A major 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 shortly after the Eukarya differentiated from bacteria and archaea. The endomembrane system is composed of membrane-bound organelles, and these organelles are made up of lipid bilayers. When the 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 facing the inside of the membrane, called the lumen. The interior of the membrane contains hydrophobic tails. This organization allows the existence of a fluid phospholipid bilayer. Also included in the membrane are proteins that aid in the function of the endomembrane system, as well as cholesterol which makes the membrane more rigid and less fluid. These membranes provide 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 separate. The endoplasmic reticulum is connected in the direction...... 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 over time 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 an environment that has limited resources the simplicity of the prokaryote could be more effective because it does not require as much energy as is needed to sustain life. The endomembrane system clearly plays a role in the biology of the cell at many levels. Whether it is macromolecule modification, macromolecule localization, cell-cell signaling, or endocytosis, it is difficult to escape the involvement of the endomembrane system in the eukaryotic cell. The complexity found in eukaryotic cells would be difficult to achieve without an endomembrane system.