Phototrophy

Phototrophy

1. Harnessing photo-excited electrons to power cell growth

2. Different Kinds (3)

   • 1. Bacteriorhodopsin proton pump
   • 2. Single-cycle chlorophyll-based photosystems I and II
   • 3. Double-cycle Z pathway

3. ultimate source of electrons driving metabolism

4. Retinal-Based Proton Pumps

   • in most cases, photosynthesis is based on chlorophyll
   • some prokaryotes use a more ancient form of phototrophy
     • Bacteriorhodopsin
       • single protein, light driven proton pump
       • absorb green light, reflect purple
       • Proteorhodopsin: homolog in marine proteobacteria
       • halophilic archaea
       • has membrane-spanning alpha helices that surround a molecule of retinal

5. Chlorophyll Photoexcitation and Photolysis

   • energy for photosynthesis comes from photoexcitation of a light-absorbing pigment
     • Photolysis: light-driven separation of an electron from a donor molecules
       • Each electron is transferred to an ETS
         • ETS generates a proton potential and NADPH
           • Proton potential drives ATP synthesis through F1F0 ATP synthase
       • All forms share a common setup (but photosynthesis is diverse
         • 1. Antenna system: maximize photon capture
           • 2. Reaction center complex: separate electron
             • 3. Electron Transport sytem: PS I, PS II, oxygenic Z patway
               • 4. Energy Carriers (make NADPH, ATP or both)

6. Chlorophylls: main light absorbing pigment

   • contains a chromophore
     • slight differences in chromophore alter their absorption spectra
   • absorbs red and blue, reflects green

7. Antenna Complex

   • maximize light absorption (chlorophyll)
   • ring forms around the reaction center where chlorophyll photoexcitation connects to the ETS

8. Thylakoids: increase photon uptake efficiency

   • extensive folding of photosynthetic membrane to maximize light absorption
   • Inside thylakoid: lumen
   • Outside thylakoid: stroma

9. Photosystems I and II

   • Photolysis and ETS occur in 3 different systems
     • Anaerobic photosystem I: NADPH
     • Anaerobic photosystem II: ATP
     • Oxygenic Z Pathway: NADHP & ATP
       • cyanobacteria and chloroplasts of green plants
         • generates 3 ATP
         • energy is used to get CO2 to build the cell