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2 Questions:metling point in unsaturated fat; nerve impulses

  1. Sep 11, 2006 #1
    Here is my first question: As we all know that, fat and oil(lipid) are triglycerides, they differ in the hydrocarbon tail with fat(saturated fat) without C=C, whereas lipid(unsaturated fat) having C=C in the hydrocarbon tail.
    My question is why does having a C=C will lower the metling point?
    And how how about their boiling points?

    My second question is why does the rate of transmission of nerve impulses is faster if the diameter of the axon is larger?
    My text book says larger diameter of the axon reduces the resistance offered by the axoplasm within; does this mean that larger diameter, the axoplasm density is lower?

    Thank you!
     
  2. jcsd
  3. Sep 11, 2006 #2

    GCT

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    Hello and welcome to physicsforums, it's the policy of the homework help forums that you start off the discussion by showing some of your work, what are your ideas regarding the answers to these questions?
     
  4. Sep 12, 2006 #3
    sorry for that.....didnt notice that@@
    For the second questions, i've made some hypothesis and about the first questions, i would suggest that since the triglyceride is having 3 hydrocarbon tail, if any of the tail is having C=C and with the rest two being saturated, then the pasking efficient in solid state will be lower, hence lowering the melting point.
    For the boiling points of the two, now i think it's not necessary to compare it as they have long hydrocarbon tail, due to the long molecular mass, there are too many variations of the tail, so better to say it's incomparable (forgive me for having such kind of casual sense.....)
     
  5. Sep 12, 2006 #4

    GCT

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    Yes the packing efficiency is relatively worse off for the unsaturated fatty acid, what does this tell you about the the strength of the intermolecular attractions between that of an unsaturated and saturated fatty acid, assuming an appropriate example? What does the overall geometry of the hydrocarbon tail for the unsaturated fatty acid look like, as compared to the saturated fatty acid? That is, how does the geometry of the unsaturated form deviate from that of the saturated component? What would be the geometry of a saturated hydrocarbon tail?
     
  6. Sep 13, 2006 #5
    First, i dont think that the packing efficiency can be a good indicator of the strength of intermolecular forces, as for the case between cis-but-2-ene and trans-but-2-ene, the cis one though having a perment dipole, its melting point is lower due to the higher packing effieiency(more symmetrical). So, if you're asking me about the relationship with the boiling in terms of packing, i couldn't tell.

    p.s. sorry, my knowledge about maths is very limited, i can only tell which one will be more more symmetrical.
     
  7. Sep 13, 2006 #6

    GCT

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    The correlation between packing efficiency and parameters related to the melting point of a molecule isn't a clear cut case, however, it is with respect to the topic of your original post. I want you to draw out the structure of an unsaturated hydrocarbon chain (one carbon double bond, draw it in steps, linking the carbons) and that of a saturated hydrocarbon chain, there's a clear cut distinction between the two.

    hint: The shape of the saturated carbon chain is somewhat linear, with regards to the shape overall; however, the unsaturated component is "v" shaped, can you tell me why?
     
  8. Sep 18, 2006 #7
    Sorry for the delayed reply, i've just done a little search, found out that C=C in unsaturated fatty acids causes a bend in the hydrocarbon tail, making the "V" shape as described by ypu, so i guess it's probably related to the dense e- cloud in the C=C, so by electron cloud repulsion, a bend is observed, right?

    And how about my second question, does it because that the axonplasm density will be lower with larger diameter or like current flows in metal wire, a larger diameter allows more charges to passes through per unit time?

    thanks!
     
  9. Sep 18, 2006 #8

    GCT

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    The overal "V" shape is due to the hybridization (one way of looking at it) of the carbon that participates in a double bond.

    For the second question, you should be able to find an equation which relates the current to the cross section diameter of a tube shaped axon.
     
  10. Sep 19, 2006 #9
    sorry, for the first point about the hybridization, could you further elaborate it? I simply have no idea.........thanks
     
  11. Sep 19, 2006 #10

    GCT

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    This is something that you should be familiar with, the geometry of the carbon with respect to its bonding atoms is different for a hypothetical sp3 and sp2 conformation. What are the bond angles for the sp3 and that for the sp2 between the atoms that are bonded to the carbon center?
     
  12. Sep 20, 2006 #11
    sp2 is plannar(120degree) and sp3 is tetrahedral(109.5degree), so what?
    Is there any picture showing the "repulsion"? It would be easier to vitrualize it, thx!
     
  13. Sep 20, 2006 #12

    GCT

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    What are the respective bond angles? This should answer your question conclusively; that is, what would be the bond angles between a carbon carbon bond, whether it is double or single, sp2 or sp3?
     
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