AP Biology
Notes: Photosynthesis II 

Two types of photosystems are located in the thylakoid membranes:
         photosystem I 
        photosystem II

        The reaction center of photosystem I has a specialized chlorophyll a molecule 
        known as p700, which absorbs best at 700 nm (the far red portion of the spectrum).
        The reaction center of photosystem II has a specialized chlorophyll a molecule 
        known as P680 which absorbs best at a wavelength of 680 nm.  
        P700 and P680 are identical chlorophyll a molecules, but each is associated
        with a different protein.  This affects their electron distribution and results in 
        slightly different absorption spectra

There are two possible routes for electron flow during the lift reactions: 
        noncyclic flow 
        cyclic flow.
Both photosystem I and photosystem II function and cooperate in noncyclic electron 
flow,  which transforms light energy to chemical energy stored in the bonds of 
NADPH and ATP 

Noncyclic electron flow
          

        Occurs in the thylakoid membrane
        Passes electron continuously from water to NADP⁺
        Produces ATP by noncylic phtophosphorylation
        Produces NADPH
        Produces O₂

Light excites electrons from P700 the reaction center chlorophyll in photosystem I.
These excited state electrons do not return to the reaction center chlorophyll but are 
ultimately stored in NADPH, which will later be the electron donor in the Calvin cycle

        *Initially, the excited state electrons are transferred from P700 to the primary 
          electron acceptor for photosystem I
        *The primary electron acceptor passes these excited state electrons to 
          ferredoxin (Fd), an iron-containing protein.
        *NADP⁺ reductase catalyzes the redox reaction that transfers these 
          electrons from ferredoxin to NADP⁺ producing reduced coenzyme--NADPH
        *The oxidized P700 chlorophyll becomes an oxidizing agent as its electron 
          "holes" must be filled; photosystem ii supplies the electrons to fill these holes.
        *Electrons ejected from P680 are trapped by the photosystem II primary 
          electron  acceptor
        *The electrons are then transferred from this primary electron acceptor to 
          an electron transport chain embedded in the thylakoid membrane.
          As these electrons pass down the electron transport chain, they lose potential 
          energy until they reach the ground state of P700.
        *There electrons then fill the electron vacancies left in photosystem I when NADP⁺ 
             was reduced
        *A water-splitting enzyme extracts electrons from water and passes them to oxidized 
         P680, which has a high affinity for electrons
        *As water is oxidized, the removal of electron splits water into two hydrogen ions 
          and an oxygen atom
        *The oxygen atom immediately combines with a second oxygen atom to from O₂. 
          It is this water-splitting step of photosynthesis that releases O₂.
        *Some electron carriers can only transport electron in the company of protons
        *The protons are picked up on one side of the thylakoid membrane and deposited 
         on the opposite side as the electrons move to the next member of the transport 
         chain
        *The electron flow thus stores energy in the form of a proton gradient across the 
          thylakoid membrane--a proton-motive force.
        *An ATP syntheses enzyme in the thylakoid membrane uses the proton-motive 
          force to make ATP.  This process is called photophosphorylation because 
          the energy required is light.
        *This form of ATP production is called noncyclic photophosphorylation

Cyclic electron flow
Cyclic electron flow is the simplest pathway, but involves only photosystem I and 
generates ATP without producing NADPH or evolving oxygen

        *It is cyclic because excited electrons that leave from chlorophyll a at the reaction 
          center return to the electron center.
       * As photons are absorbed by phtosystem I the P700 reaction center chlorophyll 
          releases excited-state electrons to the primary electrons acceptor; which in turn 
          passes them to ferredoxin.  From there the electrons take an alternate path that 
          sends them tumbling down the electron transport chain to P700.  This is the same 
          electron transport chain used in noncyclic electron flow.
        *With each redox reaction along the electron transport chain, electrons lose potential 
          energy until they return to their ground-state orbital in the P700 reaction center.
        *The exergonic flow of the electrons is coupled to ATP production by the process of 
          chemiosmosis.  This process of ATP production is called cyclic photophosphorylation.
        *Absorption of another two photons of light by the pigments send a second pair of 
          electrons through the cyclic pathway.


        Cyclic photophosphorylation supplements the ATP supply required for the Calvin 
        cycle and other metabolic pathways.  The noncyclic pathway produces 
        approximately equal amounts of ATP and NADPH, which is not enough 
        ATP to meet demand.
        NADPH concentration might influence whether electrons flow through 
        cyclic or noncyclic pathways.