Stages of Photosynthesis (Cambridge (CIE) A Level Biology)

The Two Stages of Photosynthesis

• Photosynthesis occurs in two distinct stages:

  • The light-dependent stage, which takes place in the thylakoids

  • The light-independent stage, which takes place in the stroma

Summary of the light-dependent stage 

• Reduced NADP is produced when hydrogen ions combine with the carrier molecule NADP using electrons from the photolysis of water

• ATP is produced  in a process called photophosphorylation 

  • 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

Summary of the light-independent stage

• The energy and hydrogen (from the light-dependent stage) are used

• This takes place in reactions known collectively as the Calvin cycle

• Complex organic molecules are produced, including (but not limited to) carbohydrates, such as:

  • Starch (for storage)

  • Sucrose (for translocation around the plant)

  • Cellulose (for making cell walls)

Photosynthesis Stages - Overview Diagram

Photosynthesis occurs in two, closely-linked stages: the light-dependent stage, which takes place in the thylakoids, and the light-independent stage, which takes place in the stroma