<P> The plasma membrane of a cell has multiple functions . These include transporting nutrients into the cell, allowing waste to leave, preventing materials from entering the cell, averting needed materials from leaving the cell, maintaining the pH of the cytosol, and preserving the osmotic pressure of the cytosol . Transport proteins which allow some materials to pass through but not others are used for these functions . These proteins use ATP hydrolysis to pump materials against their concentration gradients . </P> <P> In addition to these universal functions, the plasma membrane has a more specific role in multicellular organisms . Glycoproteins on the membrane assist the cell in recognizing other cells, in order to exchange metabolites and form tissues . Other proteins on the plasma membrane allow attachment to the cytoskeleton and extracellular matrix; a function that maintains cell shape and fixes the location of membrane proteins . Enzymes that catalyze reactions are also found on the plasma membrane . Receptor proteins on the membrane have a shape that matches with a chemical messenger, resulting in various cellular responses . </P> <P> The origin of the endomembrane system is linked to the origin of eukaryotes themselves and the origin of eukaryoties to the endosymbiotic origin of mitochondria . Many models have been put forward to explain the origin of the endomembrane system (reviewed in). The most recent concept suggests that the endomembrane system evolved from outer membrane vesicles the endosymbiotic mitochondrion secreted . This OMV - based model for the origin of the endomembrane system is currently the one that requires the least amount of novel inventions at eukaryote origin and explains the many connections of mitochondria with other compartments of the cell . </P>

Is the nucleus a part of the endomembrane system