Gram-Negative Bacteria Have a cell wall structure consisting of a thin peptidoglycan layer sandwiched between an inner cytoplasmic membrane and an outer membrane. The outer membrane contains LPS, phospholipids, and proteins. LPS is composed of three parts: Lipid A, core polysaccharide, and O- antigen. Gram-Positive Bacteria Have a much thicker peptidoglycan layer outside the cytoplasmic membrane. They lack an outer membrane and therefore do not have LPS. Their cell walls are primarily composed of peptidoglycan and teichoic acids.