Selective Permeability


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This illustration shows that the inside and outside of a plasma membrane are different, with the exterior covered in the spherical heads, and the interior filled with the strandlike tails.
The plasma membrane’s exterior surface is not identical to its interior surface.

Source: OpenStax Biology 2e

OpenStax Biology 2e

Plasma membranes are asymmetric: the membrane’s interior is not identical to its exterior. There is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane. On the membrane’s interior, some proteins serve to anchor the membrane to cytoskeleton’s fibers. There are peripheral proteins on the membrane’s exterior that bind extracellular matrix elements. Carbohydrates, attached to lipids or proteins, are also on the plasma membrane’s exterior surface. These carbohydrate complexes help the cell bind required substances in the extracellular fluid. This adds considerably to plasma membrane’s selective nature.

– What is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water?

Plasma membranes are amphiphilic: They have hydrophilic and hydrophobic regions. This characteristic helps move some materials through the membrane and hinders the movement of others. Non-polar and lipid-soluble material with a low molecular weight can easily slip through the membrane’s hydrophobic lipid core. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs and hormones also gain easy entry into cells and readily transport themselves into the body’s tissues and organs. Oxygen and carbon dioxide molecules have no charge and pass through membranes by simple diffusion.

– What refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene?

Polar substances present problems for the membrane. While some polar molecules connect easily with the cell’s outside, they cannot readily pass through the plasma membrane’s lipid core. Additionally, while small ions could easily slip through the spaces in the membrane’s mosaic, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes. Simple sugars and amino acids also need the help of various transmembrane proteins (channels) to transport themselves across plasma membranes.

Membranes are thin films of material having the property of selective permeability. Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis are pressure-driven membrane separation processes. Electrodialysis is a membrane process driven by an electric field. Microfiltration and ultrafiltration are true filtration processes in which particle size is the sole criterion for permeation. In reverse osmosis, selectivity is based on the chemical nature of the molecules. The majority of industrial-scale membrane separation processes occur in tangential filtration mode. Microfiltration is used to separate particles larger than 0.1 μm. Ultrafiltration retains molecules of proteins but not small peptides and carbohydrates. Nanofitration is capable of separating ions depending on their size and charge. Reverse osmosis retains practically all solutes. Solvent transport in micro- and ultrafiltration deviates from Darcy’s law, mainly because of concentration polarization and gel polarization. Membranes vary in their composition and structure. In asymmetric membranes, a very thin layer of polymer controls permeability while the rest of the membrane thickness contributes to mechanical strength. Common membrane configurations are plate-and-frame, spiral wound, tubular, and hollow fiber.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: