Light-dependent stage

Light-dependent stage

1. Photosystems

   • PS1 = pigment at primary reaction centre is type of chlorophyll a, peak absorption red light wavelength 700nm
   • PS2 = pigment at primary reaction centre is type of chlorophyll a, peak absorption red ligth wavelength 680nm

2. Role of water

   • PS2 photolysis occurs (splitting water in presence of light)
   • Source of protons for photophosphorylation
   • source of by-product O2
   • keeps plant cells turgid, allowing for function
   • donates electrons to chlorophyll, replaces lost when light strikes chlorophyll

3. Photophosphorylation

   • Generation ATP from ADP and inorganic phosphate in presence of light
   • 2 types
     • Non-cyclic photophosphorylation
       • PS1 and PS2
       • Produces ATP, O2 and reduced NADP
     • Cyclic photophosphroylation
       • PS1
       • Produces small amounts ATP

4. Non-cyclic photophosphorylation

   • 1. Photon light strikes Ps2, energy goes to primary pigment reaction centre
   • 2. Energy excites pair electrons in chlorophyll
   • 3. Energised electrons escape chlorophyll, capture by electron carrier (protein iron at centre)
   • 4. electrons replaced by electrons from photolysis
   • 5. Electrons passed along chain of electron carriers, some energy from electron released
   • 7. Electrons captured by chlorophyll a in PS1
   • 8. Protein iron sulfur complex (ferredoxin) electrons to NADP in stroma
   • 9. Protons accumulate in thylakoid space, proton gradient forms across membrane
   • 10. protons diffuse down gradient through channels associated with ATP synthase enzymes
   • 11. Flow of protons cause ADP+inorganic phosphate=ATP
   • 12. NADP accepts protons and electrons, NADP reduced by NADP reductase
   • 6. Energy pumps protons across thylakoid membrane to thylakoid space

5. Occurs in grana

6. Cyclic Photophosphorylation

   • 1. Light strike PS1 electrons gain energy, excited
   • 2. Escape chlorophyll, pass back to electron carrier, then back to PS1
   • 3. During passage, small amount ATP generated
   • 4. No photolysis = no protons, O2 and NADP produced
   • 5. ATP actively brings in K+, lowering water potential, water follows by osmosis, guard cell swells and opens stoma