9.3 C4 and CAM pathways of carbon fixation

Plants have evolved clever ways to thrive in tough environments. C4 and CAM pathways are adaptations that help plants fix carbon more efficiently in hot, dry conditions. These strategies reduce water loss and boost productivity where regular photosynthesis struggles.

C4 plants use special leaf structures to concentrate CO2, while CAM plants flip their schedule to conserve water. Both methods pump up CO2 levels around key enzymes, making photosynthesis more effective. This helps important crops and desert plants survive harsh climates.

C3 vs C4 vs CAM Photosynthesis

Biochemical Processes and Adaptations

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• C3, C4, and CAM pathways represent three distinct carbon fixation mechanisms in plants
• C3 pathway (Calvin cycle) occurs in most plants and fixes CO2 directly through RuBisCO enzyme
• C4 and CAM pathways evolved to reduce photorespiration, which decreases efficiency in C3 plants under high temperature and low CO2 conditions
• C4 and CAM initially fix CO2 into oxaloacetate (4-carbon compound) using PEP carboxylase, which has higher CO2 affinity than RuBisCO
• C4 plants spatially separate initial CO2 fixation from Calvin cycle
• CAM plants temporally separate these processes
• C4 and CAM concentrate CO2 around RuBisCO, increasing photosynthetic efficiency in hot, dry environments
• Energy requirements for C4 and CAM exceed C3, but increased efficiency offsets this cost under specific environmental conditions

Efficiency and Environmental Adaptations

• C4 and CAM plants maintain high photosynthetic rates with partially closed stomata, reducing water loss
• Initial carbon fixation in C4 and CAM operates efficiently at low CO2 concentrations
• C4 and CAM achieve higher water use efficiency compared to C3 in hot, dry environments
• These adaptations allow C4 and CAM plants to thrive in tropical grasslands, deserts, and semi-arid regions (Sahel, Sonoran Desert)
• C4 plants include important crops (corn, sugarcane) and dominate warm-season grasslands
• CAM plants often found in extreme desert environments (cacti, agaves)

Adaptations for Hot, Dry Climates

Physiological and Anatomical Adaptations

• C4 and CAM plants evolved specific traits to thrive in conditions challenging for C3 plants
• C4 plants developed Kranz anatomy, enabling spatial separation of carbon fixation processes
• Kranz anatomy characterized by concentric rings of mesophyll and bundle sheath cells around vascular bundles
• CAM plants often possess succulent leaves or stems to store organic acids produced during nighttime CO2 fixation
• Succulence in CAM plants ranges from subtle (pineapple) to extreme (barrel cactus)

Water Conservation Strategies

• Both C4 and CAM maintain productivity with partially closed stomata
• C4 plants achieve this through spatial CO2 concentration
• CAM plants temporally separate CO2 uptake (night) and fixation (day)
• These adaptations result in higher water use efficiency compared to C3 plants
• Some plants can switch between C3 and CAM photosynthesis depending on conditions (facultative CAM)
• Facultative CAM observed in some succulents (Mesembryanthemum crystallinum) and epiphytes (some orchids)

Spatial Separation in C4 Plants

Kranz Anatomy and CO2 Concentration

• C4 plants exhibit Kranz anatomy with distinct mesophyll and bundle sheath cell arrangement
• Initial CO2 fixation occurs in mesophyll cells using PEP carboxylase
• PEP carboxylase catalyzes oxaloacetate formation from PEP and bicarbonate
• Oxaloacetate quickly converts to malate or aspartate
• These 4-carbon compounds transport to bundle sheath cells
• Bundle sheath cells decarboxylate 4-carbon compounds, releasing CO2 for Calvin cycle
• This spatial separation creates a CO2 pump, concentrating CO2 around RuBisCO in bundle sheath cells
• Concentrated CO2 minimizes photorespiration and increases efficiency

Biochemical Variations

• C4 plants utilize three main decarboxylation pathways in bundle sheath cells
• NADP-ME type (corn, sugarcane) uses NADP-malic enzyme
• NAD-ME type (amaranth, millet) employs NAD-malic enzyme
• PEP-CK type (guinea grass) utilizes phosphoenolpyruvate carboxykinase
• Each type has slight variations in biochemical processes and cellular arrangements
• All types achieve the same goal of concentrating CO2 around RuBisCO

Temporal Separation in CAM Plants

Nighttime CO2 Fixation

• CAM plants open stomata at night when temperatures are cooler and humidity higher
• Nighttime CO2 uptake reduces water loss compared to daytime gas exchange
• PEP carboxylase fixes incoming CO2 into oxaloacetate
• Oxaloacetate converts to malate for storage in vacuoles
• This process results in increasing acidity in CAM plant tissues overnight
• Some CAM plants (Kalanchoe) show visible leaf movements related to this acid accumulation

Daytime Carbon Reduction

• During the day, CAM plants close stomata to conserve water
• Stored malate undergoes decarboxylation, releasing CO2
• Released CO2 enters the Calvin cycle for carbon reduction
• This process depletes organic acids, decreasing tissue acidity throughout the day
• Temporal separation allows carbon fixation with minimal water loss
• CAM plants can achieve very high water use efficiency (pineapple, agave)

Flexibility in CAM Metabolism

• Some CAM plants switch between CAM and C3 photosynthesis based on environmental conditions
• This adaptation, called facultative CAM, provides metabolic flexibility
• Observed in some succulents (Mesembryanthemum crystallinum) and epiphytes (certain orchids)
• Allows plants to optimize photosynthesis and water use under varying conditions
• Represents an intermediate evolutionary step between C3 and obligate CAM metabolism