Humber Admissions Practice Test

What process describes the diffusion of water across a selectively permeable membrane?

1. Active transport

2. Facilitated diffusion

3. Osmosis

4. Endocytosis

The correct answer is: Osmosis

Osmosis specifically refers to the movement of water molecules through a selectively permeable membrane. This process occurs when water moves from an area of lower solute concentration to an area of higher solute concentration, aiming to equalize solute levels on both sides of the membrane. Osmosis is a crucial biological process that helps maintain cell homeostasis and is driven by the principle of concentration gradients, which makes it a passive transport mechanism—meaning it does not require energy. In contrast, active transport involves the movement of ions or molecules against their concentration gradient, which requires energy input, typically in the form of ATP. Facilitated diffusion is a process that allows substances to cross membranes with the assistance of special proteins but does not apply specifically to the movement of water. Endocytosis is a different active transport mechanism that involves the engulfing of substances into the cell, which also does not pertain to the specific process of water movement. Thus, osmosis is the most accurate term to describe the diffusion of water across a selectively permeable membrane.