LITTLE GREEN MEN               Why don't animal cells photosynthesize ???
Light energy is captured by pigments, so let's compare pigmented molecules...                        chlorophyll*       vs.      hemoglobin             leghemoglobin*
► an assumption: a mutant metazoan cell whose hemoglobin's can capture light energy,                                  occurs in epithelial cells, & evolves to be as efficient as today's plants:
	best plant photosynthetic rates	=  20 mg hexose sugar/dm2leaf/hr
	average human surface area	=  170 dm2
	hexose productivity	=  [170 x 20mg = 3.4gm/hr] x 12hr = 40.8 gm/d
	1 mole glucose	=  183 gm  =   686Kc/mol
	green man productivity	=    41 gm  =   153 Kc/mol
	BMR of man	=  2,000 Kc/d   = about 8.5 % of need
We Evolve   =  increased surface area (1 dm2 projections), remain sessile,                                 peristalsis becomes vestigial, circulation is replaced, etc                ........ We are a plant, i.e., no selective pressure to photosynthesize?

 
what is   PHOTOSYNTHESIS...     a light driven phosphorylation of ADP...
                                                             ADP  +  P    ----light---->    ATP
                                                                        photophosphorylation 
  ...it's described as  AUTOTROPHIC Metabolism  that occurs in organisms,
                which produces all their organic nutrients from inorganic
                materials thru conversion of light energy into covalent bond energy...
     two autotrophic processes based upon electron sources: 
                a. chemotrophic... uses oxidation (e^-) of small inorganics  as H₂S.
                b. phototrophic ... uses light energy (photons) to make organics.          

so... What is PHOTOSYNTHESIS?     it's a cellular process* - requiring living cells...     with specialized - ionizable chlorophyll molecules          located in photosystems: PSII = P680  &  PSI = P700     that donate e-s to create a proton gradient for ATP synthase            procaryotes -  bacteria, cyanobacteria  [blue-green algae],         eucaryotes   -  all plant cells  with  chloroplasts

What are the MOLECULAR PROCESSES within Photosynthesis ?
      
►new   a.   capture of photon energy of is via PIGMENT molecules -
                    chlorophylls & accessory pigments (as carotenes & phycobilins)
 
           b.   a  REDOX reaction - [includes PHOTOLYSIS = splitting of water]
                                                         H₂O   -->   2H⁺   +  2e^-   +   1/2O₂
                     photolysis produces oxidizing power  =  O₂ 
                     captures of photons (e-) into cytochromes via plant ETC-like carrier proteins,
                     produces reducing power  =   NADPH*        [just like NADH, but with a P]
                           [cells have separate pools  of  NAD    (in cytoplasm & mitochondria)
                                                                      &   NADP (in chloroplast)]
           c.  produces ATP via photophosphorylation
                     couples e- transfer to H+ gradients  &  ATP synthase   [ADP + P = ATP ]
 
►new   d.  reduction of  carbon dioxide  (CO₂)  to  carbohydrate  (CH₂O)
                              6CO₂    +   12 H₂O^  <--->    C₆H₁₂O₆   +    6H₂O    +  6O₂^
                               CO₂    +     2H₂A     <--->      CH₂O    +       H₂O    +   2A
                     Source C   +   e^- donor            organic C       oxidized donor 

 
  There are 2 Fundamental Reaction Mechanisms of Photosynthesis:

LIGHT Reactions - photochemical reactions (molecular based)            molecular excitation of chlorophyll by light ejects an electron            electron transfers in a PHTS-ETC creates a proton gradient            a charge (H+) separation across a membrane results in            generation of ATP via PHTS-ATP synthase,            and the reduction of NADP, via an PHTS- ETC, to NADPH

  DARK Reactions - thermochemical reactions (catalyzed via enzymes)
           carbon dioxide reduction (also called fixation) -  occurs in 3 stages
                      carboxylation        CO₂ +  RuBP  (5c)      --(6C)-->    2 PGA (3c)
                      reduction               of  PGA  by  NADPH  -->     PGAL  -->   sugar
                      regeneration         of  RuBP  via  Calvin cycle  pathway
             Cellular Locations of the Photosynthetic Reactions*       

 Morphological Basis of green plant Photosynthesis...   is the CHLOROPLAST

     PLASTIDS - are double unit membrane bound organelles, most often
                          classified by pigment content and derived from...

     MERISTEMATIC Cells* give rise to Proplastids... that give rise to all other plastids

                LEUCOPLASTS        - amyloplasts    -  synthsize & store starch
                                                  aleuoplasts    -  contain stored protein (crystals)
                                                  elaioplasts     -   contain oil & fat globules - fat biosynthesis
 
              CHROMOPLASTS    -  found in flower petals, ripe fruit, senescent leaves
                                                   function in the synthesis and storage of carotenoid pigments
                                                   in flowers and fruits, and in certain leaves and roots.
 
              CHLOROPLASTS    -  is the morphological basis of photosynthesis*
                                         -  occur within the anatomy of a typical plant leaf 
                                               -  stomates control gas exchange
                                               -  likely arose by endosymbiosis with early eukaryote.
                                               -  research on synthetic chloroplasts may improve phts.
                         

 
 
    Morphological Basis of green plant Photosynthesis...   is the CHLOROPLAST

    CHLOROPLASTS - are ubiquitious to all green plants...  Structure*      
 
        SHAPE      - variable (elipsoid to ovoid; lenticular, stellate, convex)
        SIZE          - 2 to 3 um dia by 5 to 10 um long
        NUMBER   - 20 to 200 per mesophyll cell    [up to 400,000/cc]
        VOLUME   - often larger than mitochondria

    CHLOROPLASM (stroma) contains...
               pyrenoids* - are Carbon Conentrating Centers for CO₂ reduction
                70s ribosomes (prokaryotic size ribosomes)
                naked DNA -  2 to 10^-15gm ^(fentograms) of DNA/chloroplast
                            an amount equal to a bacterial cell's DNA & is circular;
                            more highly supercoiled & repetitive DNA (15% = 7 repeat motifs)
                enzymes of CO₂ fixation and lipid droplets
    images of Thylakoid  Membranes* which are responsible for light absorption
    

 
 
PIGMENTS of Photosynthesis - 
             
     1. Accessory Pigments - any non-chlorophyll pigments that absorb light energy
         a.   Carotenoids - carotenes*   and    xanthophylls*
         b.   Phycobilins  - have a straight chain porphyrin chromophore group*  +  a protein
                  ex: -  phycoerythrin & phycocyanin  - pigments of red & blue-green algae
 
  2. Chlorophylls - a,b,d, etc... [embedded in thylakoid disk membranes]*

   
Light absorption during photosynthesis by pigments:

    Electromagnetic spectrum & light* =    animation*      -->      light absorption
         
                     SPECTROSCOPY  with a   SPECTROPHOTOMETER*  
                                sample instruments:    Spectronic 20  and   cuvettes
                    ABSORPTION SPECTRA* - plot of amount light absorbed vs. wavelength
                                 but it does not tell us if pigment is involved in photosynthesis 
                    ACTION SPECTRA* - plot of physiological activity [O₂ released] vs. wavelength  

                                                                     

Molecular Excitation of Chlorophyll   ABSORPTION of Light Energy ... blue light and red light      results in electrons being excited into higher orbitals

      FATES of Absorbed Energy -  PHOTOEXCITATION
     
                1. reradiates as vibrational heat*
  
                2. reradiated as fluorescence   
  
                3. reradiated as phosphorescence* (a slower far red light re-emission)
  
                4. induced resonance* - vibrational  e^- excitation is passed from
                                                       chlorophyll molecule to chlorophyll moelcule.
  
                5. PHOTOIONIZATION - takes part in photochemical reactions:
                                electron is passed to an acceptor = ionized chl+

        
The LIGHT REACTIONS...  (or PHOTOIONIZATION)  occurs in thylakoids in...
                       animation of the light reactions of photosynthesis*
     
    Light Harvesting Complexes* are the site of the photochemical reactions.
        -   the two PHOTOSYSTEMS*  PS1 & PS2 with their complexes of protein/pigments
        -   includes: chlorophylls, reaction center molecules  P₆₈₀ & P₇₀₀,  &   e^- acceptors

PS1   -   [P700 rx center]  captures e- into coenzyme NADP+ making NADPH

PS2   -   [P680 rx center]  -  splits water (photolysis) releasing  O2  &  H+                     oxygen evolving complex* of PS-2 holds 4 Mn atoms that oxidize & lose e-'s                                 ►  oxygenic photosynthesis was a great evolutionary invention of microbial metabolism

There are two pathways of e-flow in Photosystems = anim of cyclic and non-cyclic electron flow      Electrons move thru the electron transfer chain via  Non-Cyclic & Cyclic* electron flow.                   Products of e- flow in PHTS:    non-cyclic makes   -  ATP + NADPH + O2                                                   while cyclic makes  -  ATP, but no NADPH & no O2

Chemiosmosis*  is how a chloroplast proton gradients makes ATP

Photophosphorylation is how photosynthetic cells make ATP via  ATP-synthase *

let's compare the proton gradient  orientations* in chloroplasts vs. mitochondria

  
  
DARK REACTIONS - 3 major metabolic pathways (C3, C4, & CAM)   
                    both occur in chloroplasm  (stroma) of chloroplasts...  _{dark reactions}*      
                    consume the  ATP  &  NADPH  made in light reactions
                    reduce CO₂ (fixation = reduction)   into  CH₂O   (sugars)
   
1st   C3 Calvin Cycle - [Calvin-Benson-Bassham - 1950 J Biol Chem. 185 (2): 781–7. doi:10.2172/910351]  
        C-3 biochemical pathway... Mel Calvin's used the "lollipop"* &  C¹⁴intermediates*
        C3 cycle animation* called C3 pathway, as 1st molecule made = 3-carbon carbohydrate. 
         fixation of CO₂ is catalyzed by Rubisco* -->   1-CO₂   +  5C-RuBP  --->  2-PGA
                    PGA is a sugar-carb (3-phosphoglycerate)
                    two net 3C-PGA's may combine to yield  --->  1 net glucose
           Calvin cycle can be considered as reverse of reactions of glycolysis - fig 10.18*   

           Rubisco - RuBP carboxylase is also an oxygenase with O₂ that  competes with CO₂
           Photo-Respiration - a process that releases CO₂ via Rubisco oxygenation*
                                           overcoming photorespiration* via genetic engineering.
         
  In additional to Calvin Cycle, there are additional pathways* that reduce CO₂ ... 

       3rd

email to : C.Mallery

   
     Sumanas anim - Light Rx*^{flash-view at home}

    
                  Engelman experiment (pic 2)     &    Rod cells & Cone cells Rhodopsin curve