- Selectively permeable means a membrane allows the passage of some molecules or ions and inhibits the passage of others.
- The capacity to filter molecular transport in this manner is called selective permeability.
Selective Permeability Vs Semipermeability
Both semipermeable membranes and selectively permeable membranes regulate the transport of materials so that some particles pass through while others can’t cross. Some texts use terns “selectively permeable” and “semipermeable” interchangeably, but they don’t mean exactly the same thing. A semipermeable membrane is like a filter that allows particles to pass or not according to size, solubility, electrical charge, or other chemical or physical property. The passive transport processes of osmosis and diffusion permit transport across semipermeable membranes. A selectively permeable membrane chooses which molecules are allowed to pass based on specific criteria (e.g., molecular geometry). This facilitated or active transport may require energy.
Semipermeability can apply to both natural and synthetic materials. In addition to membranes, fibers may also be semipermeable. While selective permeability generally refers to polymers, other materials may be considered to be semipermeable. For example, a window screen is a semipermeable barrier that permits the flow of air but limits the transit of insects.
Example of a Selectively Permeable Membrane
The lipid bilayer of the cell membrane is an excellent example of a membrane which is both semipermeable and selectively permeable.
Phospholipids in the bilayer are arranged such that the hydrophilic phosphate heads of each molecule are on the surface, exposed to the aqueous or watery environment inside and outside of cells. The hydrophobic fatty acid tails are hidden inside the membrane. The phospholipid arrangement makes the bilayer semipermeable. It allows the passage of small, uncharged solutes. Small lipid-soluble molecules can pass through the hydrophilic core of the layer, such hormones, and fat-soluble vitamins. Water passes through the semipermeable membrane via osmosis. Molecules of oxygen and carbon dioxide pass through the membrane via diffusion.
However, polar molecules cannot easily pass through the lipid bilayer. They can reach the hydrophobic surface, but can’t pass through the lipid layer to the other side of the membrane. Small ions face a similar problem because of their electrical charge. This is where selective permeability comes into play. Transmembrane proteins form channels that permit the passage of sodium, calcium, potassium, and chloride ions. Polar molecules can bind to surface proteins, causing a change in the configuration of the surface and gaining them passage. Transport proteins move molecules and ions via facilitated diffusion, which does not require energy.
Large molecules generally don’t cross the lipid bilayer. There are special exceptions. In some cases, integral membrane proteins allow passage. In other cases, active transport is required. Here, energy is supplied in the form of adenosine triphosphate (ATP) for vesicular transport. A lipid bilayer vesicle forms around the large particle and fuses with the plasma membrane to either allow the molecule into or out of a cell. In exocytosis, the contents of the vesicle open to the outside of the cell membrane. In endocytosis, a large particle is taken into the cell.
In addition to the cellular membrane, another example of a selectively permeable membrane is the inner membrane of an egg.