12-14: Light Dependent Reactions

Photosynthesis is the opposite of cellular respiration → takes in carbon dioxide and water, takes out carbohydrates and oxygen

Photosystem 2 Photoexcitation

  • photosystem 2 starts the light dependent reactions
  • the light antenna complex absorbs a photon and transfers it to p680 chlorophyll molecule which excites an electron
  • the electron gets transferred to the acceptor which makes the p680 positively charged
  • p680 gets its electron back from the water splitting complex
  • each water molecule donates 2 electrons so the process happens twice to produce oxygen
  • the acceptor molecules that took p680’s electron passes it to a molecule of plastiquinone (PQ)

 

Electron Transport Shuttles

  • the PQ molecule accepts an electron from photosystem 2 as well as protons (H⁺) from the stroma
  • the electron then travels through the thylakoid membrane and donates the electron to the cytochrome complex
  • at the same time, it releases the H⁺ into the lumen which increases the proton complex
  • the cytochrome complex passes the electron to another shuttle molecule PQ which carries it to photosystem 1

   

Photosystem 1 and Photoexcitation

  • when a photon of light strikes photosystem 1, an electron in the p700 molecule is excited
  • this excited electron is transferred to another acceptor leaving p700 with a positive charge
  • the electron transferred from photosystem 2 is delivered by PQ to neutralize p700

 

Ferredoxin and NADPH

  • the electron from photosystem 1 gets transferred to ferredoxin (an iron sulfur protein)
  • oxidation of ferredoxin transfers the electron to NADP+ making NADP
  • a second ferredoxin adds another electron along with H⁺ from the stroma to make NADPH
  • all of this happens with an enzyme called NADP+ reductase

 

Proton Gradient

  • all this activity has increased the proton concentration in the thylakoid lumen and decreased it in the stroma
    • splitting of water produces protons in lumen
    • PQ and cytochrome complex transport protons from storm to lumen
    • NADPH production removes protons from stroma solution
  • this proton gradient can be used for chemiosmosis

 

ATP Synthase

  • identical to ATP synthase in mitochondria
  • this time app is produced by photophosphorylation (uses light energy, no oxidation occurred to produce the proton gradient)

 

Electron Energy

  • the boost in energy provided by the photons in sunlight gradually decreases as the electrons move through ETCs of photosystem 1 and 2

 

Cyclic Electron Transport

  • photosystem 1 works independently of photosystem 2 if the electron on ferredoxin is transferred to PQ rather than NADP+ reductase
  • this continuously pumps protons into the lumen, thus producing more ATP
  • more ATP is needed than NADPH in the Calvin cycle → making this process very useful
    • 9 ATP and 6 NADPH are needed to make 1 G3P (half a glucose)