The exception is inert and impermeable. Cells are rather fluid and permeable. Cells membranes are fluid because the phospholipids that compose the membrane are made of unsaturated fats. Also, cholesterol is intercalated between these fatty acids chains hence the membrane is never compact. The permeability of the membrane is enabled by the presence of pores in the membarne made by transmembrane proteins that run from outside to the inside of the cell.