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Homework Help: Phospholipids characteristics

  1. Nov 29, 2009 #1
    1. The problem statement, all variables and given/known data

    Phospholipids have headgroups and hydrocarbon chains as follows:

    A. amphipathic, non polar
    B. non polar, polar
    C. polar, non polar
    D. charged, non polar

    2. Relevant equations

    phospholipids have a phosphate head group and fatty acid residue hydrocarbon tails.

    3. The attempt at a solution

    I have eliminated A and B as they are obviously incorrect.

    I would classify phospholipid heads as being both charged, and (thus) polar.

    C and D seem to be, both, correct answers to me.

    But of course there should be only one.
  2. jcsd
  3. Nov 30, 2009 #2


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    Your teacher hates you....

    Answers C and D are not incorrect. At what pH would the negatively charged phosphate be protonated (and uncharged)? If you were at that pH, would the phospholipid act as an amphiphile?
  4. Nov 30, 2009 #3
    The answer is not A
  5. Dec 2, 2009 #4
    since phosphoric acid is a weak acid, it's pKa should be between 2 - 6.

    Therefore at physiological pH 7.4, the acid will be dissociated, giving the negatively charged anion.

    Am I correct?

    If, the pH is very low, in strongly acidic conditions, then the phospholipid will be uncharged.
  6. Dec 2, 2009 #5


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    At that low pH would answer D apply? There is a more correct answer (independent of pH).
  7. Dec 3, 2009 #6
    Using the Le Chatelier's Principle, then there will be no charge on the phosphate because it would remain undissociated!

    Is that it?

    ooo but then, the charge will depend on the pH, while, the polarity will always be here!

    so, in a sense, it is better to say C!

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