Which statement best describes osmosis?

• A. Diffusion of water from a greater to a lesser water concentration across a selectively permeable membrane
• B. Movement of water into a solute
• C. Active transport of water across the cell membrane
• D. Diffusion of water from a lesser to greater water concentration

Answer: (A)

Osmosis is the movement of water across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration, driven by a solute gradient and without using energy. The membrane lets water pass but restricts many solutes, so water shifts toward the side with more solutes to balance concentrations. That makes the statement accurate: diffusion of water from a greater to a lesser water concentration across a selectively permeable membrane. Osmosis is a passive process, unlike active transport, and it differs from moving water into a solute or from a lower to a higher water concentration. In clinical terms, changes in tonicity (hypertonic, hypotonic, isotonic fluids) drive water movement and affect cell size, which is important in assessing dehydration, edema, and IV fluid therapy.