Plant Adaptations

C3 and C4 Plants

• Alternative mechanisms of carbon fixation evolved in hot, arid climates due to the following:
  • Dehydration: A significant challenge that requires trade-offs with metabolic processes, particularly photosynthesis.
  • Stomatal Closure: On hot, dry days, plants close stomata to conserve H₂O, but this limits photosynthesis by restricting CO₂ access and causing O₂ accumulation.
  • Photorespiration: A seemingly wasteful process that may occur under these conditions.

Leaf Anatomy (p.157)

• Each leaf contains several key structures:
  • Cuticle: A waxy layer to reduce water loss.
  • Epidermis Layer: Lets light penetrate.
  • Guard Cells: Flank each stoma and can change shape to regulate gas exchange (opening for CO₂ intake and closing to minimize H₂O loss).
  • Mesophyll Layer: Contains spongy and palisade cells; major site for photosynthesis due to its structural arrangement.
  • Vascular Bundles: Veins that transport water and minerals to leaves and carbohydrates back to roots.

Rubisco and Carbon Fixation

• Rubisco: Enzyme crucial for the Calvin cycle, catalyzing the combination of RuBP, CO₂, and H₂O.
  • Sensitivity to CO₂ Levels:
  • High CO₂: Normal function in the Calvin Cycle.
  • Low CO₂ or High O₂: Triggers photorespiration, leading to RuBP breakdown.
  • Stomata Opening: Necessary for CO₂ uptake but results in water loss.

Stomatal Function and Plant Adaptations

• Location of Stomata: Primarily on the underside of leaves to minimize direct sunlight and water loss.
• In temperate plants, stomata can fully open for maximum CO₂ uptake. However:
  • Desert Plants' Strategy: (e.g., cacti)
  • Close stomata during the day to minimize water loss, opening at night to fix CO₂ in organic acids (CAM plants).
  • Other Adaptations: Some plants (like pineapples) utilize different compartments to assist in CO₂ fixation, incorporating a C4 pathway, fixing CO₂ into oxaloacetate in mesophyll cells using PEP carboxylase.

C4 Photosynthesis Process

• Steps:
  1. CO₂ enters mesophyll cells, forming oxaloacetate, a 4-carbon compound.
  2. Oxaloacetate is converted to malate, which moves to the bundle sheath.
  3. In the bundle sheath, malate is broken down, releasing CO₂ for use in the Calvin cycle (high CO₂ concentration mitigates the effect of rubisco’s oxygenase activity).

Photorespiration as an Evolutionary Relic

• Overview of C3 plants: Initial CO₂ fixation via rubisco leads to the production of a three-carbon compound (3-phosphoglycerate).
• Process of Photorespiration:
  • Rubisco incorporates O₂ instead of CO₂, resulting in a two-carbon compound that consumes O₂ and organic fuel, releasing CO₂ without generating ATP or sugar.
• Evolutionary Context: Photorespiration may be a legacy from times with higher CO₂ and lower O₂ concentrations and can hinder the Calvin cycle, reducing carbon fixation efficiency by up to 50% in some plants on hot, dry days.

C4 Plants and Their Anatomy

• Advantages of C4 Photosynthesis: Minimizes photorespiration costs by incorporating CO₂ into 4-carbon compounds in mesophyll cells with the help of PEP carboxylase, which has higher CO₂ affinity than rubisco.
• Anatomy of C4 leaves:
  • Mesophyll Cells: Primary site for initial CO₂ fixation.
  • Bundle Sheath Cells: Site for Calvin Cycle reactivation using CO₂ from the malate breakdown.

CAM Plants

• Crassulacean Acid Metabolism (CAM): Utilized by some succulents.
  • Stomatal Timing: Open at night to absorb CO₂ and close during the day while converting CO₂ from organic acids for the Calvin Cycle.

Photosynthesis Impact

• Energy Conversion: Light energy captured in chloroplasts is stored as chemical energy in organic compounds (sugars).
• Energy and Structure Storage: Excess sugars are stored as starch in roots, tubers, seeds, and fruits.
• O₂ Production: Photosynthesis accounts for the production of oxygen in the atmosphere.