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Homework Help: General Biology - Faciliated Diffusion

  1. Oct 2, 2009 #1
    1. The problem statement, all variables and given/known data

    Explain why facilitated diffusion, unlike simple diffusion, is both specific and saturable.

    (Short essay, 4 points)

    2. Relevant equations


    3. The attempt at a solution

    Is this a good beginning to an answer? I feel like I can never apply things correctly for my prof. He will tell us facts but never the why or how, so I always feel lost when I get my homework. He really makes me angry.


    Facilitated diffusion is a passive transport of molecules. However, if a membrane is impermeable then molecules can't get through on their own. In facilitated diffusion molecules get through with the help of proteins in the form of ion channels and protein carriers. This makes them specific because the proteins only allow one certain type of molecule in. This also causes facilitated diffusion to be saturable because the proteins can only go through so fast. Their rate of concentration difference will increase, but at one point it will max out and stay constant because the carrier or channel cannot get any faster at transporting the molecules.
  2. jcsd
  3. Oct 2, 2009 #2
    By saturable I assume it means that you can create a saturated solution? Clarification on that would be good.

    You have the idea down. Facilitated diffusion is more specific because it only transports specific molecules, however I'd be more specific as to why they are needed. Think about the structure of the cellular membrane, and the common trait that certain molecules possess that allows them to pass through, (like CO2 and O2. What is different between those two and say, H2O, or Cl-?

    Also I'd mention that the direction it travels on the concentration gradient, as well as distunguish it from Active Transport. Overall it looks like you have a good start there.
  4. Oct 3, 2009 #3


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    Staff: Mentor

    I suppose saturable means that maximum speed is limited, but that just my guess, and it may be completely wrong if biologists don't follow logic and nomenclature used in electrochemistry :wink:

  5. Oct 4, 2009 #4
    He defined saturable as being that you can only go so fast before you hit a maximum speed. The example he gave us was if he was standing at the door of our classroom and to get out of the classroom you had to shake his hand. First of all, you are required to have a hand for him to shake to get out and then second of all he can only shake hands so fast, before he would become "saturable". He actually did this at the end of our class by handing out our homework sheet. You had to get the homework sheet to exit. I told him he was doing a good job of being facilitated diffusion to which he replied, "Saturable, isn't it?"

    Anyway- he flew through all the parts of the cell claiming we should remember since high school, but I graduated high school in 2003 and haven't taken Bio since 9th grade, so I don't remember the special things about the cell. I think that the cell membrane is made up of lipids? It has to do with the fact that simple diffusion can just go straight through the membrane, but facilitated can't because the ion/atom/molecule is polar... The outside parts of the membrane are hydrophilic, but in between those two pieces it's hydrophobic....So it can't break up to get through.
    Last edited: Oct 4, 2009
  6. Oct 4, 2009 #5
    Ok. I think I've got my whole answer now....

    "Simple diffusion and facilitated diffusion are both types of passive transport in cells. Simple diffusion takes place with nonpolar molecules that have no issues crossing through the hydrophobic center of the bilipid layer. However, not all ions/atoms/molecules/etc can do this. For example, a polar glucose molecule that is hydrophilic finds it extremely hard to pass through the hydrophobic bilayer. Thus, cells can have proteins that either open up a "tunnel" to travel through (called channel proteins) or a protein that carries "passengers" from one side to the other (called transport proteins). These helper proteins distinguish facilitated diffusion. The proteins are specific in what they help cross the bilayer so it isn't a free-for-all by having certain binding sites that only one type of ion/atom/molecule will be attracted to. Since only a certain item will pass through the protein it will transfer through and eventually come to a maximum speed. These items cannot pass randomly through the bilayer so they have to "wait in line" for their turn to get across. This maximum speed of movement is known as saturability.
  7. Oct 4, 2009 #6
    Seems good.

    Make sure you say phospho-lipid bilayer. There's a distinction between that and just plain old lipids.
    Last edited: Oct 4, 2009
  8. Oct 4, 2009 #7

    Ah! Very true! You are right! I will make sure to put that change in now. Thanks for the reminder!!
  9. Oct 4, 2009 #8
    I would also mention that all of these things go down a concentration gradient, and require no energy to take place. That's another large point.
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