiency
• Use PEP carboxylase as the initial carbon-fixing enzyme
  • PEP carboxylase has a higher affinity for CO2 than RuBisCO
  • Functions efficiently even at low CO2 concentrations
• Carbon fixation occurs in two distinct steps, separated spatially in different cell types
• Produce a 4-carbon compound (oxaloacetate) as the first product of carbon fixation
  • Oxaloacetate quickly converted to malate or aspartate for transport

C4 Plant Anatomy and Cell Types

• Kranz anatomy characterizes C4 plants
  • Distinct arrangement of mesophyll and bundle sheath cells around vascular tissues
  • Allows for spatial separation of initial carbon fixation and the Calvin cycle
• Mesophyll cells form the outer layer of the leaf
  • Contain PEP carboxylase for initial carbon fixation
  • Produce malate or aspartate for transport to bundle sheath cells
• Bundle sheath cells surround the vascular tissues
  • Contain high concentrations of RuBisCO
  • Site of the Calvin cycle and carbon fixation into sugars
  • Thick cell walls prevent CO2 leakage

Advantages of C4 Photosynthesis

• C4 plants concentrate CO2 around RuBisCO, reducing photorespiration
• Adapted to hot, dry environments (corn, sugarcane, sorghum)
• Higher water use efficiency due to reduced stomatal opening
• Can maintain photosynthesis at lower CO2 concentrations than C3 plants
• Require more energy (ATP) per carbon fixed compared to C3 plants
  • Trade-off balanced by increased efficiency in challenging environments

CAM Plants

CAM Photosynthesis Mechanism

• CAM (Crassulacean Acid Metabolism) plants temporally separate CO2 fixation and the Calvin cycle
• Stomata open at night to collect CO2, reducing water loss
• PEP carboxylase fixes CO2 into oxaloacetate during the night
  • Oxaloacetate converted to malate and stored in vacuoles
• During the day, malate releases CO2 for use in the Calvin cycle
  • Stomata close, conserving water while photosynthesis continues
• Named after the Crassulaceae family where it was first discovered

CAM Plant Adaptations and Examples

• Highly adapted to arid environments (cacti, pineapples, agaves)
• Exhibit extreme water conservation strategies
  • Can survive long periods of drought
  • Often have succulent leaves or stems for water storage
• Show daily fluctuations in tissue acidity due to malate accumulation
• Some plants can switch between C3 and CAM metabolism depending on environmental conditions (facultative CAM plants)
• CAM plants generally grow slower than C3 or C4 plants due to energy costs
  • Trade-off for survival in extreme environments

Comparison of CAM with C3 and C4 Pathways

• CAM and C4 both evolved to reduce photorespiration and improve water use efficiency
• CAM separates carbon fixation and the Calvin cycle temporally, while C4 separates them spatially
• CAM plants have the highest water use efficiency among the three types
• C4 plants have the highest productivity in hot, sunny environments
• C3 plants remain most efficient in moderate climates with adequate water