Photosynthesis

• Photosynthesis is the production of energy from the sun by using photons as energy
• Light phase:

  • Electron transport system and photophosphorylation
  • Generates energy through NADPH and ATP
  • Establishes a proton gradient with electron transport
• Calvin cycle:

  • Uses ATP and NADPH to reduce CO2 and form triose phosphate, starch, and sucrose
• Chlorophylls are highly effective photoreceptors because:

  • Have chromophores which are protein complexes that absorb sunlight
  • Present in PSI and PSII
  • Good at absorbing sunlight due to extended polyene structure with alternating single and double bonds allowing for light absorption
  • Strong absorption in visible region
  • Resembles heme with polycyclic planar structure, mg2+ ion to coordinate ring of nitrogens, and hydrophobic side chain
• Light harvesting complex (LHC):

  • Contains a large number of chlorophyll molecules and accessory pigments
  • Outcomes of photon absorption in accessory/antenna pigments of LHC include resonance energy transfer, photooxidation, and fluorescence
• Photosynthetic reaction center:

  • Chlorophyll molecule is the reaction center
  • Outcome of photon absorption in the special chlorophyll molecule at the reaction center is photooxidation
  • Fast, efficient, thermodynamically stable
  • Occurs in a solid state
  • High efficiency
  • Thermodynamically downhill
• 1. Z scheme electron carriers

  • NADP+ is the final electron acceptor and water is the electron donor
  • Main electron carriers discussed in class: plastoquinone (PQb), Cytb6f, plastocyanin (PC), PSI, 4Fe-4S (FB), ferredoxin (FAD), reductase enzyme, FADH2
  • Overall stoichiometry of the ETS: 8 photons absorbed, 4 electrons transferred, 12 protons
• Cyclic and non-cyclic parts of the Z-scheme:

  • Non-cyclic except for the return from Fd to PQB bypassing the reduction of NADP+
  • Condition promoting the cyclic pathway: Too much NADH is made and not being used, so PQB is saturated by Fd
• Outcomes of cyclic and non-cyclic pathways:

  • Cyclic: 2 ATPs, occurs when electrons from Fd return to PQB bypassing reduction of NADP+
  • Non-cyclic: 3 ATPs
• Direction of H+ flow:

  • E- transfer follows H+ translocation from stroma to lumen in thylakoid
  • Low pH in the lumen due to high H+ concentration, high pH in stroma
  • H+ flow out through ATP synthase leading to ATP synthesis in the stroma
• Protons generated into the lumen of the thylakoid and ATPs generated by pumping these protons out into the stroma:

  • 12 H+ and 3 ATPs
• Segregation of PSII, PSI, and ATP synthase in the thylakoid membrane:

  • PSII associated with the grana/granum
  • PSI and ATP Synthase associated with the lamella/lamallae
  • Advantage: PSI and ATP Synthase more exposed to the stroma
• LHCII complex locating to the lamellae:

  • Promotes cyclic photophosphorylation by redistributing LHCII more to the lamellae than to the grana in response to increased levels of PQBH2; regulated by phosphorylation
• Calvin Cycle:

  • Turns atmospheric CO2 into metabolic fuel
  • 3 stages: fixation, reduction, regeneration
  • ATP and NADPH used in fixation, reduction, and regeneration stages
• Reactions in stages 1 and 2:

  • Stage 1: Ribulose-1,5-bisphosphate to 3-phosphoglycerate
  • Stage 2: 3-phosphoglycerate reduced to glyceraldehyde-3-P using NADPH and ATP
• Outcome of stage 3:

  • Regeneration of Ribulose-1,5-bisphosphate
• Overall stoichiometry of the Calvin cycle:

  • Fixation of three CO2 molecules yields one GAP and regenerates 3 Ru1,5-BP
  • Consumes nine ATP molecules and six NADPH molecules
• GAP/G3P generated in the Calvin cycle is used for:

  • Regenerating rubisco and carbohydrate synthesis
• Rubisco enzymatic activities:

  • Carboxylase (fix CO2) and oxygenase (fix oxygen; wasteful; photorespiration)
  • Not a good enzyme due to slow Kcat
• Rubisco regulation by CA1P and Rubisco activase:

  • Rubisco activase changes rubisco conformation to expose Lys-201 for carbamoylation, facilitating Mg2+ coordination
• Calvin cycle enzymes not present in humans/animals:

  • Rubisco, sedoheptulose 1,7-bisphosphatase (SBPase), ribulose 5-phosphate kinase
• Calvin cycle regulation by light and thioredoxin:

  • Controlled by a thioredoxin-mediated reduction reaction, activating Calvin cycle enzymes
• Photorespiration:

  • Driven by rubisco's oxygenate activity
  • C4 plants avoid it by fixing CO2 into a 4-C compound before rubisco
• Glyoxylate cycle:

  • Active in seedlings, converts fats stored in seeds to glucose
  • Not carried out by animals due to lack of isocitrate lyase and malware synthase