Photosystem and its Components

• Photosystems

  Light-dependent reactions use two photosystems, photosystems I (PSI) and II (PSII) which are both embedded in the thylakoid membrane
• Photosystems I and II

  Named for the order in which they were discovered
• Photosystems

  • Are large complexes of proteins and pigments within the thylakoid membrane
  • Both photosystems contain special pairs of chlorophyll molecules (P700 in PSI and P680 in PSII)
• Light-dependent reactions

  Start at photosystem II
• Light Capture

  Chlorophyll molecules in plant cells capture light energy, energizing the electrons in the reaction center
• Electron Movement

  Energized electrons escape from photosystem II and move along the electron transport chain to a nearby electron acceptor
• Water Splitting

  Water molecules are split, releasing oxygen, hydrogen ions, and electrons to replace the electrons that escaped
• Energy Production


  Electrons continue down the chain, releasing stored energy (being used to create a hydrogen ion gradient)
• Hydrogen Ion Gradient

  A protein pumps hydrogen ions from the stroma into the thylakoid space, creating a concentration gradient
• ATP Synthesis

  The gradient's potential energy drive ATP synthase, which adds a phosphate to ADP, forming ATP
• Light Absorption

  When PSI absorbed more light energy, its electrons become energized
• Electron Movement

  These energized electrons escape PSI and move down the second electron transport chain
• Electron Replacement

  Electrons from the adjacent electron transport chain (near PSII) replace those in PSI
• Water Splitting

  Water molecules are split again to replace the electrons that moved from PSII
• NAPDH Production

  At the end of the electron transport chain, the energized electrons, along with hydrogen molecule, reduce NADP to NADPH
• Antenna Complex

  • Contains hundreds of chlorophyll molecules and carotenoids
  • Funnel excitation energy toward the reaction center
• Reaction Center (Complex)

  • Primary Electron Acceptor
  • Accepts all the excited electrons from the chlorophylla molecules
  • In PSII, these are p680 while in PSI, these are p700
  • Enzyme at the heart of the photosystem
  • Uses light to reduce and oxidize molecules (exchange electrons)
  • Surrounded by light-harvesting complexes
• Light Harvesting-Complex

  • Have all of the chlorophyll molecules as these can be able to capture photons from the Sun and excite their electrons
  • The electrons fall back down and get passed to the next chlorophyll molecule
  • Enhances light absorption
  • Pigments pass light energy to special chlorophyll