Final answer:
Water molecules pass through the plasma membrane by osmosis, which is a form of passive transport driven by a concentration gradient, requiring no energy from the cell.
Step-by-step explanation:
Water molecules travel through the plasma membrane unaided by a transporter, and this process is called osmosis. Osmosis is a type of passive transport, which occurs when substances move across the plasma membrane without any input of energy from the cell. The concentration gradient drives the movement of water molecules from an area where they have a higher concentration to an area of lower concentration, similar to how a ball rolls down a hill without any need for additional energy.