Absorption Spectrum and Action Spectrum in Photosynthesis

Revision of Chloroplast Structure

• Chloroplasts contain structures called thylakoids.
• Inside thylakoids are photosystems which contain photosynthetic pigments.
• Photosystems have:
  • Reaction Center: Contains two chlorophyll a molecules.
  • Accessory Pigments: Different colors to absorb various wavelengths of light.

Absorption Spectrum

• Definition: A graph showing how well each photosynthetic pigment absorbs different wavelengths of light.
• Visible light range: 400 to 700 nm.
• Example Pigments: (Names not needed to memorize)
  • Pigment A: Absorbs 420-520 nm.
  • Pigment B: Absorbs 600-650 nm.
  • Pigment C: Absorbs 600-700 nm.
  • Pigment D: Absorbs 450 nm and 650-660 nm.
• Importance: Helps understand which wavelengths of light are best absorbed, aiding in optimizing photosynthesis.

How Absorption Spectrum Affects Photosynthesis

• More light absorption increases the rate of photosynthesis.
• Light-dependent reactions produce ATP and NADPH used in light-independent reactions.

Action Spectrum

• Definition: A graph showing the overall rate of photosynthesis over a range of wavelengths of light.
• Relationship with Absorption Spectrum: The absorption spectrum of pigments affects the action spectrum.
• Example Scenario (Land Plant with 4 pigments):
  • 400 nm (Purple Light): Low photosynthesis rate.
  • 450 nm (Blue Light): High photosynthesis rate.
  • 500 nm: Moderate photosynthesis rate.
  • 600 nm: Low photosynthesis rate.
  • 700 nm: High photosynthesis rate.

Example Scenario (Seaweed with Pigments B and C)

• Different pigments result in different action spectra.
  • 400 to 500 nm: Low photosynthesis rate due to poor absorption by pigments B and C.
  • 600 nm: Moderate photosynthesis rate (Pigment B absorbs well).
  • 700 nm: High photosynthesis rate (Pigments B and C absorb well).
• Conclusion: The number and type of pigments determine the effectiveness of light absorption and the rate of photosynthesis.