Photosynthesis

• Life on earth ultimately depends on energy derived from the sun
• Photosynthesis is the only biological process that can harvest this energy
• Photosynthesis equation


  6CO2 + 6H2O → C6H12O6 + 6O2
• Uses of plants today

  • Food
  • Fuel/Biofuel
  • Therapeutic drugs
• Plants have been a key source of energy over tens of thousands of millennia
• Vinblastine and vincristine are used for treatment of leukemia and Hodgkin’s lymphoma
• Salicylic acid is isolated from bark for production of aspirin
• Alkaloids from opium are an effective analgesic
• Quinine is derived from bark for production of antimalarial drugs
• Photosynthesis occurs in the chloroplast
• Green marine sea slug

  • Elysia chlorotica
• Endosymbiosis in Elysia chlorotica

  1. Eat algae

  2. Cuts open algal filament

  3. Sucks out living chloroplast

  4. Transfers chloroplast to its own tissue

  5. Can survive up to a year without eating
• Algal genes are now found in Elysia chlorotica
• The light reactions involve pigments, photosystems, and the electron transport chain
• Photosynthetic pigments


  • Bacteriochlorophyll
  • Chlorophyll a
  • Chlorophyll b
  • Phytoerythrobilin
  • β-carotene
• Chlorophyll a is the primary photosynthetic pigment
• Chlorophyll b is an accessory pigment
• Carotenoids are accessory pigments
• The energy from pigments has three possible fates
• Fates of energy from pigments

  1. Converting extra energy to heat

  2. Transferring energy to a neighbouring chlorophyll molecule

  3. Transferring from a negatively charged high-energy electron to another nearby molecule
• The light reactions


  1. Charge separation

  2. Resonance energy transfer

  3. Electron transfer

  4. Photolysis
• Light energy drives the synthesis of both ATP and NADPH
• The oxygen evolving complex catalyzes the splitting of two water molecules
• The proton gradient drives ATP synthase to generate ATP
• The Z-scheme couples PSII and PSI
• Cyclic electron flow generates more ATP without making NADPH
• Non-cyclic vs cyclic photophosphorylation

  • Photosystems involved
  • Is photolysis involved?
  • Fate of electrons
  • Product(s)
• Non-cyclic photophosphorylation involves PSI and PSII
• Cyclic photophosphorylation involves only PSI
• Photolysis is involved in non-cyclic photophosphorylation
• ATP, NADPH, and O2 are products of non-cyclic photophosphorylation
• ATP is the product of cyclic photophosphorylation
• The Calvin-Benson-Bassham cycle


  1. Regeneration

  2. Reduction

  3. Carboxylation

  4. Output
• RuBisCO is central to life on earth
• Photrespiration occurs when RuBisCO fixes oxygen instead of carbon dioxide
• Photrespiration is an energetically costly salvage pathway
• Carbon concentrating mechanisms


  • C4 plants
  • CAM plants
  • Algae
• C4 photosynthesis involves shuttling CO2 via malate or aspartate
• CAM plants store CO2 as malic acid in the vacuole
• Carbon concentrating mechanisms involve specialized structures