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•  Light dependent reactions take place in the intermembrane space of the thylakoids

•  Light independent reactions take place in the stroma

Photosynthesis is the process by which cells synthesise organic molecules (e.g. glucose) from inorganic molecules (CO2 and H2O) in the presence of sunlight

This process requires a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, some bacteria)

  • In plants, photosynthesis occurs within a specialised organelle called the chloroplast

Photosynthesis Equation

photosynthesis equation

Photosynthesis is a two step process:

  • The light dependent reactions convert light energy from the Sun into chemical energy (ATP)
  • The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)

Step 1:  Light Dependent Reactions

  • Light is absorbed by chlorophyll, which releases energised electrons that are used to produce ATP (chemical energy)
  • The electrons are donated to carrier molecules (NADP+), which is used (along with ATP) in the light independent reactions 
  • The electrons lost from the chlorophyll are replaced by water, which is split (photolysis) to produce oxygen and hydrogen
  • The light dependent reactions occur in the intermembrane space of membranous discs called thylakoids

Step 2:  Light Independent Reactions

  • ATP and hydrogen / electrons (carried by NADPH) are transferred to the site of the light independent reactions
  • The hydrogen / electrons are combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates)
  • The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together
  • The light independent reactions occur within the fluid-filled interior of the chloroplast called the stroma

Overview of the Two Stages of Photosynthesis

photosynthesis reactions