SEMI-PERMEABLE MEMBRANES: CHARACTERISTICS, TRANSPORT, FUNCTIONS - BIOLOGY - 2025

The semipermeable membranes, also called "selectively permeable" are membranes that allow the passage of some substances, but prevent passage of other therethrough. These membranes can be natural or synthetic.

Natural membranes are the membranes of all living cells, while synthetic membranes, which can be of natural origin (cellulose) or not, are those that are synthesized for different uses.

Schematic representation of a semipermeable membrane (Source: Adam Rędzikowski via Wikimedia Commons)

An example of the utility of artificial or synthetic semi-permeable membranes are those used for kidney dialysis machines, or those used to filter mixtures in industry or in different chemical processes.

The passage of substances through a semipermeable membrane occurs by various mechanisms. In cell and synthetic membranes this can occur by diffusion through pores of different diameters, which "select" by size the substances that cross the membrane. It can also happen that substances enter by diffusion dissolving in the membrane.

In living cells, the passage of substances through the membranes can occur by means of transporters that act for or against the concentration gradients of substances. A gradient, in this case, is the difference in concentration for a substance on both sides of a membrane.

All cells on earth have membranes, these protect and separate their internal components from the external environment. Without membranes there are no cells and without cells there is no life.

Since these membranes are the most common example of semi-permeable membranes, special emphasis will be placed on these hereinafter.

characteristics

The first studies to elucidate the components of biological membranes were done using red blood cells. In these studies, the presence of a double layer forming the membranes was demonstrated and then it was discovered that the components of these layers were lipids and proteins.

All biological membranes are made up of a double lipid matrix that has different types of proteins “embedded”.

The lipid matrix of cell membranes is made up of saturated and unsaturated fatty acids; the latter give the membrane a certain fluidity.

Lipids are arranged in such a way that they form a bilayer in which each lipid, which has a hydrophilic head (which has an affinity for water) and one or two hydrophobic tails (water phobia, repels water), has hydrocarbon tails. facing each other in the center of the structure.

Phospholipids are the most abundant lipids that make up biological membranes. These include phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine, and phosphatidylserine.

Example of semi-permeable biological membrane (Source: LadyofHats via Wikimedia Commons)

Among the membrane lipids there are also cholesterol and glycolipids, all of them with amphipathic properties.

The proteins of semipermeable membranes are of several types (some of these may have enzymatic activity):

(1) those that form ion channels or pores

(2) transporter proteins

(3) proteins that bind one cell region to another and allow tissues to form

(4) receptor proteins that bind to intracellular cascades and

Transport

In a semipermeable biological membrane, transport can be by simple diffusion, facilitated diffusion, cotransport, active transport and secondary active transport.

Simple diffusion transport

In this type of transport, the energy that moves substances through the membrane is the difference in concentration that exists for those substances on both sides of the membrane.

Thus, the substances pass in a more → less sense, that is, from the place where they are more concentrated to the place where they are less concentrated.

Diffusion can occur because the substance is diluted in the membrane or passes through pores or channels. The pores or channels are of two types: those that are always open and those that open and close, that is, they are temporarily open.

Pores that are transiently open in turn can be (1) voltage dependent, that is, they open in response to a certain voltage, and (2) ligand-dependent, which must bind to some specific chemical to open.

Transport by facilitated diffusion

In this case, a transporter moves the substance to be transported from one side of the membrane to the other. These transporters are membrane proteins that can be permanently on the membrane or in vesicles that fuse to it when needed.

These transporters also work in favor of the concentration gradients of the substances they transport.

These types of transport do not require energy consumption and are therefore called passive transports, since they occur in favor of a concentration gradient.

Co-transportation

Another type of passive transport through semi-permeable membranes is called cotransport. In this case, the concentration gradient of one substance is used for the concomitant transport of another against its gradient.

This type of transport can be in two forms: symport, where the two substances are transported in the same direction, and antisport, in which one substance is transported in one direction and the other in the opposite direction.

Active membrane transport

These require energy and those that are known use ATP, which is why they are called ATPases. These transporters with enzymatic activity hydrolyze ATP to obtain the energy required for the movement of substances against their concentration gradient.

Three types of ATPases are known:

The Na + / K + pumps and the calcium pumps (calcium ATPases). These have a structure formed by an α and a ß subunit embedded within the membrane.

ATPases V and ATPases F, which have a characteristic stem shape made up of several subunits and a head that rotates around the stem subunits.

ATPases V serve to pump hydrogen ions against a concentration gradient, in the stomach and in lysosomes, for example. In some vesicles, such as dopaminergic ones, there are hydrogen bombs of this type that pump H + into the vesicles.

The ATPases F take advantage of the H + gradient so that they travel through its structure and take ADP and P and form ATP, that is, instead of hydrolyzing ATP, they synthesize it. These are found in the membranes of the mitochondria.

Secondary active transport

It is that transport that, making use of the electrochemical gradient generated by an ATPase, drags another substance against the gradient. That is, the transport of the second substance against its concentration gradient is not directly coupled to the use of ATP by the transporter molecule.

Features

In living cells, the presence of semipermeable membranes makes it possible to maintain concentrations of substances that are completely different from those of the same substances in the extracellular environment.

However, despite these concentration differences and the existence of open channels or pores for certain substances, these molecules do not escape or enter, unless certain conditions are needed or changed.

The reason for this phenomenon is that there is an electrochemical equilibrium that causes the differences in concentration across the membranes to be compensated by the electrical gradient generated by the diffusible ions and this occurs because some substances cannot get out inside the cells..

References

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