light-dependent reaction

• light dependent reactions: rely on light directly
• the light dependent reactions take place within the chloroplast, across the thylakoid membrane
• non-cyclic photophosphorylation

  1. light energy hits photosystem II in the thylakoid membrane
  2. 2 electrons gain energy and are excited to a higher energy level
  3. the excited electrons leave the photosystem and pass to the first protein in the electron transport chain (as the excited electrons leave photosystem II they are replaced by electrons from the photolysis of water
• non cyclic photophosphorylation

  4. the electrons pass down the chain of electron carriers known as an electron transport chain

  • H+ ions are actively pumped from a low conc in the stroma to a high conc in the thylakoid space generating a conc gradient across the thylakoid membrane
  • H+ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase enzymes embedded in the membrane
  • the movement of H+ ions causes the ATP synthase enzyme to catalyse the production of ATP
• non cyclic photophosphorylation

  5. at the end of the electron transport chain the electrons from photosystem II are passed to photosystem I

  6. light energy also hits photosystem I, exciting another pair of electrons which leave the photosystem
• non cyclic photophosphorylation

  7. the excited electrons from photosystem I also pass along an electron transport chain

  8. these electrons combine with hydrogen ions from the photolysis of water and NADP to form reduced NADP (NADPH)

  • the reduced NADP and the ATP pass to the light-independent reactions
• cyclic photophosphorylation

  1. light hits photosystem I
  2. electrons are excited to a higher energy level and leave the photosystem
  3. the excited electrons pass along the electron transport chain, releasing energy as they do so
• cyclic photophosphorylation

  4. the energy released as the electrons pass down the electron transport chain provides energy to drive the process of chemiosmosis

  • H+ ions are actively pumped from a low conc in the stroma to a high conc in the thylakoid space, generating a conc gradient across the thylakoid membrane
  • H+ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase embedded in the membrane
  • the movement of H+ ions cause the ATP synthase enzyme to catalyse the production of ATP
• cyclic photophosphorylation

  5. at the end of the electron transport chain the electrons rejoin photosystem I in a complete cycle

  • ATP produced enters the light-independent reaction
• light energy in the light-dependent reactions enables the splitting of water molecules in a reaction known as photolysis

  H2O -> 2H+ + 2e- + ½ O2

  the hydrogen ions and electrons are used during the light-dependent reactions while the oxygen is given off as a waste product

  • oxygen diffuses through the stomata and the plant will respire
• during the light-dependent reactions light energy is converted into chemical energy in the form of ATP and reduced NADP (NADPH)
• photophosphorylation

  photo = light

  phosphorylation = the addition of phosphate; in this case to ADP to form ATP