Unit 3

• Chlorophyll in the thylakoid membranes absorbs photons to initiate light reaction.
• Light-dependent reactions

  Use light energy to make ATP and NADPH, which are needed for the Calvin cycle
• Photosystems

  • Large complexes of proteins and pigments (light-absorbing molecules) that are optimized to harvest light
  • There are two types: photosystem I (PSI) and photosystem II (PSII)
• Reaction center chlorophylls


  P700 in photosystem I, P680 in photosystem II
• Non-cyclic photophosphorylation

  1. Electrons are removed from water and passed through PSII and PSI before ending up in NADPH

  2. Requires light to be absorbed twice, once in each photosystem

  3. Produces both ATP and NADPH
• Cyclic photophosphorylation


  Electrons follow a different, circular path and only ATP (no NADPH) is produced
• Photosystem II (PSII)


  • Special pair (P680) absorbs best at 680 nm
  • Primary electron acceptor is pheophytin
  • Gets electrons from water
• Photosystem I (PSI)


  • Special pair (P700) absorbs best at 700 nm
  • Primary electron acceptor is chlorophyll A0
  • Gets electrons from PSII via electron transport chain
• Electron transfers in light-dependent reactions

  Are driven by, and made possible by, the absorption of energy from light
• Phospholipid bilayer


  A structure consisting of two layers of phospholipid molecules that forms the basic structure of biological membranes
• Protein channel


  • A structure within the phospholipid bilayer membrane that allows the passage of specific molecules or ions
• Stroma


  The fluid-filled space outside the thylakoid membrane in a chloroplast
• Thylakoid lumen


  The internal space within the thylakoid membrane of a chloroplast
• Pheophytin


  The primary electron acceptor in Photosystem II
• P680 special pair


  The chlorophyll a molecules in the reaction center of Photosystem II that absorb light and become excited
• Light absorption in Photosystem II

  1. Light hits the protein channel

  2. Electrons from the P680 special pair are excited and transferred to the Pheophytin, primary acceptor
• Water splitting in Photosystem II

  1. H2O produces 1/2 O2 plus 2 hydrogen ions and 2 electrons

  2. Electrons are transferred into the protein channel
• Chlorophyll A sub 0


  The primary electron acceptor in Photosystem I
• P700 special pair


  The chlorophyll a molecules in the reaction center of Photosystem I that absorb light and become excited
• Light absorption in Photosystem I

  1. Light hits the protein channel

  2. Electrons from the P700 special pair are excited and transferred to the Chlorophyll A sub 0, primary acceptor
• During the light-dependent reactions, an electron that's excited in PSII is passed down an electron transport chain to PSI (losing energy along the way). In PSI, the electron is excited again and passed down the second leg of the electron transport chain to a final electron acceptor.
• The light-dependent reactions involve two photosytems (II and I) and an electron transport chain that are all embedded in the thylakoid membrane.
• Light that is harvested from PSII causes an excited electron of the chlorophyll a special pair to be passed down an electron transport chain (Pq, Cyt, and Pc) to PSI.
• The passing of the electron in the first part of the electron transport chain causes protons to be pumped from the stroma to the thylakoid lumen.
• Protons diffuse out of the thylakoid lumen through the enzyme, ATP synthase, producing ATP in the process.
• Once the electron reaches PSI, it joins its chlorophyll a special pair and re-excited by the absorption of light. It proceeds down a second part of the electron transport chain (Fd and NADP+ reductase) and reduces NADP+ to form NADPH.
• The electron lost from the chlorophyll a special pair is replenished by electrons flowing from PSII.
• In some cases, electrons break this pattern and instead loop back to the first part of the electron transport chain, repeatedly cycling through PSI instead of ending up in NADPH. This is called cyclic photophosphorylation.
• Cyclic electron flow results in the production of ATP (because protons are pumped into the thylakoid lumen), but do not result in the production of NADPH (because electrons are not passed to NADP+ reductase).
• Chloroplasts seem to switch from linear to cyclic electron flow when the ratio of NADPH to NADP+ is too high (when too little NADP+ is available to accept electrons).
• Cyclic electron flow may play a photoprotective role, preventing excess light from damaging photosystem proteins and promoting repair of light-induced damage.
• Photosynthesis

  1. Oxidation of carbohydrates

  2. Reduction of carbohydrates

  3. Production of oxygen gas