Unlike the cell membrane that is found in every cell, the cell wall is much less common, found in Archaea (which have walls of pseudomurein) and Eubacteria (which have walls of pepditoglycan). The cell wall surrounds the cell membrane, and in the case of plant, fungi and bacteria cell walls, does not usually determine the materials that can and cannot enter the cell; this is mostly reserved for the cell membrane, located below the cell wall. The cell wall provides and maintains the shape of the cell and serves as a protective barrier, as well as maintaining cell integrity and osmatic pressure in a hypotonic environment.
Diffusion.
Diffusion (also known as passive transport) is the movement of molecules from a region of higher substance concentration, to a region of lower substance concentration until both regions are evenly distributed with the molecules; this process requires no energy from the cell. An example of diffusion occurring within a living cell would be the gas exchange happening to a single celled organism such as an amoeba. The amoeba takes in oxygen, and releases carbon dioxide. When molecules or particles move from a high concentration area to a low concentration area, they create a diffusion gradient. In laymen's terms, that simply means that the particles move further from the concentrated area, resulting in fewer within a particular space.
Another form of diffusion is known as facilitated diffusion'. This process uses a carrier, generally in the form of a protein molecule, which assists in the movement of the solute molecules. Many cells utilise this process for the uptake of certain sugars, amino acids, and vitamins. Some molecules also cross cell membranes by active transport', which requires a specialised carrier molecule, as well as a source of energy.
Osmosis.
Osmosis is a special type of diffusion. It is the movement of water molecules from a weak solution to a strong solution.
