Until now you have only observed oxygen and carbon dioxide diffusing through a cell membrane. The way those two molecules directly pass through the cell membrane is actually quite uncommon. Most molecules are unable to pass through the membrane, so some kind of hole must be made to allow them to diffuse through. This is done using one or more proteins to create a pore in the membrane that allows only specific molecules or ions to pass through.

To the right is the model of such a pore. This one allows water to pass through a cell membrane.

Explore the model by using the animation controls and use the "snapshot" button below to take pictures that will help you answer the questions below.