In eukaryotic cells that lack a cell wall, the plasma membrane is the external covering of the cell. In function and basic structure, the eukaryotic and prokaryotic plasma membranes are very similar. There are, however, differences in the proteins. In eukaryotes, carbohydrates serve as receptor sites that assume a role in such functions as cell-to-cell recognition. These carbohydrates also provide adherence for bacteria. Eukaryotic plasma membranes also contain sterols, complex lipids not found in prokaryotic plasma membranes (with the exception of the mycoplasmas) (Costerton, 2001).
Sterols seem to be associated with the ability of the membranes to resist lysis due to increased osmotic pressure. Substances can cross eukaryotic and prokaryotic plasma membranes by simple diffusion, facilitated diffusion, osmosis, or active transport. Group translocation does not occur in eukaryotic cells. However, eukaryotic cells can utilize an additional mechanism called endocytosis. This occurs when a segment of the plasma membrane surrounds a particle or large molecule, encloses it, and brings it into the cell.
Endocytosis is one of the ways that viruses can enter animal cells (Thomas, 2004). Two very important types of endocytosis are phagocytosis and pinocytosis. During phagocytosis, cellular projections called pseudopods engulf particles and bring them into the cell. Phagocytosis is used by white blood cells to destroy bacteria and foreign substances.
In pinicytosis, the plasma membrane folds inward, bringing extracellular fluid into the cell along with whatever substances are dissolved in the fluid.