Final answer:
Water moves down its concentration gradient during osmosis, from higher to lower water concentration, until an equilibrium is reached.
Step-by-step explanation:
In which direction does water move across membranes, up or down the concentration gradient? The correct answer is b) Down the concentration gradient. Water moves from an area of high water concentration to one of low water concentration, which is also the side of higher solute concentration.
Osmosis is a special type of diffusion that describes the movement of water across a semipermeable membrane. This process continues until the concentration gradient of water goes to zero or the hydrostatic pressure of the water balances the osmotic pressure, facilitating equilibrium within living systems. Noteworthy is the role of aquaporins in assisting the efficient transport of water across cellular membranes, such as in red blood cells and kidney tubules.