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    I get the whole depolarisation of one part of the axon (although, still not really too sure what causes it to start?), but a bit confused about the whole transmitting of the action potential down the neurone.

    What I understand that happens is 'local circuits' are set up, which means the sodium ions in the depolarised area travel inside the neurone to the part ahead of it because it gets attracted to the negative resting potential. If this is so, why do the sodium ions not travel backwards aswell to the part of the neurone in the refractory period, because this place has an even more negative value and therefore will attract the sodium ions even more strongly.

    Another thing I was wondering, is how do myelinated sheaths actually stop the action potential being generated where they are? Because, assuming I am right in saying the sodium ions do travel inside the neurone, they will still travel to the parts of the neurone surrounded by the myelin sheath. But in the text book it says saltatory conduction occurs due to the sodium ions 'jumping' from node to node. So should I just assume the myelinated sheath prevents the sodium and potassion ion channels opening?

    Any thoughts?!

    Thanks!
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    saltatory conduction....

    the Na+ diffuse through the axon between the myelin sheath

    the myelin sheath is an electrical insulator

    prevents movement of ions across the axon membrane

    therefore only at the nodes of Ranvier is the axon potential generated

    this is what it means when the action potential 'jumps' from node of Ranvier to node of Ranvier

    i dont really want to confuse you (and myself) by answering the first question because i haven't got a clue why exactly Na+ ions won't diffuse backwards as well as forwards
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    Thanks! I was hoping that was the answer, because I just realised fats are non-polar and therefore will not allow the passage of ions.

    As to the first question, I asked my teacher and she had no idea either. So in the exam do you just say they don't flow backwards because the distribution of the ions is not yet back to normal?
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    hum... perhaps the concentration of Na+ ions is greater or equal to the Na+ concentration behind therefore it only diffuses forward where Na+ conc is less... am speculating a bit though
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    I get the whole depolarisation of one part of the axon (although, still not really too sure what causes it to start?)
    It originiates from a stimulus; for example, pressure and subsequent deformation of the Pacinian copuscle in the skin will open voltage-gated channels and sodium ions are allowed to flood in, depolarising the membreane.
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    hum... perhaps the concentration of Na+ ions is greater or equal to the Na+ concentration behind therefore it only diffuses forward where Na+ conc is less... am speculating a bit though
    Yes, that would make sense, because the refractory period is caused by the flowing out of potassion ions, so presumably there is still a very high concentration of sodium ions in that part of the neurone. Therefore they will only flow forwards down their concentration gradient. Hence the part I said about the sodium ions flowing down an electric gradient is only really partly true, they really flow down their concentration gradient. Agree?
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    yeh i agree, they would flow down an electrochemical gradient then
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    yeh i agree, they would flow down an electrochemical gradient then
    Yes, like the hydrogen ions in chemiosmosis and companion cells, or sodium ions in the proximal convoluted tubule, or calcium ions at the presynaptic knob. I would laugh if a question about electrochemical gradients comes up in the OCR unifying concepts paper tomorrow! Do you do OCR?
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    nope, edexcel... (i do OCR B physics and it's the most stupid exam i've ever done in my life) what is chemiosmosis... i'm sure i asked someone this before tho

    sodium ions are actively pumped out of the proximal convoluted tubule

    calcium ions diffuse into the presynaptic knob though i don't know if it's down an electrochemical gradient or not
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    Chemiosmosis happens across the double-envelope of the mitochondrial membrane. Hydrogen ions are actively pumped into the intermembranal space, then flow back down their electrochemical gradient through ATP synthase enzymes on the inner mitochondrial membrane , which funnily enough synthesise ATP.

    Does anyone know if this also happens in chloroplasts?
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    (Original post by Jen05)
    Chemiosmosis happens across the double-envelope of the mitochondrial membrane. Hydrogen ions are actively pumped into the intermembranal space, then flow back down their electrochemical gradient through ATP synthase enzymes on the inner mitochondrial membrane , which funnily enough synthesise ATP.

    Does anyone know if this also happens in chloroplasts?
    Yes, in the thylakoid membrane (look on page 4 of the Biology 2 OCR book if you have it)
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    look on page 4 of the Biology 2 OCR book if you have it
    Yep, found it. Thanks!
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    [QUOTE=Jen05]Thanks! I was hoping that was the answer, because I just realised fats are non-polar and therefore will not allow the passage of ions.

    As to the first question, I asked my teacher and she had no idea either. So in the exam do you just say they don't flow backwards because the distribution of the ions is not yet back to normal?[/QUO

    i think they do flow backwards. It's just that the postsynaptic neurone has no actylecholine and so therefore can not sent the action potential do another neurone in that direction. This is listed as one of the functions of the synapse
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    This is listed as one of the functions of the synapse
    Thats true, but I think that we are meant to think the point of the refractory period is to make sure that the direction of the action potential is only in one direction. So in the exam, I guess you will have to be able to put both points down, even though they contradict each other.
 
 
 
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