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Photosynthesis

Discussion in 'DAT Discussions' started by osimsDDS, Jun 20, 2008.

  1. osimsDDS

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    Is this correct:

    Light Dependent Reactions:

    non-Cyclic photosynthesis: formation of O2 by splitting of H20 (PSII) and NADPH (PSII)...ATP formed by PSI

    cyclic photosynthesis: Just regeneration of ATP by PSI

    Calvin Cycle:
    Takes 6CO2 and fixes it to 6 Rubisco molecules to get 12 PGA. Then it takes 12 PGA, 12 ATP, and 12 NADPH and makes 12 PGAL. Then takes 2 PGAL and makes 1 glucose molecule and 6 ATP with the remaining 10 PGAL are used to regenerate 6 RuBP (rubisco).

    Thats basically the main jist of photosynthesis...correct me if iam wrong anywhere..thanks
     
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  3. DRHOYA

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    Very good. Although in non-cyclic phosphorylation, there is ATP generated with PSII as well, as when PSII electron is first excited, it travels down a gradient to genertate ATP and end up to fill the hole in PSI................then H20 is split to generate O2, H+ and electrons to fill the hole in PSII.


    Also, I think you bookkeeping is correct, although I have a hard time following. Are you talking about just 1 single turn?? I think for 1 single turn 3CO2's ---->steps (6-carbon molecules)---->6molecules of PGA---->6NADPH used up---->6molecules of PGAL (high energy) ----> 1 PGAL goes off to possible form glucose and other sugars----->5 PGAL's --->3 ATP consumed--->3 molecules of Ribulose biphosphate------->starts over with 3CO2's. Thats all I can summarize for ONE SINGLE TURN....but idk if its correct.
     
  4. DRHOYA

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    Also man, while were at it......I'll add the C4 pathway. Starts off in mesophyl cells where C02 combines with PEP to go to Malic acid, which is then allowed to be transported to the bundle-sheath cells. In the bundle sheath cells, malic acid dumpes off the CO2 which can enter the Calvin cycle and produce energy. The PEP generated from malic acid when CO2 leaves then goes back to the mesophyl cells and combines with more CO2. This pathway is like a reserve for CO2 to be useful in plants that live in dry climates where their stomata are generall closed to conserve water. Hope that is correct and helps.:thumbup:
     
  5. osimsDDS

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    I think you need 6 cycles to make 1 glucose molecule...2 PGAL = 1 glucose molecule
    I believe every cycle produces 1 CO2 molecule and therefore you need 6 turns to make 6 CO2 that then bind to Rubisco (5-carbon molecule) to make a 6-carbon molecule (PGA)
     
    #4 osimsDDS, Jun 20, 2008
    Last edited: Jun 20, 2008
  6. osimsDDS

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    1 SMALL mistake...its not malic acid its malate...malic acid is in CAM plants where they store the excess CO2 during the night in the form of malic acid in the vacoule...
     
  7. DRHOYA

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    Ok that makes sense. Since your book-keeping used 2 PGAL to generate glucose........you basically went around the cycle twice (b/c what I stated was for 1 single turn)

    Also, thanks for the correction between malic acid and malate........but in the C4 pathway page in Schaum's.....it says "malic acid"??? (even has it diagramed)
     
  8. osimsDDS

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    haha in Cliff's AP bio book it has it diagramed too and it says malate for C4 and malic acid for CAM...w/e i DOUBT they will ask that on the test but if they do it prob wont be a big difference it wont be both malic and malate as answers...at least i hope not that would be tough
     
  9. DRHOYA

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    Haha yeah very true. I thought I saw a ghost when that diagram showed up in Schaums. I wish I could scan in a pic of it.......maybey I'll try that...but the pic would be too large of file. Oh well.
     
  10. harrygt

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    TO DRHOYA and OSIM: Nice refreshing of the memory guys! Thanks!
    Osim, the photosystems are responsible for exciting the electrons only. NADPH is produced in the thylakoid membrane of the chloroplast [or you can also say in stroma (I have seen both as correct answers(] once e-, and H+ joing NADP+. this happens after the passing of electrons from PSII, but you cant really say that NADPH is produced by PSII.
    The same for ATP. ATP is formed by the ATP-synthase enzyme which is in the membrane of the thylakoid. H+ ions move out of the thylakoid in the direction of the gradient through this enzyme which is also a canal. Their movement makes the F1 unit of the enzyme spin which transfers energy to ADP + P to make ATP. So I don't see why you would say ATP is made by PSI.
    The same for non-cyclic production of ATP. PSI only decomposes water and also excites the electrons to help the H+ pumps bring some H+ in the thylakoid by active transport. You can't say ATP is made by PSI. I have taken this course twice, and never seen such an argument in any book or by any teacher. Let me know, if you are citing it from Schuam or any other book.
    Thanks anyway. Nice posts
     
  11. osimsDDS

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    you are correct...what i believe i was trying to say is that the two cycles of the light reactions facillitate the machinery to end the light reactions with those products (ATP, NADPH, O2, etc...)

    P.S.- can you explain how the "holes" of each photosystem are closed iam having trouble visualizing that...thanks
     
  12. harrygt

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    What do you mean by the 'holes' of photosystems? The photosystems don't have any canals or holes as you say.
     
  13. DRHOYA

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    Osims DDS. This helps me to visualize it. We'll start with PSI. Ok, so PSI contains electrons (n electrons). When light strikes this chlorophyll-a molecule in PSI, an electron will be excited to a higher energy level, and thus travel down the gradient producing an ATP and NADPH (we'll assume its going to be non-cyclic for now). Since in non-cyclic, this electron does not return to PSI....it is incorpoarted to reduce NADP+......PSI is "down" an electron = "electron hole". Now, PSII comes into play. When PSII is struck with light, one of its electrons are excited to a higher energy level, and thus move down the gradient generating an ATP, and end up falling to PSI.....thus replacing the electron hole in PSI. Now PSII has an eletron hole. To fill this, H2O is split to yield an H+ (go to proton gradient) O2, and an electron which fills the hole in PSII..................................Now it is complete. Hope this helps man. :thumbup:
     
  14. osimsDDS

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    fing beautiful, understood it perfectly...except 1 question, why dont you start with psII (p680) in non-cyclic?? i thought electrons go from psII to psI...
     
  15. DRHOYA

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    Well technically I think neither is a "start" point, b/c all processes would be happening simultaneously due to light hitting the photosystems. The non-cyclic vs. cyclic has to do strictly with the path of electrons. Just think of starting with PSI. Follow the path of the original electron (as a reference point) through cyclic.......it end up back in PSI. If you follow it through non-cyclic......it reduces NADP+ so it does not return to its original origin of PSI......thus generating the electron holes we discussed above. Essentially the action of PSII replacing the electron lost in noncyclic from PSI can't happen until PSI has an electron hole.........so I always thought of starting the whole system with PSI.
     
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  17. nixon13

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    So let me get this straight. Light hits chlorophyll in photosystem 1. The excited e- are transfered to NADP+ to generate NADPH and also some ATP. Since the e- went to NADP+, there are electron holes in photosystem 1. Then light strike photosystem 2 and e- from chlorophyll are excited and get transfered to photosystem 1. This fills the holes for photosystem 1 but leaves holes in photosystem 2. The holes from photosystem 2 are filled when the absorbed energy from e- is used to split H20 to H+ and O2. The holes are then filled from the e- carried with the e- from H+??

    Does this sound right?
     
  18. DRHOYA

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    ^ What you described was "non-cyclic". There is also cyclic. Also, when electrons from PSII are excited and travel down the cascade to PSI, ATP is also produced.
     
  19. nixon13

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    k cool thanks guys i love you.
     
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