The structure that separates the cell content from the external environment is the plasma membrane. This is a lipo-protein bilayer, which can be visible under the electron microscope.
It reveals numerous enfolding and differentiation. The plasma membrane also acts as a means of connection among the neighbouring cells. The main function of the plasma membrane is to regulate the movement of materials into and out of the cell.
It includes the inward movement of water and other macromolecules by the process of endocytosis and the the outward movement of cell products by exocytosis. The plasma membrane is surrounded by a cell wall in plant cells and by a cell coat in animal cells.
The cell wall gives rigidity and mechanical strength to the plant tissues. Cellulose fibers and occasionally, Lignin constitute the cell wall structure. The cell walls are connected with one another by plasmodesmata, which allow communication among cells of the tissue.
In most animal cells, the plasmamembrane is covered by a cell coat made of glycoproteins and glycolipids that may extend the thickness of the cell membrane. The cell coat carries out several functions besides protection of the cell. It is involved in molecular recognition of identity of the cells.