BIO 104 4/24/96

Photosynthesis:
We will be looking at the differences between C3 and C4 carbon fixation and
CAM photosynthesis:
Photorespiration:
1) Carbon dioxide combines with RuBp to form 2 PGA:  this is accomplished by
the enzyme RuBp carboxylase.
     -This enzyme is not endowed with a plethora of knowledge:
          it is a stupid enzyme
          -The enzyme cannot tell the difference between 
           oxygen and carbon dioxide.  The result of this is that
           the RuBp molecule is wasted along with the RuBp carb-
           oxylase.  This enzyme is very inefficient in an 
           oxygen rich environment.  
          -If the enzyme does react with oxygen instead of carbon
           the result is a 3 carbon molecule and a 2 carbon
           molecule.  This reaction is called the RuBp carbo-
           xylase reaction.
     There are 2 different ways of fixing this problem:
          The C3 and C4 pathways
          -C4 is involved because it has a mechanism for over-
           coming photorespiration.
          C4 photosynthesis overcomes photorespiration by 
          involving 2 cell types:
          1) Mesophyll Cells (Parenchyma Cells)
               a. CO2 is first fixed into a 4 carbon compound called
                    malate using another enzyme (PEP carboxylase)
                     - very efficient! -that fixes the CO2 to form a
                     4 carbon molecule.  This is a very smart
                     enzyme since it fixed CO2 in the presence of Oxygen

               b. NADPH2 and ATP are used up during this process.
          -The 4 carbon malate travels via plasmodesmata into the 
           Bundle-Sheath Cells
          2) Bundle Sheath Cells
               a. Four carbon product, Malate, diffuses into the 
                    sheath cells
               b. CO2 is released from Malate and fixed using
                    the Calvin-Cycle

     SEE FIGURE 8.13  

     In the Bundle-Sheath cells there is an enzyme that takes the
     CO2 from the malate to be used in the Calvin-Cycle and then
     to form sugars.
          -These types of plants are most successful in hot/dry
           climates.
     CAM plants (desert plants for example) fix CO2 at night when
     the stoma are open and store the CO2 in organic acids. 
     The CO2 is refixed during the day using the Calvin cycle (when the 
     stoma are closed) Remember ATP and Reduced NADP are made using
     light energy (the light-dependent reactions) Also remember that ATP
     and reduced NADP are used in the Calvin cycle.

At this point Dr. Barstow gave his "Little Green Humanoid from
Mars" lecture.
The worksheet given out in class is available at the BLC and the
Internet.
It should be noted that these notes are fine but you really missed a fine
lecture by not being there! 
For note purposes see the video of lecture and see the worksheet.