Photosynthesis Overview

Photosynthesis is essential for life on Earth, enabling plants to convert sunlight, carbon dioxide, and water into glucose and oxygen. This process has been developed over 450 million years.

Two Types of Reactions

• Light-Dependent Reactions
• Light-Independent Reactions (Calvin Cycle)

These two processes are crucial to photosynthesis, with the Calvin Cycle being a significant component of the light-independent reactions.

Photosynthesis Requirements

• Water
• Carbon Dioxide
• Sunlight

How Plants Absorb Resources

• Water:
  • Absorbed by roots in vascular plants through xylem to leaves.
• Carbon Dioxide:
  • Enter and exit through stomata (tiny pores on leaves).
• Sunlight:
  • Absorbed by chlorophyll, a pigment in plant cells.

Plant Cell Structure

• Chloroplasts
  • Key organelles where photosynthesis occurs, containing:
    • Thylakoids: Membranous sacks where chlorophyll is located.
    • Grana: Stacked thylakoids.
    • Stroma: Fluid surrounding thylakoids.

Light-Dependent Reactions

1. Photon Absorption:
   • Sunlight excites an electron in chlorophyll (photoexcitation).
   • Part of Photosystem II (PSII), a complex of proteins and lipids.
2. Electron Transport Chain:
   • Energized electrons move through various protein complexes, losing energy, which is harnessed to create ATP and NADPH.
3. Water Splitting:
   • Water molecules are split to replenish lost electrons, producing oxygen as a byproduct.
4. ATP Formation:
   • Created through the enzyme ATP synthase as protons flow through the thylakoid membrane.
5. NADPH Creation:
   • Electrons are re-excited in Photosystem I (PSI) to form NADPH.

After these reactions, chemical energy is stored in the form of ATP and NADPH, and oxygen is released.

Calvin Cycle (Light-Independent Reactions)

• Occurs in the Stroma of chloroplasts and utilizes ATP and NADPH produced in the light-dependent reactions.

Steps in the Calvin Cycle

1. Carbon Fixation:
   • CO2 is attached to Ribulose bisphosphate (RuBP) using the enzyme Rubisco.
2. Reduction Phase:
   • ATP adds a phosphate group; NADPH donates electrons, producing Glyceraldehyde 3-phosphate (G3P).
3. Regeneration of RuBP:
   • Some G3Ps are used to regenerate RuBP to keep the cycle going.
   • For every three RuBP molecules, six G3P are made, but only one G3P exits the cycle.

Importance of G3P

• G3P can be converted into glucose, cellulose, and starch, making it the ultimate product of photosynthesis.

Conclusion

• Photosynthesis transforms sunlight, water, and carbon dioxide into essential compounds for life, demonstrating the complexity and efficiency of plant systems.

Further Learning

• Review selected references and consider rewatching parts of the lecture for better understanding. Questions can be left in the comments.