The cell membrane surrounds the protoplasm of a cell and, in animal cells, physically separates the intracellular componentsfrom the extracellular environment. Fungi, bacteria and plants also have the cell wall which provides a mechanical support for the cell and precludes passage of the larger molecules. The cell membranealso plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues. The barrier isdifferentially permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can bepassive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.
The cell wall is the tough,usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also actsas a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell. They are found in plants, bacteria, fungi, algae, andsome archaea. Animals and protozoa do not have cell walls.
Why scientists described the cell membrane as a “fluid mosaic”?
According to the fluid mosaic model of S. J. Singer and GarthNicolson 1972, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less easily. This picture may be valid in the space scale of 10 nm.However, the plasma membranes contain different structures or domains that can be classified as protein-protein complexes; lipid rafts, and pickets and fences formed by the actin-based cytoskeleton....