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

The correct answer is the process known as osmosis. Osmosis specifically refers to the movement of water molecules through a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This process is vital for maintaining the balance of fluids in cells and is a passive transport method, meaning it does not require energy. Osmosis occurs naturally in biological systems, ensuring that cells maintain their shape and function effectively by regulating their internal environment. This ability of water to flow through membranes is critical for various physiological processes, including nutrient absorption and waste elimination.

In contrast, diffusion involves the movement of particles from an area of higher concentration to an area of lower concentration but does not specifically pertain to water. Active transport requires energy to move substances against their concentration gradient and does not typically involve the passive movement of water. Endocytosis is a process that cells use to engulf large particles or fluids, which is entirely different from the selective movement of water through a membrane. Thus, osmosis uniquely captures the specific function of water movement across semi-permeable membranes in cellular contexts.