The biomembranes are structures, very dynamic and selective mainly lipid nature, part of the cells of all living beings. In essence, they are responsible for establishing the boundaries between life and the extracellular space, in addition to deciding in a controlled way what can enter and leave the cell.
The properties of the membrane (such as fluidity and permeability) are directly determined by the type of lipid, the saturation and length of these molecules. Each type of cell has a membrane with a characteristic composition of lipids, proteins and carbohydrates, which allows it to carry out its functions.
The currently accepted model for describing the structure of biological membranes is called “fluid mosaic”. It was developed in 1972 by researchers S. Jon Singer and Garth Nicolson.
A mosaic is the union of different heterogeneous elements. In the case of membranes, these elements comprise different types of lipids and proteins. These components are not static: on the contrary, the membrane is characterized by being extremely dynamic, where lipids and proteins are in constant motion. ‘
In some cases we can find carbohydrates anchored to some proteins or to the lipids that make up the membrane. Next we will explore the main components of membranes.
Lipids are biological polymers made up of carbon chains, whose main characteristic is insolubility in water. Although they fulfill multiple biological functions, the most outstanding is their structural role in membranes.
The lipids that are capable of forming biological membranes are composed of an apolar portion (insoluble in water) and a polar portion (soluble in water). These types of molecules are known as amphipathic. These molecules are phospholipids.
How do lipids behave in water?
When phospholipids come into contact with water, the polar portion is the one that actually comes into contact with it. In contrast, the hydrophobic “tails” interact with each other, trying to escape the liquid. In solution, lipids can acquire two patterns of organization: micelles or lipid bilayers.
Micelles are small aggregates of lipids, where the polar heads are grouped “looking” at the water and the tails are grouped together inside the sphere. The bilayers, as their name implies, are two layers of phospholipids where the heads face the water, and the tails of each of the layers interact with each other.
These formations occur spontaneously. That is, no energy is needed to drive the formation of micelles or bilayers.
This amphipathic property is, without a doubt, the most important of certain lipids, since it allowed the compartmentalization of life.
Not all membranes are the same
In terms of their lipid composition, not all biological membranes are the same. These vary in terms of the length of the carbon chain and the saturation between them.
By saturation we mean the number of bonds that exist between the carbons. When there are double or triple bonds, the chain is unsaturated.
The lipid composition of the membrane will determine its properties, particularly its fluidity. When there are double or triple bonds, the carbon chains “twist”, creating spaces and decreasing the packing of lipid tails.
The kinks reduce the contact surface with neighboring tails (specifically the van der Waals interaction forces), weakening the barrier.
In contrast, when chain saturation is increased, van der Waals interactions are much stronger, increasing the density and strength of the membrane. In the same way, the strength of the barrier can increase if the hydrocarbon chain increases in length.
Cholesterol is another type of lipid formed by the fusion of four rings. The presence of this molecule also helps to modulate the fluidity and permeability of the membrane. These properties can also be affected by external variables, such as temperature.
In a normal cell, slightly less than half the composition of the membrane is proteins. These can be found embedded in the lipid matrix in multiple ways: fully immersed, that is, integral; or peripherally, where only a portion of the protein is anchored to lipids.
Proteins are used by some molecules as channels or transporters (of the active or passive pathway) to help large, hydrophilic molecules cross the selective barrier. The most striking example is the protein that works as a sodium-potassium pump.
Carbohydrates can be attached to the two molecules mentioned above. They are generally found surrounding the cell and play a role in general cellular marking, recognition, and communication.
For example, cells of the immune system use this type of marking to differentiate what is their own from what is foreign, and thus know which cell should be attacked and which should not.
How are the limits of life established? Through biomembranes. Membranes of biological origin are responsible for delimiting the cellular space in all forms of life. This compartmentalization property is essential for the generation of living systems.
In this way, a different environment can be created inside the cell, with the necessary concentrations and movements of materials that are optimal for organic beings.
Additionally, biological membranes also establish limits within the cell, originating the typical compartments of eukaryotic cells: mitochondria, chloroplasts, vacuoles, etc.
Living cells require constant input and output of certain elements, for example ion exchange with the extracellular environment and excretion of waste substances, among others.
The nature of the membrane makes it permeable to certain substances and impervious to others. For this reason, the membrane, together with the proteins within it, act as a kind of molecular “gatekeeper” that orchestrates the exchange of materials with the environment.
Small molecules, which are not polar, can cross the membrane without any problem. In contrast, the larger the molecule and the more polar it is, the difficulty of the step increases proportionally.
To give a specific example, an oxygen molecule can travel through a biological membrane a billion times faster than a chloride ion.
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- Stein, W. (2012). The movement of molecules across cell membranes . Elsevier.