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. This drives the uptake of other species by secondary activetransport. Cell wall - Primary function is to prevent bursting from osmotic shock. Overall structure is analogous to girder-supported bridge. Cell wall is a rigid, multilayered sheet.