Light

• Light dependent stage


  The light-dependent stage of photosynthesis
• Light dependent stage


  1. How energy from light is harvested and used to drive the production of chemicals which can be used as a source of energy for other metabolic processes (ATP and reduced NADP) with reference to electron carriers and cyclic and non-cyclic

  2. Photophosphorylation

  3. The role of water
• Photosynthesis


  Photosynthesis takes place in two stages both in the chloroplast:
• Light-dependent Stage


  1. Photosynthesis occurs in the thylakoid membrane of the chloroplasts - using light energy to make chemical energy, ATP and NADPH (reduced NADP)

  2. Light-independent Stage - The Calvin Cycle (Photosynthesis occurs in the stroma of chloroplasts, this is where the necessary enzymes are located). Light is not directly used, carbon dioxide is fixed and used to build complex organic molecules
• Light-dependent stage


  Occurs in the grana (thylakoids)
• Light-dependent stage


  1. Light harvesting at the photosystems

  2. Photolysis of water

  3. Photophosphorylation (Non-cyclic and Cyclic)

  4. Formation of reduced NADP
• Photosystems


  • Embedded in the thylakoid membranes
  • Funnel shaped structures that contain photosynthetic pigments
  • Pigments are held in place by proteins
  • Photosynthetic pigments absorb certain wavelengths of light and reflect others
• Types of photosystem


  • Photosystem I (PSI) - pigment at the primary reaction centre is a type of chlorophyll a which absorbs most light at a wavelength of 700nm, P700 (red light)
  • Photosystem II (PSII) - pigment at the primary reaction centre is a type of chlorophyll a which absorbs most light at a wavelength of 680nm, P680 (red light)
• Light dependent stage


  1. Light is harvested (large SA, lots of chloroplasts, photosynthetic pigments present)

  2. Enzymes, that in the presence of light, splits H2O into protons (H+), electrons and oxygen - this is called photolysis

  3. Some O2 made is used in respiration. But when the rate of photosynthesis outweighs the rate of respiration, excess O2 diffuses to the atmosphere via the stomata

  4. Water is important in to plants as it: Is a source of H+ ions, Is a source of e-s, Produces O2, Keeps the plant cells turgid
• Photophosphorylation


  Formation of ATP from ADP + Pi in the presence of light
• Types of photophosphorylation


  • Non-cyclic (PSI and PSII) - ATP, Oxygen and reduced NADP made
  • Cyclic (PSI) - Small amounts of ATP made
• Light Dependent Reaction: Non-cyclic

  1. Light energy excites electrons in pigment molecules in PSII (P680)

  2. Energy is passed along pigment molecules until it reaches the primary pigment reaction centre

  3. The energy causes a pair of electrons to be excited from the chlorophyll a

  4. Electrons from water photolysis replace these

  5. These electrons are picked up by electron carriers (in the thylakoid membrane) containing iron ions - Fe3+ Fe2+ Fe3+

  6. Energy from this is used to pump protons from the stroma into the thylakoid space

  7. This creates a concentration gradient of protons

  8. Electrons reach another molecule of chlorophyll a in PSI (P700)

  9. Electrons are passed from PSI to ferredoxin

  10. Ferredoxin passes the electrons to NADP in the stroma

  11. Protons move down the concentration gradient through proton channels attached to ATP synthase (Chemiosmosis)

  12. ADP + Pi -> ATP

  13. Protons accepted by NADP which is reduced (catalyzed by NADP reductase)

  14. ATP and reduced NADP (NADPH) are passed on to the light independent stage in the stroma
• Light Dependent Reaction: Cyclic

  1. Light strikes PSI

  2. Pair of electrons in the chlorophyll a molecule are excited and pass to ferredoxin

  3. They are passed to an electron carrier system

  4. They return back to PSI

  5. Some ATP is made from the electron flow through the electron carrier system
• Cyclic Phosphorylation


  1. Some cells e.g. guard cells only have PSI so only carry out cyclic photophosphorylation. Small amount of ATP is used to pump K+ ions in to change the water potential

  2. ATP may also be used in the guard cells (their chloroplasts only contain PSI)

  3. This will bring in potassium ions, lowering the water potential and causing water to follow by osmosis

  4. This causes the guard cells to swell and open the stomata
• Z Pathway or Z Scheme

  Shows movement of electrons and H+ ions
• Products of the light-dependent reaction

  • Products that pass into the light-independent reaction: reduced NADP, ATP
  • Products that leave the plant: Oxygen (from photolysis)
  • Products that are re-used in another part of the light-dependent reaction: H+ ions, electrons