Photosynthetic Carbon Fixation

I.  Light Independent Reactions

II. Calvin Benson Cycle (C3 Photosynthesis)

     A. Chloroplast stroma

     B. 6 CO2 + 12 NADPH2 +18 ATP -> 
      
        1 C6H2O6 + 12 NADP + 18 ADP + 18 Pi + 6 H2O

     C. One complete cycle fixes one molecule of CO2
        
        1. Six complete cycles makes one molecule of glucose

        2. PGA = Phosphoglycerate

        3. PGAL = Glyceraldehyde phosphate

     D. Ribulose bisphosphate (RuBP)

        1.  High CO2 -> 2 PGA

        2.  Photorespiration 

               a.  High O2 -> 1 PGA + phosphoglycolate

               b.  phosphoglycolate -> PGA + CO2

               c.  20-30 % of Carbon fixed is lost 


III. Hatch & Slack (C4) Photosynthesis

     A. Spatial separation of initial CO2 fixation and C3 cycle

     B. Mesophyll Cells 
         
        1. "normal" looking chloroplasts

        2. CO2 + pyruvate + NADPH + H+ + 2 ATP ->

           malate + NADP+ + 2 ADP + 2 Pi

        3. PEP carboxylate is insensitive to O2

     C. Malate transported to 

     D. Bundle Sheath Cells

        1. chloroplasts lack grana

        2. malate + NADP+ -> pyruvate + NADPH + H+ + CO2


        3. Calvin Cycle proceeds in CO2 enriched environment

     E. Pyruvate transported to Mesophyll Cells

     F. More efficient under conditions of high light and high    
        temperature

         1. PEP carboxylase more efficient than RuBP under these
            conditions

         2. CO2 concentrated at site of Calvin Cycle effectively
            eliminates photorespiration

IV. Crassulacean Acid Metabolism = CAM

     A.  Temporal separation of initial CO2 fixation and C3 Cycle

     B.  Night

         1. Stomata open = rate of water loss reduced

         2. CO2 + pyruvate + NADPH + H+ + 2 ATP ->

            malate + NADP+ + 2 ADP + 2 Pi
         
         3. Malate stored in vacuole = pH decreases

     C.  Day

         1. Stomata closed = minimum water loss

         2. Light reaction provide energy

         3. Malate retrieved from vacuole and converted = pH      
            increases

         4. malate +NADP+ -> pyruvate + NADPH + H+ +CO2

         5. Calvin cycle

     D.  Adapted to conditions of high light, high temperature, and
         extreme dryness

          1. Water conserved

          2. Succulent plants

          3. Slow growth