1.  Photosynthesis is a light-driven redox process.  The entire process occurs in several steps, because there is insufficient energy to boost e- from H₂O directly into NADP+.
NET RX: H₂O + NADP⁺ + ADP + P_i --->1/2 O₂ + NADPH + H⁺ + ATP
i. H₂O ---> 2H⁺ + 2e- + 1/2O₂
ii. NADP+ 2H⁺ + 2e- ---> NADPH + H⁺
iii. ADP + P_i ------> ATP

2.  Organisms obtain energy from oxidation-reduction reactions.

3. PLANTS USE TWO PHOTOSYSTEMS WITH DIFFERENT FUNCTIONS: PHOTOSYSTEM I AND II
PS II, P680:   P680* PHEO ---> P680⁺ PHEO^-
    P680⁺ + e- ----> P680
    H₂O ---> 2H⁺ + 2e- + 1/2O₂
PS I, P700:       P700* A ---> P700⁺ A^-
    2 ELECTRONS- + NADP⁺ + 2H⁺--> NADPH + H⁺
EXPT. EVIDENCE

4. THE TWO RX CENTERS ARE CONNECTED BY AN ELECTRON TRANSPORT CHAIN SIMILAR TO THAT IN THE MITOCHONDRIA
REVIEW:  ORGANISMS OBTAIN ENERGY FROM OX/RED RX

5.  ELECTRON TRANSPORT AND WATER SPLITTING GENERATES A PROTON MOTIVE FORCE THAT IS USED TO MAKE ATP.
~50% OF LIGHT ENERGY IS TRANSFERRED TO CHEMICAL ENERGY

8-10 PHOTONS ARE REQUIRED TO RELEASE 4e- FOR EVERY O₂ RELEASED
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Lecture Review
Organisms obtain energy from oxidation-reduction reactions.
    Tendency to transfer an electron depends on the redox potential of each redox couple.  The reducing potential is a measure of the readiness with which an atom/molecule takes up an electron.   A negative red. pot. indicates the atom/mol has a lower affinity for electrons than H₂/2H+.  A positive potential indicates it has a higher affinity for electrons than H₂/2H+.

Table.  Mid point redox (reducing) potentials of selected redox couples from the respiration and photosynthesis.
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                                            E_m (V)
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Ferredoxin ox/red                        -0.42
 2H+/NAD+/ NADH                   -0.32
NADP+ +2H+/ NADPH + H⁺      -0.32
Ubiquinone                                 +0.040
Cyt c ox/cyt c red                       +0.220
S + 2H+ /H₂S                            +0.23
1/2O₂+ 2H+/ H₂O                     +0.82
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Useful equations to analyze energy changes in redox reactions:
    DE = E (acceptor) - E (donor)         E = reducing potential (V)
    DG = - zFDE                               DE = difference in the reducing potential (v)
                                                        z = number of electrons transferred
                                                        F = Faraday's constant, 23 kcal/V.mol
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Exercises
1.   Oxidation-reduction reactions require or release energy.  In photosynthesis, the oxidation of water leading to the reduction of NADP+ requires energy.

a) How much energy is theoretically needed to transfer 2e- from water to NADP+.
Start with the two half reactions.  Note number of electrons transfered.
redox pot. of water oxidation:   = +0.82 V
  NADP+/NADPH  =  -0.32
Recall: DG = -nFDE

b) Splitting of water and electron transport also result in the formation of a proton electrochemical gradient.  This is used to make 2 ATP for every 2e- transferred.  How much energy in kcal is required?  Assume DG required to form 1 ATP = +10 kcal/mole.

c)  What is the total energy (in kcal) required to form 1 NADPH and 2 ATP?

d)  If 1 photon of red light = 40  kcal, how many photons are needed for 2 e- transfer?

2.  See Fig. 7-11 (Taiz & Zeiger, 2002 or 2006).  By varying the energy of each light flash, Emerson and Arnold found a unicellular green alga, Chlorella, produced 1 O₂ per 9-10 quanta.  More light energy per flash did not result in more oxygen.  Why?
    When 2 H₂O is oxidized to yield 1 O₂, how many electrons are released?  If one photon of light can excite just one electron, why are 9-10 quanta required to yield  1 O₂  .
    They also found that the maximum yield was 1 O₂ per 2500 chlorophyll molecules.   Given that the reaction center pigment P680+  or P700+  can be  reduced by only one electron at a time.   How many chlorophyll molecules are associated with one reaction center pigment?