The compound that allows materials to move through cell membranes by osmosis is water.

Here's why:

* Osmosis is the movement of water molecules across a semi-permeable membrane from an area of high water concentration to an area of low water concentration.

* Cell membranes are semi-permeable, meaning they allow some substances to pass through while blocking others.

* Water molecules are small enough to pass through the phospholipid bilayer of the cell membrane, facilitated by special protein channels called aquaporins.

While other substances might be involved in the overall process of cell transport, water is the key player in osmosis.