living matter is at least 60% water it makes sense then the cell membranes are made of phospholipids because

The lipids form an insoluble barrier.

Phospholipids, a type of lipids, don't dissolve in water. Membrane-forming lipids have like other lipids hydrophobic region, but also in addition, they have hydrophilic region. In the cell membrane, they are organized in a lipid bilayer. In each layer, hydrophilic heads face a solution, and hydrophobic tails are inside the layer. That way, phospholipids are in contact with water, but still present a barrier because of their hydrophobic tails inside the layer.