A cell membrane is considered waterproof primarily due to its unique structure, which is made up of a double layer of phospholipids. Each phospholipid molecule has a hydrophilic (water-attracting) ‘head’ and two hydrophobic (water-repelling) ‘tails’. This arrangement creates a barrier that prevents water-soluble substances from passing freely through the membrane.
The hydrophobic tails face inward, away from the water, while the hydrophilic heads are positioned outward towards the watery environment inside and outside the cell. This bilayer effectively acts as a barrier that blocks the passage of polar molecules and ions, making it waterproof to a large extent.
In addition to the phospholipid bilayer, various proteins embedded in the membrane also contribute to its functionality by facilitating the transport of specific molecules that cannot easily cross the hydrophobic layer. However, the fundamental waterproof quality comes from the phospholipid structure itself.