Light Dependent and Light Independent Phases: Study Notes

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PHOTOSYNTHETIC PIGMENTS

The photosynthetic pigments of higher plants are divided into two groups: the chlorophylls (primary pigments) and the carotenoids (accessory pigments).

• Chlorophyll absorbs red and blue light and reflects green light.
  • Each chlorophyll molecule has a hydrophilic head and a hydrophobic tail.
  • The head contains a magnesium ion, which is essential for absorbing or harvesting light.
  • The hydrophobic tail anchors the molecule to the thylakoid membrane.
• Different types of chlorophyll have varied side chains on the head, which alters their absorption spectra. Chlorophyll a is the most common.
• Carotenoids are pigments that appear yellow, orange, red, or brown. The most common carotenoid is beta-carotene.

ABSORPTION AND ACTION SPECTRA

• An absorption spectrum is a graph that shows the absorbance of different wavelengths of light by a pigment.
• An action spectrum is a graph that represents the rate of photosynthesis at different wavelengths of light. It indicates the effectiveness of various wavelengths in driving photosynthesis, which is related to their absorption and energy content. Shorter wavelengths contain more energy. The action spectrum illustrates the amount of photosynthesis occurring at each wavelength.

Photosystems

Component

• Antenna Complex:
  • Contains 200-300 pigment molecules (such as chlorophyll).
  • Collects light energy across various wavelengths.
• Light Harvesting Complex:
  • Transfers light energy to the reaction center for conversion into chemical energy.
• Reaction Center:
  • Contains chlorophyll a and a primary electron acceptor.
  • Responsible for the primary energy conversion process in photosynthesis.
• Photosystem I (PSI):
  • Absorbs light at 700 nm.
  • Passes electrons through the electron transport chain, resulting in the formation of NADPH.
• Photosystem II (PSII):
  • Absorbs light at 680 nm.
  • Transfers electrons to the plastoquinone pool and subsequently to the cytochrome complex.

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