The document summarizes the structure and function of the cell envelope of Gram-negative bacteria. It discusses three main layers: (1) the cytoplasmic or inner membrane composed of phospholipids, proteins and carbohydrates, (2) the peptidoglycan cell wall composed of polysaccharides and cross-linked peptide chains, and (3) the outer membrane containing phospholipids, lipopolysaccharides and porin proteins. Between the inner and outer membranes is the periplasmic space containing the peptidoglycan layer and degradative enzymes. The cell envelope protects bacteria from their environment while allowing selective transport of molecules.