light-dependent stage

• photolysis


  • splitting of water molecules into protons (H+), electrons and oxygen using light energy
• ATP synthase


  • enzyme responsible for forming ATP for combining ADP and Pi
• NADP


  • Co-enzyme involved in photosynthesis, which carries hydrogen atoms
• Photophosphorylation


  • process by which light energy is used to add a phosphate group to ADP to form ATP
• Photosystem II


  • the photosystem that absorbs light energy and drives photolysis
• 3 ways H+ gradient is maintained

  1. splitting of water
  2. ETC pumping H+ to thylakoid
  3. reduction of NADP
• cyclic phosphorylation

  • produces ATP only
  • electrons not passed onto NADP
  • excited electrons pass to an electron acceptor and back to chlorophyll
  • electrons recycled back to Photosystem I
• non cyclic phosphorylation

  • produces ATP, NADPH and oxygen
• phase 1: Absorption of light 


  • light energy is absorbed by chlorophyll, exciting the electrons to a higher energy level
  • excited electrons are transferred down the ETC (electron transport chain) where they lose energy
• Phase 4: production of NADPH 


  • light energy excites electrons in PSI
  • these electrons and a hydrogen ion are transferred to NADP to form NADPH
• phase 3 : production of ATP


  • energy lost from electrons is used to create a proton gradient in the thylakoid and stroma
  • protons move down concentration gradient via an enzyme called ATP synthase
• phase 2: photolysis of water 


  • some oxygen released into atmosphere via stomata
  • electrons in PSII need to be replaced
  • H+ ions accepted by coenzyme NADP which becomes reduced NADPH
• chemiosmosis


  • flow of H ions through ATP synthase enzymes - force of the flow allows production of ATP
• water 


  • source of protons (hydrogen ions) that will be used in phosphorylation
  • donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
  • is the source of the by product, oxygen
  • keeps plant cells turgid, enabling them to function
• Z scheme