A Brief Look at the Endomembrane System

A main difference that sets eukaryotic cells apart from prokaryotic cells is that eukaryotic cells contain a highly ordered and complex endomembrane system. It is thought that the endomembrane system evolved very shortly after Eukarya diverged from bacteria and archaea. The endomembrane system is composed of membrane bound organelles and these organelles are formed by 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 that consist of hydrophilic polar heads and hydrophobic tails. The only difference in the two layers is the orientation of the phospholipids. While the hydrophilic heads of one layer face towards the cytosol the other layer has hydrophilic heads that face the inside of the membrane, called the lumen. The interior of the membrane contains the hydrophobic tails. This organization allows for a fluid phospholipid bilayer to exist. 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. Theses membranes provide structural and functional division within the cell. The endomembrane system is crucial in the processing and sorting of macromolecules, 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 direc...

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...ications applied to proteins in the ER involves the addition and removal of sugar molecules. Often time’s misfolded proteins have to be glycosylated repeatedly until the correct folding is induced. This process requires a lot of energy and this is typically not a problem for the eukaryotic cell but in a environment that has limited resources the simplicity of the prokaryote could be more effective because it does not require near the amount of energy to sustain life.

The endomembrane system clearly plays a role in the biology of the cell at many levels. Whether it be the modification of macromolecules, localization of macromolecules, cell to cell signaling, or endocytosis the involvement of the endomembrane system is hard to escape in the eukaryotic cell. The complexity found in eukaryotic cells would be difficult to achieve without an endomembrane system.