Plant and bacterial cell membranes are surrounded by a thick cell wall.
Bacterial cell wall
The bacterial cell is enclosed within a wall that differs chemically from the cell wall of plants. The cell wall contains a rigid framework of polysaccharide chain cross linked with short peptide chains and its outer surface is coated with lipopolysaccharide. The pili, found in some bacteria are extensions of the cell wall. In some bacteria the cell wall is surrouned by an additional structure called a capsule. The cell wall and capsule confer shape and form of the bacterium and also act as a physical barrier to the cell membrane. In the absence of cell wall and capsule is mechanically fragile and the bacteria would rupture.
Plant cell wall
The cell wall is a thick polysaccharide containing structure immediately surrounding the plasma membrane. In multicelllar plants, the plasma membrane of neighboring cells are separated by these walls, and adjacent plant cell have their walls fused together by a layer called the middle lamella. The cell wall serves both as a protective and a supportive unit for the plant. The degree to which the cell wall may be involved in the regulation of the exchange of materials between the plant cell and its surroundings is difficult to assess but is most likely restricted to macromolecules of considerable size. As in animal cells,most of the regulation of exchanges between the cytoplasm and the extracellar surrounding of plant cells is a function of the plasma membrane.
The cell wall protects bateria against swelling in hypotonic media. It is porous and allows most small molecules to pass. Some of the pili are hollow and serve to transfer DNA from sexual conjugation.