Re: Do water molecules enter a cell by diffusion or facilitated transport?

  Hello.  In answer to your question, yes, water normally enters cells by 
diffusion.  However, diffusion of water across cell membranes is also 
regulated by a large set of special proteins called aquaporins. (or just 
porins, as they're sometimes called.)  
   Lets start with the diffusion question.  For the large majority of cells, 
water enters and leaves the cell purely by diffusion.  C02, O2, and H20 are 
the three things that are generally considered to be freely diffusible 
across the cell membrane.  The direction that water moves through the cell 
membrane is determined by what the osmolarity of the cell and the outside 
solution are.  In general, water will move towards the higher osmolarity. 
(it will try to "dilute" the high concentration of salt.)  So, if the 
osmolarity of the cell is higher than the solution it is in, water will flow 
into the cell through the membrane, making it blow up like a balloon.  If 
the cell isn't able to control the amount of water that comes into it, it 
will pop.  
  So, I said that in general, water is freely diffusible across membranes, 
but this isn't entirely true.  It still takes water a little bit of work to 
get across the membrane.  Some specialized cells and tissues in our bodies 
express specialized proteins that allow water and other small hydrophilic 
("water-loving") compounds to diffuse even more rapidly across the membrane.  
These are known as aquaporins.  These proteins make plasma membranes about 
100-200 times more permeable to water than pure phospholipid membranes.  
Aquaporins are actually found in lots of tissues, and the reasons they're 
there aren't really known yet.  One good example of where aquaporins 
function, though, is in the kidney.  
   Our kidneys job is to filter our blood and remove wastes, such as urea.  
This urea is concentrated into urine, which has a very high salt 
concentration.  One of the jobs of the kidney is to reabsorb water out of 
urine so that it doesn't dehydrate us.  One of the major players in this 
system are aquaporins, which help to reabsorb water from the urine.