What process describes the movement of water across a semi-permeable membrane?

The correct answer is the process known as osmosis, which specifically refers to the movement of water molecules across a semi-permeable membrane. Osmosis occurs when water moves from an area of lower solute concentration to an area of higher solute concentration, aiming to balance the solute levels on both sides of the membrane. This movement is essential in many biological processes, including nutrient absorption and maintaining cell turgor in plants.

In osmosis, only water molecules can pass through the semi-permeable membrane, while solutes cannot, making this process distinct from other forms of transport. It relies on the principles of concentration gradients but does not require energy input from the cell, which is a critical feature that differentiates it from active transport processes. Understanding osmosis is fundamental to grasping how cells regulate their internal environment and interact with their surroundings.