CIE A Level Biology (9700) 2019-2021

Revision Notes

13.1.1 Stages of Photosynthesis

The Two Stages of Photosynthesis

  • Photosynthesis occurs in two stages: the light-dependent stage, which takes place in the thylakoids, and the light-independent stage, which takes place in the stroma
  • During the light-dependent stage of photosynthesis:
    • Reduced NADP is produced when hydrogen ions combine with the carrier molecule NADP using electrons from the photolysis of water
    • ATP is produced (from ADP and Pi by ATP synthase in a process called photophosphorylation (ADP + Pi → ATP)
    • Photophosphorylation uses the proton (H+) gradient generated by the photolysis of water
    • Energy from ATP and hydrogen from reduced NADP are passed from the light-dependent stage to the light-independent stage of photosynthesis
  • The energy and hydrogen are used during the light-independent reactions (known collectively as the Calvin cycle) to produce complex organic molecules, including (but not limited to) carbohydrates, such as:
    • Starch (for storage)
    • Sucrose (for translocation around the plant)
    • Cellulose (for making cell walls)

Exam Tip

Remember, the whole purpose of the light-dependent reactions is to produce ATP and reduced NADP, which are then used to complete the process of photosynthesis through the light-independent reactions.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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