Field Botany: Photosynthetic Processes

Field Botany

Variation in Photosynthetic Processes

Overview

• Photosynthesis can vary through different processes, primarily categorized based on carbon fixation methods.

• Major types of photosynthesis:

  • Photorespiration in C3 Photosynthesis

  • Alternative Photosynthetic Procedures

  • Spatially: C4 Photosynthesis

  • Temporally: CAM Photosynthesis

• Overall, these variations are significant for plant adaptation to environmental conditions.

C3 Photosynthesis

• Key Components:

  • Light energy

  • Chlorooplast structure:

  • Outermembrane of chloroplast

  • Innermembrane of chloroplast

  • Primary Processes:

  • Calvin Cycle

    • Uses RuBisCO enzyme to fix carbon dioxide (CO₂) into sugars.

    • ATP and NADPH produced during light reactions aid in carbon fixation.

    • Energy-transduction reactions take place in the thylakoid membrane, while carbon-fixation reactions happen in the stroma.

• Ensuring Efficiency:

  • During the day:

  • When CO₂ concentration is high (stomata open), RuBisCO captures CO₂ efficiently leading to carbon gain.

  • When CO₂ concentration is low (stomata closed), RuBisCO captures oxygen leading to Photorespiration which results in:

  • No energy production

  • Net loss of carbon.

• Environmental Conditions:

  • Reduced transpiration occurs with stomata closure, impacting CO₂ uptake.

Photorespiration in C3 Photosynthesis

• Mechanism:

  • When [CO₂] is low, RuBisCO can bind to O₂ instead leading to photorespiration:

  • Outcome: Generates 2-phosphoglycolate instead of 3-phosphoglycerate, leading to a subsequent pathway that results in energy loss.

  • This signifies inefficiency and resource wastage under certain conditions.

C4 Photosynthesis

• Key Characteristics:

  • C4 plants exhibit spatial separation of CO₂ uptake and carbon fixation which enhances efficiency:

  • Initial Fixation of CO₂: Occurs in mesophyll cells forming 4-carbon acids (malate or aspartate)

  • Release of CO₂: 4-carbon acids are transported to bundle-sheath cells where CO₂ is released to enter the Calvin cycle.

• Advantages:

  • Typically found in environments with high light intensity, high temperatures, and drought prevalence.

  • Water Use Efficiency: On average, C3 plants lose 400-500 g of H₂O per gram of CO₂ fixed, while C4 plants lose only 250-300 g of water.

• Families:

  • Presence in 46% of grass species and several plant families.

CAM Photosynthesis

• Key Differences:

  • CAM plants utilize a temporal separation for CO₂ uptake and carbon fixation:

  • At night, stomata open and CO₂ is fixed into 4-carbon acids.

  • During the day, stomata close, and the CO₂ is released internally to feed the Calvin cycle while minimizing water loss.

• Water Loss Efficiency: CAM plants lose only 50-100 g of H₂O per gram of CO₂ fixed, making them highly efficient in arid conditions.

• Adaptation: Most common in succulent species and epiphytes found in drought-resistant environments.

• Growth Rate: CAM plants typically have slow growth due to energetic costs involved in CO₂ reconstitution and limited vacuole storage capacity for 4-carbon acids.

Activity

• To understand photosynthetic variation, compare similarities and differences between C3, C4, and CAM pathways to explain co-occurrence of these photosynthetic types in the Sonoran Desert.

Ethical and Practical Implications

• Understanding these photosynthetic variations aids in agricultural practices targeted at improving drought resistance and productivity in different environments, ensuring sustainable management of plant resources.