Stages of Photosynthesis (Cambridge International A-Level Biology)

The Two Stages of Photosynthesis

Spec point — 13.1.2 · explain that energy transferred as ATP and reduced NADP from the light-dependent stage is used during the light-independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules

• 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 a proton (H⁺) gradient, set up when energy from electrons passing along the electron transport chain is used to pump protons into the thylakoid space

• 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 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

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