Question

How can water pass through the lipid bilayer?

Answer 1

Water passes through the lipid bilayer by diffusion and by osmosis, but most of it moves through special protein channels called aquaporins.

Explanation:

Water can diffuse through the lipid bilayer even though it's polar because it's a very small molecule.

Water can also pass through the cell membrane by osmosis, because of the high osmotic pressure difference between the inside and the outside the cell.

That doesn't mean that it's an easy process, because the solubility of water in lipid is about 1 molecule of water per million molecules of lipid.

But the outside concentration of water is very high (about 50 mol/L), and the surface area to volume ratio of the cell is very large, so this is an important cellular process.

Here's a video simulation of a single water molecule diffusing through a lipid bilayer.

However, the "straight through the membrane" passage is relatively slow.

Most of the water passes through channel proteins called aquaporins.

Aquaporins selectively conduct water molecules in and out of the cell, while preventing the passage of ions and other solutes.

Every second about a billion water molecules pass in single file through a channel in the middle of an aquaporin.

Here is a slow-motion video simulation of water flowing through an aquaporin.

The yellow dot represents a single molecule of water moving through the channel.