Net movement of water across a cell membrane occurs from a region of high water concentration to a region of low water concentration. This movement is driven by osmosis, which is the diffusion of water across a selectively permeable membrane.

Here's a breakdown:

* Water concentration: Water concentration is influenced by the concentration of solutes (dissolved substances) in a solution. A solution with a high concentration of solutes has a lower concentration of water molecules, and vice versa.

* Selectively permeable membrane: The cell membrane allows water to pass through freely but restricts the passage of most solutes.

* Osmosis: Water molecules move from an area where they are more concentrated (lower solute concentration) to an area where they are less concentrated (higher solute concentration) to try and equalize the concentration on both sides of the membrane.

In summary, water moves across a cell membrane from a hypotonic solution (high water concentration, low solute concentration) to a hypertonic solution (low water concentration, high solute concentration).