Principles Module 28


  • Discuss the two major interdependent steps of photosynthesis
  • Recognize that photosynthesis is, in many ways, the reverse of respiration
  • Describe the first step of how light energy is transformed into chemical energy

Key points

  • Photosynthesis transforms light energy into chemical energy
  • Net reaction: CO2 + H2O + light → (CH2O)n + O2
  • Photosynthesis is two interdependent pathways
    • light-dependent reactions
      • light energy used to produce ATP and NADPH, temporary high-energy molecules
    • light-independent reactions
      • ATP and NADPH ‘spent’ to reduce carbon, making carbohydrates
  • Light energy begins to be transformed into chemical energy when it strikes pigments in the leaf
    • pigments absorb light at specific wavelengths based on their structure
    • light absorbance results in the rearrangement of electrons on a molecule
      • molecular excitation is when the electrons are rearranged, conserving the energy of light
      • photo-oxidation is when an electron is lost off a molecule, travels to a nearby molecule called an electron acceptor

Questions for Practice

  • In what ways are the light-dependent reactions dependent upon the carbon reactions?
  • What would happen if the plant were unable to take in CO2 for some reason?
  • What is the role of NADPH in photosynthesis?