Final answer:
Large differences in water concentration are not required to drive water across a membrane; this is a false statement. Water moves across membranes through osmosis, which can occur with even slight differences in water concentration, facilitated by membrane proteins such as aquaporins.
Step-by-step explanation:
The notion that large differences in water concentration are required to drive water across a membrane is false. Water can move across a membrane through a process called osmosis, which is the diffusion of water from an area of high water concentration (low solute concentration) to one of low water concentration (high solute concentration), even if the differences in concentration are minuscule. This movement occurs until the concentration of the solute equates on both sides of the membrane or until osmotic pressure is balanced by hydrostatic pressure. Osmosis is an example of facilitated diffusion, where water transport across the cell membranes is facilitated by proteins called aquaporins, making the process efficient even with minor differences in water concentration.
For instance, consider a semipermeable membrane with different solute concentrations on either side. Water will naturally move to the side with the higher solute concentration (and a lower concentration of free water molecules) to try and equalize the concentration gradient. Thus, osmosis does not require a large water concentration difference; it relies on the presence of a concentration gradient, no matter how small, across a semipermeable membrane.