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
Examiner Tip
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.
