Understanding Photosynthesis Processes

Nov 25, 2024

Photosynthesis Lecture Notes

Introduction

  • Photosynthesis is the process by which plants produce their own energy.
  • Unlike animals, plants use sunlight, water, and carbon dioxide to produce energy.
  • Photosynthesis means 'building with light' - photo (light) + synthesis (to build).
  • Plants are crucial as they form the foundation of the food chain.

Stages of Photosynthesis

  1. Light Reactions

    • Location: Occur in the thylakoid membranes within chloroplasts.
    • Function: Convert solar energy into chemical energy.
    • Pigment: Chlorophyll absorbs sunlight. Exists in two forms, A and B.
      • Structure: Porphyrin ring helps in unique absorption of sunlight.
      • Function: Excites electrons to an excited state.
    • Photosystems:
      • Photosystem II: Starts the electron transport chain, produces ATP via proton gradient.
      • Photosystem I: Converts NADP+ into NADPH.
    • Overall Reaction: Requires sunlight and water, produces oxygen, ATP, and NADPH.
  2. Calvin Cycle

    • Location: Occurs in the stroma of chloroplasts.
    • Nature: Anabolic process (building up), unlike the catabolic citric acid cycle.
    • Phases:
      • Phase 1: Carbon Fixation
        • Enzyme: Rubisco captures CO2.
        • Reaction: CO2 is attached to ribulose bisphosphate (RuBP), forming an unstable 6-carbon compound, splitting into two 3-phosphoglycerate.
      • Phase 2: Reduction
        • Process: 3-phosphoglycerate receives a phosphate from ATP, is reduced by NADPH to become glyceraldehyde 3-phosphate (G3P).
      • Phase 3: Regeneration
        • Some G3P goes on to form glucose; some regenerates RuBP.
    • Requirements: 9 ATPs and 6 NADPH for net 1 G3P output.

Summary of Photosynthesis

  • Light Reactions: Light + water → Oxygen + ATP + NADPH.
  • Calvin Cycle: CO2 + ATP + NADPH → Organic compounds (sugars).
  • Photosynthesis is essentially the reverse of cellular respiration, involving redox reactions where electrons flow oppositely.

Conclusion

  • Photosynthesis allows plants to produce their own food, supporting entire ecosystems by forming the base of the food chain.