Water crosses the membrane osmotically when the external osmolarity differs from internal. The driving force is the osmotic pressure difference Δπ.
Water flux: J_w = Lp·A·(Δπ - ΔP), where ΔP is turgor pressure and Lp is hydraulic conductivity.
Hyperosmotic shock → water efflux → cell shrinks (plasmolysis). Hypoosmotic → swelling, possible lysis. Regulatory volume decrease/increase (RVD/RVI) involves solute release/uptake.