Light Reactions

Principles Module 29

Principles Module 30–1


  • Describe the components in the cell necessary to carry out the light reactions
  • Explain the process of how light energy results in the formation of NADPH and ATP
  • Discuss the flow of electrons from PSII to PSI

Key points

  • The structure of chlorophyll allows it to absorb light in the blue and red parts of the spectrum
    • chlorophyll is not free-floating in the chloroplast, but rather is bound to membrane proteins in the thylakoid
    • the long fatty-acid chain on chlorophyll allows it to associate with the hydrophobic regions of these proteins
  • Photochemistry (the removal of an electron from chlorophyll) only occurs at photosynthetic reaction centers
    • reaction centers are surrounded by light harvesting complex proteins that act as antennas to harvest light
    • transfer of energy between light-harvesting complex chlorophylls and reaction center chlorophylls is by resonance energy transfer
  • Electron flow through the reaction center creates a H+ gradient
    • flow is linear, with electrons re-supplied by the hydrolysis of H2O and reducing NADP+ to NADPH
    • the H+ gradient drives the formation of ATP from ADP and Pi
    • the energy for this electron flow comes, ultimately, from light

Questions for Practice

  • Compare and contrast the electron transport system of photosynthesis with that of respiration. Identify the electron donors, transporters, membranes, H+ gradient, and terminal electron acceptor.
  • Describe or draw the photosynthetic electron transport system.
  • What role do the light harvesting complex proteins play in the light reactions?