A main difference that distinguishes eukaryotic cells from prokaryotic cells is that eukaryotic cells contain a highly ordered and complex endomembrane system. The endomembrane system is thought to have evolved very soon after the Eukaryan divergence of bacteria and archaea. The endomembrane system is composed of membrane-bound organelles and these organelles are formed of lipid bilayers. When bilayers fold, they create separate compartments that do not include the cytosol. This folding is an energetically favorable process. These folded membrane compartments are topologically equivalent to the extracellular matrix, which, as we will see, is a very important aspect. The lipid bilayer contains two identical layers consisting of hydrophilic polar heads and hydrophobic tails. The only difference between the two layers is the orientation of the phospholipids. While the hydrophilic heads of one layer face the cytosol, the other layer has hydrophilic heads that face the interior of the membrane, called the lumen. The interior of the membrane contains the hydrophobic tails. This organization allows the existence of a fluid phospholipid bilayer. The membrane also contains proteins that contribute to the functioning of the endomembrane system as well as cholesterol which makes the membrane stiffer and less fluid. These membranes ensure structural and functional division within the cell. The endomembrane system is crucial in macromolecule processing and sorting, macromolecule localization, and cell-to-cell signaling. The nuclear envelope is the beginning of the endomembrane system. The nuclear envelope is a lipid bilayer that surrounds the nucleus and keeps the cytosol and nucleus separated. The endoplasmic reticulum is connected directly...... middle of paper ......ications applied to proteins in the ER involve the addition and removal of sugar molecules. Often, misfolded proteins must be repeatedly glycosylated until correct folding is induced. This process requires a lot of energy and this is generally not a problem for the eukaryotic cell, but in a resource-limited environment the simplicity of the prokaryote might be more efficient because it does not require as much energy to sustain life . The endomembrane system clearly plays a role in the biology of the cell at several levels. Whether it is macromolecule modification, macromolecule localization, cell-to-cell signaling, or endocytosis, the involvement of the endomembrane system is difficult to escape in the eukaryotic cell. The complexity found in eukaryotic cells would be difficult to achieve without an endomembrane system.