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Name: Gage
Status: Student
Grade:  6-8
Location: KY
Country: United States
Date: Fall 2010


Question:
What in the cell membrane attracts water?



Replies:
Gage,

It's what 'on', not 'in' the cell membrane, that attracts water. If you look at a cut-away of the cell membrane, it looks like a sandwich in a way. Think of one side of the sandwich as the 'inside' -- facing the cytoplasm of the cell -- and the other side as the 'outside', facing the extracellular matrix. The 'bread' of this sandwich -- the layers facing the cytoplasm or facing outside the cell -- is made of ions that attract water, and allow the cell to contact water. The inside of the cell is also water, so the membrane must have ions on the inside surface too. But between the inside surface and the outside surface (the 'meat' of the sandwich) is made of stuff that is not attracted to water, but those are shielded from the water by the ions, just like the break shields the greasy meat from your hands.

The actual molecules of cell membranes are 'lipids', but with an ionic head group. Think of them as a pushpin, with a head and a long 'tail' The long tails attract each other to form a flat sheet (called a lamella), with the ionic head groups facing outward. You can search for images of this on the internet or in biology books that depict it very well.

To answer your explicit question, basically, the ions on the one end of the lipid molecules are able to interact with water molecules very readily, and that is what attracts the water. A more detailed explanation would require some thermodynamics -- but not sure of your grade level, so let me know if you want more detail.

I don't know your grade level, so I'm speaking in general terms - let me know if you want a more technical description. There are several other posts on AAS that address other aspects of cell membrane function and content:

http://www.newton.dep.anl.gov/askasci/mole00/mole00430.htm

http://www.newton.dep.anl.gov/askasci/mole00/mole00750.htm

http://www.newton.dep.anl.gov/askasci/mole00/mole00949.htm

Hope this helps,
Burr



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