JB_Photosynthesis_Has_Evolved_Through_Time

Introduction to Photosynthesis

• Photosynthesis has evolved throughout time.

• Expected Outcomes:

  • Describe how the history of the Earth has influenced the evolution of photosynthetic organisms.

  • Explain why not all plants have carbon concentrating mechanisms.

Structure of Leaves in Photosynthesis

• Photosynthesis occurs in the leaves featuring:

  • Leaf Cross Section:

    • Vein

    • Mesophyll cells

    • Stomata (for gas exchange - CO2 in, O2 out)

  • Chloroplast Components:

    • Mesophyll cell (5 µm)

    • Thylakoid lumen and granum

    • Stroma (1 µm)

Light Reactions and the Calvin Cycle

• Light Reactions:

  • Inputs: Light energy, H2O

  • Outputs: ATP, NADPH, O2

• Calvin Cycle:

  • Inputs: CO2, ATP, NADPH

  • Outputs: CH2O (sugar)

Rubisco Enzyme in the Calvin Cycle

• Rubisco is vital for the Calvin Cycle, specifically as:

  • Active site: Ribulose-1,5-bisphosphate carboxylase/oxygenase.

• Function:

  • Catalyzes the conversion of CO2 into organic molecules.

Stages of the Calvin Cycle

• Rubisco catalyzes the following steps:

  1. Fixation of CO2 with RuBP.

  2. Reduction of 3-phosphoglycerate.

  3. Regeneration of RuBP from G3P.

Inefficiency of Rubisco

• Rubisco is described as an inefficient enzyme due to:

  • Slow reaction rate: Limited to only a few reactions per second.

  • Lack of specificity: Can also bind to O2, leading to photorespiration.

    • Photorespiration: Results in approximately 20% error rate.

Reactions Catalyzed by Rubisco

• Carboxylase Activity (Photosynthesis):

  • RuBP + CO2 -> 2 x 3-phosphoglycerate.

• Oxygenase Activity (Photorespiration):

  • RuBP + O2 -> 3-phosphoglycerate + 2-phosphoglycolate (CO2 released, ATP used).

Factors Affecting Rubisco Activity

• Rubisco's oxygenase activity increases when:

  • Temperature rises (gases have decreased solubility).

  • O2 concentration increases relative to CO2.

Alternate Photosynthetic Pathways

• C3 Pathway:

  • Most common among plants.

  • Involves three stages in the Calvin cycle yielding three-carbon compounds.

• C4 Pathway:

  • Concentrates CO2 in bundle sheath cells.

  • Involves two types of cells: mesophyll and bundle-sheath cells.

  • Requires PEP carboxylase enzyme to form a 4-carbon compound.

C4 Plant Anatomy

• C4 plants exhibit Kranz anatomy which features:

  • Specialized bundle sheath cells surrounding vascular tissue.

• Differences include:

  • Increased CO2 concentration leading to optimized photosynthesis in hot climates.

CAM Pathway Overview

• CAM (Crassulacean Acid Metabolism):

  • An alternative pathway for photosynthesis evolved for dry conditions.

  • CO2 is collected at night and stored as malic acid, used during the day for photosynthesis.

Environmental Adaptations of Photosynthetic Pathways

• Ideal conditions for:

  • C4 plants: Generally in hot, arid environments.

  • CAM plants: Thrive in hot, dry climates, where they can minimize water loss.

Conclusion

• Understanding the evolution and mechanisms underlying photosynthesis is crucial to comprehending plant biology and their adaptations to different environments.