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Selective permeability is the ability of some membranes to allow only specific solutes to pass from one side to the other. That is, it refers to the ability that it confers to choose or select the solutes that can pass and those that cannot, thus regulating the transport of molecules and ions through the membrane.
A membrane that exhibits selective permeability acts like the gatekeeper of an exclusive club, carefully monitoring who gets in and who doesn’t to make sure that only the right people are inside. In addition, it is also responsible for removing those molecules that must come out. This type of transport can be carried out either passively (down the concentration gradient and without the need for energy) or actively (against the concentration gradient and through ATP or GTP hydrolysis).
Selectively permeable versus semipermeable membranes
A related term that is often heavily confused with selective permeability is semipermeability . Indeed, many biologists and health sciences professionals, as well as biology and medicine texts, use both terms as if they were the same, when this is not entirely true.
Semipermeable membranes are those that restrict the passage of solutes based on properties such as their size, their polarity, and their electrical charge. In this sense, a semipermeable membrane that allows a neutral solute of a certain size to pass will allow all neutral solutes of that size or those that are smaller to pass, but it will not allow larger neutral molecules to pass.
This is the operating principle of the semi-permeable membranes used for the desalination of seawater by reverse osmosis . These are polymeric synthetic membranes with very small pores that only allow the passage of water molecules and not dissolved ions or other larger neutral solutes.
On the other hand, a membrane that is selectively permeable might be permeable to a molecule such as glucose, but not permeable to another, even smaller carbohydrate. The reason is that, in the case of selective permeability, this selectivity is much more specific than in the case of semipermeability.
Perhaps the confusion or the reason why biologists often use both terms interchangeably is because the cell membrane is in turn one of the best known examples of semipermeable membranes and selectively permeable membranes. In fact, in the cell, semipermeability and selective permeability almost always go hand in hand and work together to control transport into and out of the cell and thus maintain the complex balance that keeps each cell alive and efficient.
Selectivity mechanisms
A fundamental difference between selectivity and semipermeability is the mechanism by which particles are or are not allowed to pass from one side of the membrane to the other. In the case of semipermeable membranes, osmosis and simple diffusion are the main transport mechanisms. Osmosis occurs when water molecules cross the membrane through pores formed by proteins called aquaporins, moving from the compartment that is more dilute to the one that is more concentrated in solutes.
On the other hand, the membrane is formed by a phospholipid bilayer with hydrophilic phosphate groups exposed on both sides of the membrane while the hydrophobic tails of fatty acids are concentrated in the center. This prevents polar solutes and ions from crossing the membrane, but small nonpolar solutes such as oxygen and carbon dioxide can freely diffuse from one side to the other.
Instead, the selectivity of membranes with respect to the passage of solutes is almost always mediated by one or more membrane proteins. Selective permeability involves either facilitated diffusion or active transport .
facilitated diffusion
Facilitated diffusion is a type of passive transport mediated by carrier proteins . In a typical case, these proteins bind to the solute (or substrate) on one side of the membrane. As the solute binds, the protein changes conformation, pushing the solute across the membrane and releasing it on the other side.
Example of selective permeability by facilitated diffusion
- The classic example of this type of glucose transport mechanism is mediated by a family of proteins called glucose transporters (it’s easy to remember). There is a whole family of transporter proteins called SLC2 that are responsible for the selective transport of specific carbohydrates, inorganic cations and anions, and other solutes.
Active transport across the membrane
In the case of active transport , this refers to the transport of solutes across the membrane against their concentration gradient. By going against this gradient, energy must be supplied for the process to take place, which is why it is called “active” transport.
There are two main types of active transport, which are primary active transport (when an enzyme called a pump transports solute directly against its gradient) and secondary active transport (in which one pump transports another solute against its gradient, and then this gradient provides the energy to transport a second solute against its gradient while the first moves down its gradient.)
Examples of selective permeability by active transport
- As an example of primary active transport we can mention the sodium/ potassium pump, which uses the energy contained in an ATP molecule to simultaneously transport three sodium ions out of the cell and 2 potassium ions in, in both cases against each other. of their concentration gradients.
- Another example of active transport is the proton pump in the inner mitochondrial membrane. In this particular case, the energy needed to move the protons against their concentration gradient comes from the redox reactions of the aerobic respiration chain . This type of transport makes the inner membrane of the mitochondria a selectively permeable membrane.
- Finally, as an example of secondary active transport is the selective permeability of the membrane to calcium ions mediated by the antiporter sodium calcium. This antiporter uses the sodium concentration gradient generated by the sodium potassium pump to pump one calcium ion out of the cell while 3 sodium ions enter it.
References
Fluence Corp. (September 11, 2019). Water Treatment Membranes and Their Processes. Retrieved from https://www.fluencecorp.com/es/membranas-de-tratamiento-de-agua/
Pérez, JM and Noriega B., MJ (). TRANSPORT THROUGH MEMBRANE. Open Course Ware. Retrieved from https://ocw.unican.es/pluginfile.php/879/course/section/967/Tema%25204-Bloque%2520II-Transporte%2520a%2520traves%2520de%2520Membrana.pdf
Selective permeability (sf). Medical dictionary. Retrieved from https://www.cun.es/diccionario-medico/terminos/permeabilidad-selectiva
Sagle, A. & Freeman, B. (2004). Fundamentals of Membranes for Water Treatment. Retrieved from https://texaswater.tamu.edu/readings/desal/membranetechnology.pdf
Semi-permeable membrane (sf). Journal of Membrane Science & Technology. Retrieved from https://www.longdom.org/peer-reviewed-journals/semipermeable-membrane-6018.html
