green men       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?

tulip - next






what is   PHOTOSYNTHESIS...     a light driven phosphorylation of ADP...
       ADP  +  P    ----light---->    ATP
photophosphorylation'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?

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

cyanobacteria  [blue-green algae],
eucaryotes   -  all plant cells  with  chloroplasts

rose - next page        









What are the MOLECULAR PROCESSES within Photosynthesis ?
new   a.   capture of photon energy of is via PIGMENT molecules -
chlorophylls & accessory pigments (as carotenes & phycobilins
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,
reducing power  =   NADPH*        [just like NADH, but with a P]
[cells have separate pools  of  NAD    (in cytoplasm & mitochondria)
(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)
2    +   12 H2O^  <--->    C6H12O6   +    6H2O    +  6O2^
2    +     2H2A     <--->      CH2O    +       H2O    +   2A
                          - next page       
             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

  DARK Reactions - thermochemical reactions (catalyzed via enzymes)
           carbon dioxide reduction (also called fixation) -  occurs in 3 stages
        CO2RuBP  (5c)      --(6C)-->    2 PGA (3c)
reduction               of  PGA  by  NADPH  -->     PGAL  -->   sugar
         of  RuBP  via  Calvin cycle  pathway
pansy - next page             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

   -  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.
iris -
                        next page                         










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
   - 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
ribosomes (prokaryotic size ribosomes)
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)
of CO2 fixation and lipid droplets
flower - next page    images of Thylakoid  Membranes* which are responsible for light absorption














PIGMENTS of Photosynthesis - 
1. Accessory Pigments - any non-chlorophyll pigments that absorb light energy
- carotenes*   and    xanthophylls*
- have a straight chain porphyrin chromophore group*  +  a protein
ex: phycoerythrinphycocyanin 
- 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  
                        bell - next page










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

                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.

PHOTOIONIZATION - takes part in photochemical reactions:
lilly - next page                                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
                                  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
CO2 (fixation = reduction)   into  CH
O   (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
                    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 ... 

daffodil - next page                                                              







2nd  C4 PATHWAY - Hatch & Slack described as a  C4 pathway* in sugarcane, because
                                  the 1st stable molecule with 14C was a 4-carbon acid OAA.

            1 CO2 PEP (3C)  combine into --->   4C acid [OAA]*  (in mesophyll cells)
            4C acid    --->    breaks down into
 PYR (3C)CO2 
(in bundle sheath cells)
            & this CO2 is now fixed into
CH2O in the Calvin cycle   (as described above) 
        Different leaf anatomy...  C3-anatomy* [
anim]   vs.  C4-anatomy*    [SEM leaf-xs]
found in tropical grass species under arid and water stress conditions.  
2 enzymes:
      a)  PEP Carboxylase... produces 4C acid OAA (recall Krebs)
                                 b)  Rubisco... produces two 3-PGAs

          pathway results in spatial separation of acid & sugar production
             and PEP-carboxylase has a better enzyme efficiency

3rd   CAM PATHWAY - Crassulacean Acid Metabolism Photosynthesis:
             evolved in succulents (cacti & pineapple) as an adaptation to arid dry conditions,
                     when stomata close during the day and are open at night.
             Using C4 enzyme mechanisms (PEP carboxylase) the CAM path first makes the
                     4C acid (oxaloacetate) by reductions at night and store it until daylight,
                     then release CO2 for the Calvin cycle reactions to make PGA... [C4 v. CAM
  Summary image of Photosynthesis
Making Connections
  &  Summary animation of Photosynthesis
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     copyright c2023     .   
       charles Mallery,          Biology 150, Department of Biology,   U. of Miami,  Coral Gables, FL 33124






















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     Sumanas anim - Light Rx*flash-view at home

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