Water crosses the plasma membrane primarily by means of osmosis, which is the movement of water molecules through a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
Osmosis is a critical process in maintaining cellular homeostasis. The plasma membrane, while it is selectively permeable, allows water to pass through more readily than many solutes. Although water can diffuse directly through the lipid bilayer, the process is facilitated mainly by specialized proteins known as aquaporins. These channels specifically facilitate the rapid movement of water in and out of the cell, which is especially important in cells that require large amounts of water transport, like kidney cells.
In summary, osmosis plays a key role in allowing water to cross the plasma membrane smoothly, ensuring that cells can maintain their internal environment effectively.