Question

How does water pass through the cell membrane?
A- through water carrier proteins
B- through a water ion pump
C- through channel proteins just for water
D- directly through the lipid bilayer

Answer 1

Water is a tiny molecule, basically just an oxygen atom with two extra electrons and two extra protons. It can diffuse through surfaces on it's own simply with the concentration gradient (from high concentration to low).

It does not require energy when working "with the flow" (ion pumps are for working against the flow), it isn't a molecule so large it needs a protein to shuttle it though the membrane, and it doesn't need any special proteins at all!

The only question is if the surface is hydrophobic ("repels" water), hydrophilic ("attracts" water), or both? As long as it's hydrophilic or partly hydrophilic, water will diffuse through, no problem.

So, if process of elimination isn't obvious enough, you tell me: What is the phospholipid bilayer the makes up cell membranes?