Question

the diffusion of water molecules across a semipermeable membrane and down a concentration gradient is known as .

Explanation:

In biology (a subfield of Natural Science), the process where water molecules diffuse across a semipermeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration) down a concentration gradient is defined as osmosis. This matches the description in the question.

Answer:

osmosis