Which part of a phospholipid is charged, or polar?
The phosphate heads
The phosphate heads are polar, which allows them to mix with water. This is why they are on the external and internal part of the cell membrane (because there's a lot of water that cells come into contact with externally, and there's a lot of water internally too). The lipid tails are the non-polar section of the phospholipid bilayer, which is why they're on the inside (so they'll be less exposed to water).