plants

• ATP and NADPH


  Essential reducing power/electron donor for anabolic reactions and anabolic balance in all organisms, coenzyme that regulates cellular homeostasis
• Multi-protein complexes required for light energy capture


  • Photo-system I (P700): Absorb light
  • Photo-system II (P680): Convert light into chemical energy
  • ATP-synthase: Makes ATP
  • Cytochrome b6f: Provides electronic connection between the PSI and PSII, generates electrochemical proton gradient for ATP synthesis
  • Light harvesting complex (antenna): Protein+chl that transfers the energy to the chl a (reaction center)
• Light energy capture


  1. Light hits the antenna, the chlorin ring captures the photons, energy is transferred

  2. Light absorption (diverse): Sunlight hits the photosystems, and the chl vibrates, enough vibration causes e- to be free from the molecule

  3. Light energy into chemical energy: The photon capture excited the chl, which loses an e- to become oxidized chl+, it is reduced back by stripping an e- from water, producing O2 and protons

  4. As the excited e- travels through the electron transport chain, it loses energy, the energy is used to pump hydrogen ions across from the stroma into the lumen of the thylakoids

  5. To replace the e- lost from the PSII, water is split (photolysis), adding more protons to the lumen, electron gradient is created

  6. More light hits the PSI, to excite the low energy electron (dangerous, cation radical) back up to an excited state

  7. The excited electron goes through an electron transport chain again (ferredoxin) to reduce NADP+ (electron acceptor) to NADPH

  8. The protons in the lumen goes back to the stroma through the ATP synthase to produce ATP
• P680 (PSII)


  • Strong oxidant and a reductant (strong enough to pull electrons from H2O)
• P700 (PSI)


  • Strong oxidant (strong enough to donate electrons to NADP+)
• 1. scheme: Arranged according to the redox potential (ability to transfer electron)
• PSI can function without PSII, only producing ATP (cyclic electron transport in bacteria)
• Electron transport chain


  • Plastoquinone: Mobile e- carrier, accepts e- from PSII
  • Plastocyanin: Mobile, copper containing e- carrier, accepts e- from cytb6f
  • Cytochrome b6f: Membrane embedded multiprotein complex, Fe-S, proton uptake from stroma, proton transfer to lumen, does not require light energy
• Regulation of light energy capture


  • Non-photochemical energy quenching: Regulate the amount of excitation energy delivered to the photosystems
  • State transition: Redistributing excitation energy between photosystems
  • Redundant electron transport pathways: Maintain redox balance in chloroplasts
  • In combination with antioxidant systems
• Photophosphorylation


  1. Cyclic photophosphorylation: High-energy electron released from P700 flows in a cyclic pathway, produces proton-motive force, pumping H+ ions across the membrane and producing a concentration gradient that can be used to power ATP synthase during chemiosmosis

  2. Non-cyclic photophosphorylation: Two-stage process involving two different chlorophyll photosystems in the thylakoid membrane, produces O2, ATP, and NADPH + H+
• The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions
• The photosynthesis is carried out in the chloroplasts, organelles inside the plant cell that are 3-10nm in diameter, made out of 3 membranes, the outer membrane, inner membrane and the thylakoid membrane
• Chloroplasts are green due to their absorption of red and blue light, reflecting to our eyes in an enriched wavelength of about 550nm, making us see it green
• The light reactions take place in the thylakoids and the dark reactions in the stroma
• Some organisms lack chloroplasts but can still perform photosynthesis with the help of some unicellular organisms called prochlorons in a symbiotic way
• Photosynthetic pigments


  • Chlorophyll a
  • Chlorophyll b
• Excited chlorophyll


  1. Re-emitting that photon (fluorescence)

  2. Converting excess energy to heat (non-chemical quenching)

  3. Converted to damaging triplet state (ROS)
• Lamella


  Part of the lumen
• Chloroplasts


  Green due to their absorption of red and blue light, reflecting to our eyes in an enriched wavelength of about 550nm, making us see it green
• Photosynthesis


  1. Light reactions take place in the thylakoids

  2. Dark reactions take place in the stroma
• Some organisms lack chloroplasts but can still perform photosynthesis by using prochlorons in a symbiotic way
• Light is absorbed in photosynthetic pigments
• Photosynthetic pigments


  Chlorophyll a and b, with a porphyrin ring comprising a Mg atom in its center and a hydrocarbon tail that anchors it to the photosynthetic membrane
• Excited chlorophyll returning to ground state

  1. Re-emitting the photon (fluorescence)

  2. Converting excitation energy into heat

  3. Transferring the energy to another molecule

  4. Transferring the energy to the photosynthesis, causing chemical reactions
• Light harvesting antennas (LHC)


  A group of pigments (Carotenoids, Chlorophyll a,b) that collect light and transfer it to the reaction center complex
• Photochemical reaction centers (RC)


  Where the reductive and oxidative reactions take place for long term energy storage
• Having several pigments sending energy into a single RC makes the process much more convenient and efficient, as the enzymes will be active for most of the time
• Carotenoids


  Upon contact with light, they get excited and begin a cascade of energy transmission, transferring electrons to e- acceptors and then getting reduced by an e- donor
• Photosystem I


  Absorbs light in the 700nm wavelength or far red light, producing a weak oxidant and a strong reductant
• Photosystem II


  Absorbs light in the 680nm wavelength, producing a strong oxidant and a weak reductor
• 1. scheme for ejected electrons from chlorophyll


  1. Photosystem II obtains light as energy, exciting it and causing it to create strong oxidants that will oxidize water, releasing protons into the lumen

  2. Cytochrome b6f receives the electrons and delivers them to the photosystem I, which in turn will create strong reductants, reducing NADP+ into NADPH through the action of the enzymes ferredoxin and NADP reductase

  3. ATP-synthase will produce ATP as protons retract back to the stroma
• Electron carriers


  Pheophytin, plastoquinones (Qa,Qb), cytochrome b6f complex, plastocyanin, chlorophyll (A0), A1 quinone, iron-sulphur proteins, ferredoxin (FeSx->FeSa->FeSb->fb), FNR (Ferredoxin-NADP- reductase)
• Photophosphorylation


  The phosphorylation of ADP to form ATP using the energy of sunlight
• Cyclic photophosphorylation


  Occurs on the stroma lamella, or fret channels. The high-energy electron released from P700 flows in a cyclic pathway, producing a proton-motive force that can be used to power ATP synthase during chemiosmosis. This pathway produces neither O2 nor NADPH
• Non-cyclic photophosphorylation


  A two-stage process involving two different chlorophyll photosystems in the thylakoid membrane. First, a photon is absorbed by chlorophyll pigments surrounding the reaction core center of Photosystem II, exciting an electron in the pigment P680 which is transferred to the primary electron acceptor, pheophytin. The energy of P680+ is used to split a water molecule into 2H+ + 1/2 O2 + 2e-. The electrons then pass through Cyt b6 and Cyt f to plastocyanin, using energy from Photosystem I to pump hydrogen ions (H+) into the thylakoid space. This creates a H+ gradient, making H+ ions flow back into the stroma of the chloroplast, providing the energy for the (re)generation of ATP. The excited electrons are transferred to ferredoxin-NADP+ reductase, which uses them to catalyze the reaction NADP+ + 2H+ + 2e- → NADPH + H+, leading to a net production of 1/2O2, ATP, and NADPH + H+
• The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow
• Water movement


  • Gravity
  • Pressure flow
  • Diffusion
  • Osmosis
• Pressure flow


  Solvent and solute move together until pressure equilibrium
• Diffusion


  Solute move in a Brownian motion from high to low concentration until concentration equilibrium, short distance only
• Osmosis


  Solvent (water) moves from lower solute concentration to the higher solute concentration until equilibrium through a membrane