Cellular process - bacteria, blue-green, and             eukaryotic cells with chloroplasts   Capture of light energy by pigments -       chlorophylls  &  accessory pigments     Capture e’s  as reducing power in NADPH -   Reduction of CO2 to CH2O

 

 

 

 

 

 

2 Fundamental Reaction Mechanisms
LIGHT Reactions (photo-chemical reactions)                  molecular excitation chlorophyll by light... charge separation                  hydrolysis of H2O   releasing    2H+ and ½O2                   generation of proton motive force (H+ gradient) across thylakoid membranes                  reduction of NADP to NADPH via an ETS
DARK Reactions (thermo-chemical reactions)                        CO2 fixation via  (reduction stages - reverse of glycolysis)                                carboxylation: CO2  +  RuBP  -->  2 PGA   [ 1C + 5C --> 2 (3C) ]                                reduction PGA with NADPH   -->   PGAL                                regeneration of RuBP via HMP  (5C sugar) pathway  -->  RuBP
6CO2 + 12H2O* --> C6H12O6 + 6H2O + 6O*2

    
Photosynthetic Electron Flow...   what happens to the photoionized e-

 Overview 4 stages PHTS* 1) light absorption, 2) e^- transport & PMF, 3) ATP synthesis, & 4) carbon  reduction
  
 Photosystems* - chlorophylls, accessory pigments, reaction center, primary e-acceptors
     PS-I   and   PS-II -  contain Reaction Centers...  mcb 12.34a*  &  mcb12.34b  &  PSII* 
        >  path of e- flow within   PS-II  & PS-I is -->  noncyclic electron flow   mcb 12.37*
                         1)   captures e- into coenzyme NADP⁺ ---> NADPH 
                         2)   releases O₂ from splitting of H₂O   O₂-evol*               Karp 6.9 non-cyclic
     >  path of e- flow PS1 - cyclic electron flow        mcb 12.36*             Karp 6.16 cyclic
  
     Morphological basis  &  mechanism of action...        
                        mcb 12.29*  &   mcb 12.42*  &    some possible locales within thylakoids
                        cooperation of PSII & PSI within the complexity of the e- reactions...
                                                             Karp fig 6.10 PS-II    &    Karp fig 16.14 PSI   &    Karp fig 6.13
   
      ATPase makes ATP (just like in mitochondria)...   summary figure* 
                       chemiosmosis & proton gradient generate a PMF                  mcb 12.22*
                 comparison of locations of chemiosmosis (chlp & mito)          mcb 12.22*

  
 Dark Reactions of Photosynthesis... 
         occur in stroma (chloroplasm)
                consume ATP and NADPH made in light reactions
                reduces (fixation) CO₂ into CH₂O (sugars)
         Radiocarbon tracing C-atoms during dark reaction --> ¹⁴CO₂  &  Calvin "lollipop*"
                     &  paper chromatography  was used to identify pathway.   1961 Nobel
  
           3 different pathways to make sugar
   
                   1.  CALVIN cycle or the C3-pathway
                       1 CO₂ + 5C RuBP ---> (2) 3C sugars (PGA)   mcb 12.43*
                           (2) 3C sugars combine ---> 1 net glucose        mcb 12.44*
                                 RuBP carboxylase  -  Rubisco &  pic pic -  [50% of leaf protein]
   
                                 but  Rubisco is inhibited by O₂ -->  Photo-respiration* - [definition]
                                                
 

 
 
   2.  Hatch & Slack Pathway...    C4-pathway of Carbon Fixation...
   
        1 ¹⁴CO₂ + 3C PGA --> 4C-acid  [OAA] --> malate in mesophyll cells   (Hugo Kortschak-1965)
                                     4C-acid ---> 3C (PYR)  +  CO₂    (in  mesophyll cells)  and
                                     CO₂ is reduced in Calvin Cycle of bundle sheath cells to glucose.
              pathway (mcb 12.46b*)  &  leaf anatomy differences  (mcb 12.46a*)
              PEP-carboxylase (fig + fig)          &        cellular location of pathways - karp fig 6.21
 
       
        KEY:    Rubisco's -  K_m(CO₂) is about 10mM in C₃ plants, but about 30mM  in C₄ plants
  
           Several groups of angiosperms (corn, sorghum, sugarcane) as well as certain dicots, including
            pigweed (Amaranthus) & halophytes such as Atriplex (saltbush) have developed the
            adaptations which allow CO₂ uptake & formation of a 4-carbon molecule  [OAA]  instead
            of the two 3-carbon PGA's of the Calvin cycle.  Hence these plants are called C₄ plants.
     details of C-4 pathway.         Plants that have only the Calvin cycle are thus C₃ plants.
 

 
    
   3.  CAM Pathway  (C4- Crassulacean Acid Metabolism)    -   CAM plants...
  
        are also C₄ plants but don't separate C₄ & C₃ pathways in different parts of leaf (spatially),
        but rather separates them in time.    CAM was 1st studied in the plant family Crassulaceae.

 
  
   These temporal and anatomical adaptations also enable these plants to thrive in conditions of
         (1) high daytime temperatures,    (2) intense sunlight, &      (3) low soil moisture.
  
   The CAM path occurs in a wide variety of plant species, mainly in arid and tropical regions...
   
   Some examples of CAM plants: Crassulaceae (Sedum, Kalanchoe), Cactaceae (cacti),
        Bromeliaceae (pineapple) & all epiphytic bromeliads including Spanish moss,
        Orchids, the century plant, yucca, and  the "ice plant" that grows in sandy
        parts of the scrub forest biome.
  
   CAM plants represent only about 10% of the world's flora (C3 are 85% & C4 are 5%),
  
                    Same pathway & carboxylase enzymes as in C4, but within the same cell..
                            shows temporal  not spatial differences
                            regulated by stomatal uptake.
                                                             Comparative figure   C4 v. CAM*
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PHOTORESPRATION...     RUBISCO catalyzes two different reactions:                              adding CO2 to ribulose bisphosphate -   a  carboxylase activity                              adding   O2 to ribulose bisphosphate -  an  oxygenase activity.             mcb 8.43*
Which one predominates depends on the relative concentrations of O2 & CO2 with                              high CO2 :  low   O2  favoring the carboxylase action                                  high   O2 :  low CO2  favoring the oxygenase action
The light reactions liberate O2 & more O2 dissolves in the cytosol of the cell at higher temps. Thus,     high light intensities & high temps (above ~ 30°C) favor the oxygenase second reaction. The uptake     of O2 by RUBISCO forms two 3-carbon molecules:     [reaction]
1)  one is 3-phosphoglyceric acid  [3PGA] just as in the Calvin cycle                   2)   the other is 2P-glycolate.
	Cellular basis - Karp fig 6.22 (p239)...  the glycolate enters peroxisomes where it uses O2 to form intermediates that enter mitochondria where they are broken down to CO2;  so this process uses O2 and liberates CO2 just as cellular respiration does, thus it's called photorespiration. It undoes the anabolic work of photosynthesis, reducing the net productivity of a plant.  For this reason, much effort so far largely unsuccessful - has gone into attempts to alter crop plants so that they carry on less photorespiration. Speculation: the problem may solve itself. If atmospheric CO2  concentrations continue to rise, perhaps it will enhance the net productivity of the world's crops by reducing losses to photorespiration.

 

 

 

 

 

Details of the C4 cycle:    [leaf anatomy]
After entering through stomata, CO2 diffuses into a mesophyll cell.  Being close to the leaf surface, these cells are exposed to high levels of O2, but these cells have little RUBISCO, whose Km is less efficient here than in bundle sheath cells, so cannot start photorespiration or the Calvin cycle dark reactions. the CO2 is inserted into a 3-carbon compound (C3) called phosphoenolpyruvic acid (PEP) forming the 4-carbon compound oxaloacetic acid (C4).           [C4 cycle]             Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells are deep in the leaf interior so atmospheric oxygen cannot diffuse easily to them; often have thylakoids with reduced photosystem II complexes (the one that produces O2). Both of these features keep oxygen levels low. Here the 4-carbon compound (malic/aspartic acids) is broken down into CO2 carbon dioxide, which enters the Calvin cycle to form sugars and starch. Pyruvic acid (C3) - which is transported back to a mesophyll cell where it is converted back into PEP.
	C4 plants represent only 5% of world's species, but fix 18% of world's organic carbon
	C4 plants are well adapted to (and likely to be found in) habitats with high daytime   temperatures  and intense sunlight... including: crabgrass,  corn (maize),  sugarcane, &  sorghum.