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admin
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Joined: 08 Dec 2003
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 Posted: Wed Jul 15, 2009 2:08 pm Post subject: GS-1 Biological Sciences Question Q. 7 |
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Last edited by admin on Sat Feb 27, 2010 12:03 am; edited 1 time in total |
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elisha98449
Joined: 24 Jul 2009
Posts: 2
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| Posted: Thu Jul 30, 2009 2:45 am Post subject: GS1 B7 Question |
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| Isn't simple diffusion diffusion through the lipid bilayer along its concentration gradient without a need of a protein membrane channel or an intracellular protein? Electrolytes are charged, so they're hydrophilic and can't simply diffuse through the bilayer . Does this not require facilitated diffusion? If not then what is the distinction? |
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admin
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| Posted: Thu Sep 03, 2009 2:35 pm Post subject: |
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klee92181
Joined: 06 Mar 2008
Posts: 17
Posted: 07 Mar 2008 08:04 pm Post subject: Na+ considered simple diffusion?
| Quote: | Hi,
As electrolytes (Na+) are transported across the membrane, is it considered simple or facilitated diffusion. The GS1 CBT says it is simple diffusion, but aren't all ions considered big and hydrophylic, hence they use special ion channels to cross the membrane?
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mcat_premed3832
Joined: 19 Oct 2006
Posts: 164
Posted: 07 Mar 2008 10:40 pm Post subject:
There is a simple answer and a more complex one.
| Quote: | The simple answer is that the MCAT rewards reasoning over memorization. When you evaluate the question in the context of GS-1 (presumably you are referring to question 7), there is no other logical option as described in the Explanation.
The more complex answer is that it depends. To begin with, most ions are considered very small (after all Na, K and Cl are atoms not molecules). Certainly, their charge presents a problem crossing the membrane but not their size. But as you would know from your review, the Vm in neurons is in part due to the leakiness of K+ across the plasma membrane (of course driven by simple diffusion).
So there is a simple diffusion gradient but because of charge, the membrane is semi permeable. If any channels are open within the membrane, then ions will flow according to the electrochemical gradient (which is diffusion). And, of course, active transport uses ATP to move ions against their gradients. |
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seanoconno1879
Joined: 16 Jul 2011
Posts: 1
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| Posted: Sat Jul 16, 2011 7:51 pm Post subject: |
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| Simple diffusion occurs only with hydrophobic molecules and does not use membrane proteins to assist (as in the case of leaky K+). This question needs revision, as facilitated diffusion would be the correct answer. |
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