So... in the plasma membrane of an animal cell..

It comprises of phosolipids right? The phosphorus is hydrophilic, and the fatty acids hydrophobic. I asked my teacher how the water diffused in and out of the membrane then, and she said. "The water molecules are small enough that they just go through"

Now. I know I'm the student, but that doesn't fit with me? Then I learnt about the transport protiens. And that made more sense.

But, is that the defenite way, and if so, what really happens in the process of water going in and out of the membrane?

You may find that you are both right in your own way. Your teacher may have been referring to the process of osmosis, by which water can enter the membrane. However, if we take a look at the details of how this works, there are specific transport proteins that help to form channels that allow water to pass, but not ions or other solutes. You may want to read up on acquaporins if you are interested : hope this helps
24 November 2011
Thank you :)
27 December 2011
The water molecules are small and can diffuse through the adjacent phospholipid molecules.
29 January 2012
Recall that the adjacent phospholipd molecules are not joined by bonds but are aligned in a bilayer - it is fluid. Therefore small molecules such as water are able to diffuse across the bilayer down their specific concentration gradient (water potential gradient for water!) and in the context of water, state that this is the process of osmosis. Water molecules can freely diffuse through channel proteins but not carrier proteins.
29 January 2012
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