BSC1010 Lecture 9

Photosynthesis Overview

• Photosynthesis Equation:

  • Input: 6 CO₂ + 6 H₂O + Light Energy

  • Output: C₆H₁₂O₆ + 6 O₂

• Cellular Respiration Equation:

  • Input: C₆H₁₂O₆ + 6 O₂

  • Output: 6 CO₂ + 6 H₂O

Leaf Anatomy

• Key Structures:

  • Chloroplasts: Organelles where photosynthesis occurs.

    • Stroma: Fluid-filled space inside chloroplasts.

    • Thylakoid: Membrane-bound structures where light reactions occur.

      • Grana: Stacks of thylakoids.

  • Mesophyll: Tissue in leaves where chloroplasts are located.

  • Stomata: Pores on leaf surface for gas exchange (CO₂ in, O₂ out).

Photosynthesis Process

Light Reactions

• Components:

  • Occur in the thylakoid membranes.

  • Involves sunlight, H₂O, NADP⁺.

• Steps:

  • Water (H₂O) is split; O₂ is released.

  • Light energy excites electrons in Photosystem II (P680) and Photosystem I (P700).

  • Electrons move through the Electron Transport Chain (ETC) to produce ATP and NADPH.

Calvin Cycle (Light-Independent Reactions)

• Components:

  • Occur in the stroma.

  • Utilizes CO₂, ATP, NADPH.

• Process:

  • Carbon fixation occurs using rubisco enzyme.

  • Produces sugar (G3P); requires 3 turns of the cycle for one net G3P.

Pigments in Photosynthesis

• Main Pigments:

  • Chlorophyll a and Chlorophyll b: absorb light energy.

  • Chlorophyll a (P680 and P700) are involved in photosystems.

  • Carotenoids: Accessory pigments assisting in light absorption.

• Function of Pigments:

  • Reflect green light, contributing to why most plants appear green.

  • Absorb specific wavelengths of light, utilized in the photosynthesis process.

Energy Diagrams for Light Reactions

• Non-Cyclic Pathway:

  • Photons energize electrons in Photosystem II; water molecules are split generating O₂ and H⁺.

  • Electrons travel the ETC producing ATP through oxidative phosphorylation.

  • Electrons in Photosystem I produce NADPH.

• Cyclic Pathway:

  • Only involves Photosystem I; ATP is produced, but NADPH is not.

  • Less efficient than the non-cyclic pathway.

C3 Photosynthetic Pathway

• Characteristics:

  • CO₂ is initially fixed into a 3-carbon compound (3-PGA) via rubisco.

  • High photorespiration occurs under hot, dry conditions due to closed stomata.

  • Examples: rice, wheat, soybeans.

C4 and CAM Pathways

C4 Photosynthetic Pathway

• Mechanism:

  • Fixes CO₂ into a 4-carbon compound (oxaloacetate) in mesophyll cells.

  • Light-independent reactions occur in bundle-sheath cells.

  • Higher ATP cost but minimizes photorespiration.

  • Examples: grasses and palms.

CAM Photosynthetic Pathway

• Mechanism:

  • CO₂ fixation occurs at night; stored as malate.

  • Light-dependent reactions happen during the day.

  • Efficient in water conservation, producing less glucose.

  • Examples: cacti and bromeliads.

Conclusion

• Theoretical Relationship between Photosynthesis and Cellular Respiration:

  • Photosynthesis converts light energy into chemical energy, producing glucose that can be utilized in cellular respiration to generate ATP.