Cytoplasmic membrane . The cytoplasmic membrane is a selectively permeable lipid bilayer . It is composed mostly of phospholipids . The cytoplasmic membrane serves as the boundary of the internal part of the cell. It also functions in nutrient import, waste product export, metabolism, and detection of environmental cues.
All cells have a cytoplasmic membrane.
Cell walls provide structural rigidity and protection from osmotic lysis . Nearly all procaryotes have cell walls. An exception is bacteria in the genus Mycoplasma , they lack cell walls. Many antibiotics, such as penicillin, work by inhibition of bacterial cell wall synthesis. Because animal cells do not have cell walls , these types of antibiotics are usually harmless to animals.