Science Buddies: "Ask an Expert"

Hello,

A student asked me how osmosis works. after i described the process they wondered (and left me wondering) why the water isnt repelled by the cells membranes hydrophobic tail. Instead of speculating, I wanted to find a definite answer. Can you help me with this?

Hello there!
Your question initially had me confused as well because you're right. Water is repelled by the hydrophobic tails of the phospholipid bilayer, so it would not be able to go through an impermeable membrane. However, osmosis occurs through a semi-permeable membrane with aquaporins, "small transmembrane proteins similar to those responsible for facilitated diffusion and ion channels" (https://en.wikipedia.org/wiki/Osmosis). So basically, the osmosis is possible because water diffuses through these holes in the bilayer opposed to actually crossing the phospholipids themselves.

I hope that helps!
-Kayli Masuda