Water movement and phospholipid memebrane.

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student12
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Water movement and phospholipid memebrane.

Post by student12 » Thu Jun 01, 2006 12:05 pm

My text book mentions that small uncharged molecules such as water can pass through the phospholipid membrane. It then say that phospholipid membrane acts as a barrier separting the internal environment from the external environment.

How can it act as a barrier if water passes freely?

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Post by MrMistery » Thu Jun 01, 2006 5:58 pm

Actually only a small amount of water can pass freely through the membrane. Cells that need to transport lots of water and do it fast have so called water channels, or aquaporins.

The membrane is a barrier only to some molecules: large ones, charged, polar etc. Small nonpolar molecules such as O2 and CO2 can pass freely, and a certain amount of water can also pass.
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Post by student12 » Fri Jun 02, 2006 4:42 am

If only a small amount of water can pass freely then why does red blood cell burst when placed in distilled water?

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Post by MrMistery » Fri Jun 02, 2006 5:26 pm

cause red blood cell has about 200.000 aquaporins in it's membrane ;)
Aquaporins in red blood cells function as always-open channels, they can not be closed.
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Post by Nite » Mon Jun 05, 2006 1:49 pm

if an animal cell has little or no aquaporins, it will not burst in a hypotonic solution no matter how long the cell is placed in it??
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Post by victor » Mon Jun 05, 2006 2:16 pm

theorically, yes..:lol:
but, is there any cell which doesn't aquire any aquaporins??
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Post by MrMistery » Mon Jun 05, 2006 6:47 pm

actually, no!
Not all cells have aqauporins. Epidermis cells i think is an example of cells without aquaporins. if an animal cell has no aquaporins, it will still burst into a hypotonic solution, only it would do it slowly. Water still moves through the membrane until the solute potential is reached(or the cell bursts), only it does it much slower than through water channels. I don't know numbers, unfortunately.

Also, recent studies on plant cell stomata show that some aquaporins can actually be closed and opened. The front-running hypothesis is that the regulation is done by phosphorilating them, and Ca ions act as second messengers in the signal transduction pathway.
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Post by bookworm29 » Tue Jun 13, 2006 6:54 pm

well it mostly acts as a barrier for larger molecules that want to pass through which either have to pass with help of passive or active transport

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