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.