Semipermeable membrane

Semipermeable membrane is a type of synthetic or biologic, polymeric membrane that allows certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability. Many natural and synthetic materials which are rather thick are also semipermeable. One example of this is the thin film on the inside of an egg.

Biological membranes are selectively permeable, with the passage of molecules controlled by facilitated diffusion, passive transport or active transport regulated by proteins embedded in the membrane.

A phospholipid bilayer is an example of a biological semipermeable membrane. It consists of two parallel, opposite-facing layers of uniformly arranged phospholipids. Each phospholipid is made of one phosphate head and two fatty acid tails. The plasma membrane that surrounds all biological cells is an example of a phospholipid bilayer. The plasma membrane is very specific in its permeability, meaning it carefully controls which substances enter and leave the cell. Because they are attracted to the water content within and outside the cell (or hydrophillic), the phosphate heads assemble along the outer and inner surfaces of the plasma membrane, and the hydrophobic tails are the layer hidden in the inside of the membrane. Cholesterol molecules are also found throughout the plasma membrane and act as a buffer of membrane fluidity. The phospholipid bilayer is most permeable to small, uncharged solutes. Protein channels are embedded in or through the phospholipids, and, collectively, this model is known as the fluid mosaic model. Aquaporins are protein channel pores permeable to water.

Information can also pass through the plasma membrane when signaling molecules bind to receptors in the cell membrane. The signaling molecules bind to the receptors, which alters the structure of these proteins. A change in the protein structure initiates a signaling cascade. G protein-coupled receptor signaling is an important subset of such signaling processes.

Because the lipid bilayer is semipermeable, it is subject to osmotic pressure. When the solutes around a cell become more or less concentrated, osmotic pressure causes water to flow into or out of the cell to equilibrate. This osmotic stress inhibits cellular functions that depend on the activity of water in the cell, such as the functioning of its DNA and protein systems and proper assembly of its plasma membrane. This can lead to osmotic shock and cell death. Osmoregulation is the method by which cells counteract osmotic stress, and includes osmosensory transporters in the membrane that allow K+ and other molecules to flow through the membrane.

Artificial semipermeable membranes see wide usage in research and the medical field. Artificial lipid membranes can easily be manipulated and experimented upon to study biological phenomenon. Other artificial membranes include those involved in drug delivery, dialysis, and bioseparations.

The bulk flow of water through a selectively permeable membrane because of an osmotic pressure difference is called osmosis. This allows only certain particles to go through including water and leaving behind the solutes including salt and other contaminants. In the process of reverse osmosis, water is purified by applying high pressure to a solution and thereby push water through a thin-film composite membrane (TFC or TFM). These are semipermeable membranes manufactured principally for use in water purification or desalination systems. They also have use in chemical applications such as batteries and fuel cells. In essence, a TFC material is a molecular sieve constructed in the form of a film from two or more layered materials. Sidney Loeb and Srinivasa Sourirajan invented the first practical synthetic semi-permeable membrane. Membranes used in reverse osmosis are, in general, made out of polyamide, chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small molecules that cannot be filtered.

Reverse osmosis membrane modules have a limited life cycle, several studies have endeavored to improve the performance of the process and extend the RO membranes lifespan. However, even with the appropriate pretreatment of the feed water, the membranes lifespan is generally limited to five to seven years.