Overall Production of Light-Dependent Reactions

• The principal purpose of light-dependent reactions is to provide the energy for the carbon fixation that occurs in the subsequent light-independent reactions. The source of this energy is light. Light-dependent reactions can be summarized as transforming 12 water molecules, 12 NADP+ and 18 phosphates into six oxygen molecules + 12 NADPH + 18 ATP in the presence of light.
  
  

Oxygen Production

• Light-dependent reactions of photosynthesis begin with a photon of light striking pigments in the photosystem -- a complex arrangement of pigments that transfers the light energy to an electron. The central pigment in photosystems is chlorophyll, which transfers an excited electron to a subsequent electron transport chain. A magnesium complex within the chlorophyll replaces the lost electron from water molecules. As the water molecules are stripped of electrons, they form oxygen gas and protons. Every four electrons removed produces one oxygen molecule from two water molecules. Oxygen is released as a byproduct.
  
  

ATP Production

• After leaving the photosystem, the energized electron travels through an electron transport chain that uses the electron's energy to concentrate protons on one side of a membrane, creating a proton gradient. The protons flow down their gradient through channels in the membrane formed by an enzyme called ATP synthase. This enzyme uses the flow of protons to attach a phosphate group to a molecule of ADP (adenosine diphosphate) to form ATP. ATP provides the energy needed for light-independent reactions of photosynthesis, during which carbon dioxide is fixed into organic molecules.
  
  

NADPH Production

• The presence of a second type of photosystem allows organisms to carry out cyclic or non-cyclic photophosphorylation. In cyclic photophosphorylation, the second photosystem re-energizes the electron and is sent back through the electron transport chain to make more ATP. In non-cyclic photophosphorylation, the re-energized electron is used to convert NADP+ to NADPH. NADPH is an energy carrier molecule needed along with ATP in light-independent reactions. This junction in the process allows plants to make NADPH or to make additional ATP as needed or if insufficient NADP+ is available.