The molecule that moves across the membrane during osmosis is water.

Here's why:

* Osmosis is the movement of water across a semipermeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration).

* The membrane itself acts as a barrier to the movement of solutes, but it allows water molecules to pass through.

* The driving force for this movement is the difference in water potential between the two regions.

So, while solutes can influence the direction of water movement, it's ultimately the water molecules themselves that are moving across the membrane during osmosis.