Why would you not expect water to move easily through a memb

Why would you not expect water to move easily through a membrane? How do they do it?

What does it mean that phospholipids are amphipathic?

Solution

Cell membrane comprise of phosphipid bilayers. Phospholipids are polar at one end (that means they are hydrophilic or water loving at one end) and non polar (that menas hydrophobic or water fearing) at the other end. This bipolar nature of phospholipids is called as amphipathic.A phopspholopid has a glycerol molecule attached at its head with two long fatty acid hydrocarbon chains. Long hydrocarbon tail of the lipid part has non-polar bonds and is hydrophobic in nature whereas head part of it, where phosphate group is attached is hydrophilic in nature. These amphiphatic molecule comes together to form bilayer structures of cell membrane.

This phopholipids are thus being hydrophobic at one end form a barrier for water to enter the cell. Majorly water passes in the cell through special channel proteins called aquaporins. Aquaporins act as transmembrane proteins which act as gated water channels thereby facilitating the rapid transport of water acoss the cell membrane, even against the concentration gradient.Only a small amount can pass through the phospholipid bilayer by the process of osmosis (along the concentration gradient).