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Homework Help: Understand the Photosystems

  1. Aug 26, 2008 #1
    Hi, i'm having trouble on understand the Photosystems as my book is not really clear about it.

    It says that in the non-cyclic photophosphorylation electrons are released from photosystem II and are released then the electrons are accepted by the primary electron acceptor then by plastoquinone then cytochrome b6f complex then plastocyanin and then to photosystem I. In photosystem I the high energy electron is ejected and passes onto electron acceptor ferredoxin and then NADP+ reductase and the NADP+ reductase makes NADPH and release it to the stroma.

    Then it says in the cyclic photophosphorylation, electron are released from the photosystem I and the electron are passed on to an electron acceptor called FeS. FeS then passes the electron to ferredoxin.

    I'm confused, aren't FeS ferredoxin? and it also says that in cytochrome b6f complex, the flow of electrons provides energy to pump H+ across the thylakoid membrane. What does this means?

    Thank you.
  2. jcsd
  3. Aug 26, 2008 #2
    Re: Photosystems

    Well it looks like non-cyclic photophosphorylation is described clearly. The electrons from Photosystem II (PSII) passed through the electron transport train (ETC) and ended up at Photosystem I (PSI). They are reenergized by light at that point and taken to another primary electron acceptor. Ferrodoxin is the last electron acceptor (as you'll see, it plays a part in both cyclic and non-cyclic photophosphorylation) and in non-cyclic phosphorylation, it reduces NADP+ reductase.

    Now FeS is Iron Sulfide and is not ferrodoxin. I think the book mentioned FeS as just a electron acceptor, and nothing more. Basically in noncyclic photophosphorylation, we start at PSII as we normally do and light energizes the electrons. Then they high energy electrons are taken by a primary electron acceptor (probably FeS, the name of the molecule isn't highly important) and are passed on to the Electron Transport Chain. Ferredoxin and Cytochrome are both carrier proteins in the ETC and they pass the electrons down from one carrier protein to another. As the electrons move down, they lose energy and this energy is used to phosphorylate about 1.5 ATP molecules.

    Then the electrons arrive at PSI and get reenergized again and taken to another primary electron acceptor and this time these highly energized molecules can be passed down the ETC again (hence the term cyclic photophosphorylation) or passed on to the last electron acceptor ferrodoxin, and this goes back to my first paragraph on noncyclic photophosphorylation.

    Thus in summary, ferrodoxin is the last electron acceptor and also a carrier protein and plays a role in noncyclic and cyclic photophosphorylation, respectively.

    As for how the flow of energy pumps H+ ions, I would suggest reading up on the chemiosmotic theory.
  4. Aug 27, 2008 #3
    Re: Photosystems

    Thanks. Really made things clear.
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