Eukaryotic cells have internal membranes, including membrane bound organelles such as nucleus, and are larger. Viruses are a special case, as they are acellular by definition. Some variable cells are pleiomorphic.
Membrane structure is composed of a lipid bilayer involving phospholipids.
Two basic types of phospholipid distinguish Bacteria and Arachaea. Proteins include many enzyme systems which facilitate transport of large molecules. A primary function of cell membrane proteins is transport. Diffusion can be passive or facilitated. Facilitated diffusion uses carrier proteins to obtain saturation kinetics of uptake.