The endomembrane system is composed of the different membranes that are suspended in the cytoplasm within a eukaryotic cell.These membranes divide the cell into functional and structural compartments, or organelles.he plasma membrane is a phospholipid bilayer membrane that separates the cell from its environment and regulates the transport of molecules and signals into and out of the cell. Embedded in the membrane are proteins that perform the functions of the plasma membrane.