Phospholipids are the substance/structure that is the building block of the cell membrane. Without phospholipids, we wouldn't even have a cell membrane and wouldn't really have any way how to shield and guard or control th epassage of substances across the cell surface. The cell membrane is at the same time partially permeable; this means it will let certain ions that can be found in the intra and extracellular fluid in and out, while others will be kept from moving so freely.