Photosynthesis Lecture Notes

I. Introduction

• Definition: Process by which plants use sunlight, carbon dioxide (CO2), and water (H2O) to produce sugar (carbohydrates like starch).
• Byproduct: Oxygen (O2) is released.
• Chemical Equation:
  • 12 H2O + 6CO2 + light energy → 6O2 + C6H12O6 + 6H2O
  • Reverse of aerobic respiration, essential for life on Earth.
• Location: Occurs in chloroplasts with a double membrane structure:
  • Stroma: Fluid interior.
  • Thylakoid System: Interior folded membrane forming grana.
• Occurrence: In leaves of plants and many protists (algae).
• Process: Two main steps:
  • Light-dependent reactions: Requires sunlight to split H2O into O2 and H+, producing ATP.
  • Light-independent reactions: Uses carbon from CO2 to synthesize sugar (Calvin Cycle).

II. Pigments - How Plants Capture Light Energy

• Nature of Pigments:
  • Pigments give color by reflecting/transmitting light and absorbing other colors.
  • Light travels in various wavelengths, visible spectrum being 380-750 nm (ROY G BIV).
  • Red has the longest wavelength; violet the shortest.
  • Different pigments absorb various wavelengths, reflecting/transmitting others.
• Chlorophyll: Primary photosynthetic pigment:
  • Blue-green color, reflects green/yellow light, absorbs violet and red.
• Accessory Pigments:
  • Chlorophyll b: Yellow-green, absorbs blue/orange wavelengths, transfers energy to Chlorophyll a.
  • Carotenoids: Yellow/orange, visible in fall when chlorophyll disintegrates, absorb damaging light.

III. Capturing Sunlight Energy

• Chlorophyll Arrangement: Form "antenna complex" in thylakoid membrane with central reaction center.
• Photon Absorption: Photons hit pigment, elevating electrons to excited state, then return to ground state, transferring energy until it reaches the reaction center.

IV. Light-Dependent Reactions

• Purpose: Harvests energy, splits water, releases electrons, O2, and H+.
  • Responsible for oxygenating the atmosphere.
• Location: Thylakoid membrane compartments (grana).
• Electron Flow:
  • Non-cyclic Flow:
    • Involves Photosystem II (P680) and Photosystem I (P700).
    • Light energy excites electrons, which travel via the electron transport chain, aiding in ATP formation.
    • NADP+ is reduced to NADPH, carrying electrons to the next stage.
  • Cyclic Flow:
    • Involves only Photosystem I, produces ATP but no NADPH.
    • Compensates for higher ATP needs in light-independent reactions.
• ATP Formation:
  • Achieved through chemiosmosis.
  • H+ ions from water and electron transport buildup, creating a gradient.
  • Ions flow through ATP Synthase, converting ADP to ATP, releasing it into the stroma.
  • ATP and NADPH provide energy for the Calvin Cycle.