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Specific Osmosis Question

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Specific Osmosis Question

Postby Protist » Sat Sep 24, 2011 8:18 pm

When water passes through the cell membrane, does it need a channel? If not, how is it that water can pass through the hydrophobic portion of the cell membrane?

I've seen an animation via my online textbook resources that shows water molecules passing through the cell membrane without the help of a channel. I've also seen other resources showing a protein channel that allows water to move across the membrane.

I've searched a little bit on the bio online forum search, but I could not find this specific question.

Thanks!
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Re: Specific Osmosis Question

Postby chrisPbacon » Fri Sep 30, 2011 10:36 pm

Funny you asked this because I just recently read something regarding your very exact question.

Firstly, Osmosis is the diffusion of water across a differentially permeable membrane due to concentration differences. That is to say, water (H20) goes from a higher concentration to a lower concentration, or from outside the cell (higher concentration), to inside the cell (lower concentration).

Secondly, water passively moves through a membrane channel protein now called an aquaporin, which is why water can cross a membrane more quickly than expected. This is a newer finding, but allow me to elaborate. The plasma membrane of animal cells is composed of a phospholipid bilayer in which proteins are embedded (integral proteins) or also occur on the cytoplasmic side (peripheral proteins). And the hydrophobic tails that make up the inside of the membrane, which you stated are "water-fearing." The aquaporin, as I stated previously is a channel protein. Channel proteins allow substances to simply move across the membrane freely.

This might sound confusing so here is a picture to illustrate what I mean:
The channel protein is the red object.
The membrane itself is depicted by the purple spheres.

Image

Source:
Mader, Sylvia S.
Biology / Sylvia S. Mader. -- 10th ed.
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