Cell membranes consist of a phospholipid bilayer, which acts as a hydrophobic barrier. Conversely, the concentration of sodium ions is greater outside the cell than inside. The lipid carrier involved in carrying building blocks for bacterial cell wall synthesis across the cell membrane is the target for vancomycin. In order to access the cell, the drug must be sufficiently hydrophobic to cross the membrane. There are various antibacterial and antifungal compounds that destroy cells by building helical structures through the cell membrane, thus forming a tunnel through which ions and small polar molecules can flow in an uncontrolled manner. However, the antibiotic polymyxin B shows some selectivity against bacterial cells over animal cells and has been used against some Pseudomonas infections. The specific target for vancomycin is a lipid carrier in bacteria which is responsible for carrying the building blocks required for cell wall synthesis across the cell membrane.