How does osmosis occur?
Osmosis is the net movement of water across a selectively permeable membrane driven by a difference in water's chemical potential. In simple cases water moves toward a higher concentration of nonpenetrating solutes. Membrane permeability and pressure differences affect the outcome.
Membranes and osmotic pressure
Water molecules move in both directions, so osmosis concerns the net flow. Osmotic pressure is related to the pressure difference required to balance that flow. Equal total solute concentrations do not necessarily have identical effects on cells, because penetrating and nonpenetrating solutes behave differently. Tonicity accounts for this distinction when describing cell swelling or shrinking.
In cells and water treatment
Plant cell walls help withstand internal pressure and support turgor; animal cells lack that wall. Reverse osmosis applies pressure to push water against its usual osmotic direction, separating certain solutes with a suitable membrane. It requires energy and does not remove every contaminant equally. Experiments must specify membrane properties, temperature, pressure, and solute type rather than concentration alone.