Cell Wall: Nearly all prokaryotes have a protective cell wall that prevents them from bursting in a hypotonic environment (an aqueous environment with a lower concentration of solutes than are found within the cell). The composition of cell walls vary depending on the type of organisms, but most cell walls contain a combination of the major organic molecules—proteins, carbohydrates and lipids. Bacteria have a unique molecule called peptidoglycan in their cell wall. Archaean cell walls do not contain this molecule. Cell wall composition of bacteria allow scientists to classify them as either Gram-positive or Gram-negative.