Light dependent reactions

• Light dependent reactions (LDR)

  Take place in the thylakoid membrane and require constant solar energy
• Light independent reactions (LIR)


  Occur in the stroma and require no direct solar energy
• LDR


  • Uses light energy to make two molecules needed for the next stage of photosynthesis: ATP and NADPH (electron carrier)
• The light reactions take place in the thylakoid membranes
• LDR - Photosystems

  • Made up of pigments (i.e. chlorophyll) and proteins in a complex
  • When a pigment absorbs a photon, it is raised to an excited state
  • Antenna complex includes all surrounding pigment molecules that gather light energy
  • Transfers light energy to reaction centre
• LDR - Photosystems

  • Collectively, the pigment molecules collect energy and transfer it towards the reaction center
  • The reaction center of a photosystem contains a unique pair of chlorophyll molecules called the special pair
  • Once energy reaches the special pair, it passes an entire electron to the primary electron acceptor
• Electron Transport Chain 1

  1. Light is collected by special pair P680 and an electron is donated to the primary electron acceptor

  2. When an electron leaves PSII, it is transferred first to plastoquinone (Pq), then to a cytochrome complex (Cyt), and finally to plastocyanin (Pc)

  3. As the electron travels, some of the energy is used to pump protons across the stroma to the thylakoid lumen