Final answer:
Osmosis is the movement of water through a semipermeable membrane from a higher concentration to a lower concentration, often facilitated by proteins known as aquaporins, until equilibrium or pressure balance is reached.
Step-by-step explanation:
The net diffusion of water down its own concentration gradient across a semipermeable membrane is called osmosis. Osmosis occurs when water moves from an area where it is more concentrated to an area where it is less concentrated, attempting to equalize concentrations on both sides of the membrane. This movement continues until equilibrium is reached or until the osmotic pressure is balanced by the hydrostatic pressure.
For instance, when a semipermeable membrane separates two solutions with different solute concentrations, water will naturally move to the side with the higher solute concentration (hypertonic side), because that is where the water concentration is lower. This process is essential for maintaining proper cellular function and is facilitated by aquaporins, which are proteins that allow water to travel through cell membranes more easily.