Photosynthesis

Photosynthesis

1. 2 Step process

2. Cells synthesize organic molecules from inorganic molecules in presence of sunlight

   • In chloroplast

3. Light Dependent Reactions

   • In intermembrane space of thylakoids
   • Use photosynthetic pigments to convert light into chemical energy
     • 1. Excitation of photosystems by light energy
       • 2. Production of ATP via ETC
         • Reduction of NADP+ and photolysis of water
           • E-electrons from PSI may transfer to carrier to reduce NADP+
             • Forms NADPH-; needed for LI reactions
               • Electrons lost from PSI replaced by de-energized from PSII
                 • Electrons lost from PSII replaced by those released via photolysis
                   • Water is split by light energy into H+ and O
         • E-Electrons from PSII transfer to ETC in membrane
           • Electrons passed through lose energy; energy used to translocate H+ into thylakoid
             • Build up of protons creates electrochemical gradient
               • H+ return to stroma via ATP synthase (Chemiosmosis)
                 • ATP synthase uses H+ passage to catalyze ATP synthesis (Photophosphorylation)
                   • De-energized electrons from PSII go to PSI
       • When photosystem absorbs light, delocalized electrons become excited
         • Excited electrons transfer to carriers within thylakoid membrane

4. Light Independent Reactions (Calvin Cycle)

   • In Stroma of chloroplasts
   • 1. Carboxylation of ribulose bisphosphate
     • 2. Reduction of GP
       • 3. Regeneration of RuBP
         • 1/6 TP used to form 1/2 sugar
           • 2Cycles required to produce sugars
         • 5/6 TP recombine to regenerate stocks of RuBP
           • Requires energy from hydrolysis of ATP
       • GP converts to Triose Phosphate (TP) using NADPH + ATP
         • NADPH transfers H to compound;hydrolysis of ATP provides energy
           • Each GP needs 1NADPH+1ATP to form TP (1Cycle=6of each)
     • RuBP carboxylase catalyzes attachment of CO2 to RuBP
       • =Unstable 6C; breaks into glycerate-3-phosphate (GP)
     • 1 Cycle=6GP
   • Calvin Cycle
     • Formation of Organic from inorganic molecules
     • RuBP is carboxylated by CO2 forming a hexose biphosphate
       • Hexose Biphosphate breaks into GP
         • GP converted by ATP + NADPH to TP
           • Tp used to form organic molecules OR recombined to RuBP

5. Photophosphorylation

   • The production of ATP by light dependent reactions
   • Cyclic
     • Use of only PSI + no reduction of NADP+
       • When PSI absorbs light, E-electron may enter ETC
         • De-energized electron returns to PS, restoring supply
     • Produces steady supply of ATP w/ sunlight
       • ATP cannot be stored
   • Noncyclic
     • Uses both PSI and II; reduction of NADP+
       • When PSII absorbs light, E-electrons enter ETC
         • Photolysis replaces electrons lost in PSII
       • Photoactivation of PSI results in release of electrons; reduces NADP+
         • PSI electrons replaced by PSII
     • Produces NADPH + ATP
       • Long term energy storage

6. Chloroplast

   • Convert light into chemical energy
     • ATP (Dep.) or Organic Comps (Indep.)
   • In leaves but not roots
   • Thylakoids: maximize H gradient
   • Grana: increase SA:Vol
   • Photosystems: maximize light absorption
   • Stroma: contain enzymes; suitable pH for CC
   • Lamellae: connect thylakoid stacks; maximizes photosynthetic efficiency