Here's why:
* High Abundance: Water makes up a significant portion of the cell's internal environment and the external environment.
* Small Size: Water molecules are relatively small, allowing them to pass through the membrane's phospholipid bilayer with ease.
* Polarity: While water is polar, it can still move across the membrane through special channels called aquaporins. These channels are specifically designed for water transport.
While other molecules like oxygen and carbon dioxide are also important for cell function, they don't cross the membrane as frequently as water due to their lower abundance and/or different transport mechanisms.
