LIPID BILAYER: CHARACTERISTICS, STRUCTURE, FUNCTIONS - BIOLOGY - 2024

The lipid bilayer is a thin, bimolecular, laminar membrane of amphipathic lipids, that is, they contain a hydrophobic portion and another hydrophilic portion. It has a very low permeability for ions, as well as for most of the molecules soluble in water, however it is very permeable to water.

In aqueous solutions, polar lipids, such as phosphoglycerides, associate to form different types of aggregates, called micelles, lipid monolayers and bilayers. In these structures, the heads of the polar lipids, which are hydrophilic, are directed externally to be in contact with water, while the tails (hydrophobic) are all arranged at the opposite end.

Diagram of possible lipid arrangements at the edge of a pore through a lipid bilayer. Taken and edited from: MDougM.

Living beings have cell membranes made up mainly of phospholipids and glycolipids, forming a lipid bilayer. This bilayer constitutes a permeability barrier that allows regulating the internal content of salts and electrolytes of the cell. In order to achieve this, they have structures called ion pumps.

The first scientists to propose the lipid bilayer model for cell membranes were Drs. Evert Gorter and F. Grendel (1925), from Leiden University, Holland, a model that was confirmed in 1950 by electron microscopy studies.

There are several current and potential uses of lipid bilayers, but to date, the most successful commercially speaking has been the use of artificial vesicles (liposomes) in medicine, for the administration of drugs to cancer patients.

characteristics

Lipid bilayers are very thin and fragile lamellar structures that present some biologically important characteristics such as:

Permeability

One of the main characteristics of the lipid bilayer is its selective permeability. Indeed, these membranes are very impervious to ions and most polar molecules, with water being an important exception, as it can easily pass through the membrane.

An example of this selective permeability is sodium and potassium, whose ions cross the membrane more than a million times slower than water. On the other hand, indole, a heterocyclic organic compound, crosses the membrane at a speed a thousand times higher than tryptophan, another molecule similar to this structurally.

Even before knowing the double nature of the membrane, the scientist Charles Overton pointed out (1901) that the permeability coefficients of small molecules is directly related to the relative solubility that they present in organic solvents and in water.

Asymmetry

Each of the layers that make up the membrane is structurally and functionally different from the other. A functional example of this asymmetry is the sodium-potassium pump. This pump is present in the plasma membrane of the vast majority of cells of higher organisms.

The Na ⁺ - K ⁺ pump is oriented in such a way that it expels Na ⁺ from inside the cell, while introducing the K ⁺ ions. Additionally, this transport medium needs energy in the form of ATP for its activation and can only be used if it is inside the cell.

The components of each layer are also different, membrane proteins are synthesized and inserted asymmetrically into the bilayer, like lipids, but the latter, unlike proteins, do not present an absolute asymmetry, with the exception of glycolipids.

In the case of erythrocytes, for example, sphingomyelins and phosphatidylcholines are located in the outer layer of the membrane, while phosphatidylethanolamine and phosphatidylserines are internally positioned. Cholesterol is a constituent, however, of both layers.

One of the causes of the asymmetry in the distribution of phospholipids is that most of these constituents are synthesized within the cell and therefore are initially included in the inner layer, and from there some of them will migrate to the outer layer with the help of enzymes called flipases.

Fluency

Lipid bilayers are not rigid structures, but on the contrary, they are fluid and dynamic structures, where lipids and many proteins are constantly moving laterally.

Lipids diffuse laterally into the membrane at an average rate of 2 µm per second. The lateral displacement of the proteins in the bilayers, on the other hand, can vary depending on the type of protein; while some are as fast as lipids, others remain practically immobile.

Transverse diffusion, also called flip-flop, on the other hand, is much slower for lipids, and has never been observed in proteins.

On the other hand, the fluidity of the membrane can vary depending on the relative ordering of the fatty acids of the lipids. When all fatty acids are ordered, the bilayer is in a rigid state, while in the fluid state they are relatively disordered.

These changes may be due to variations in temperature; the transition from solid state to fluid state occurs abruptly when the temperature exceeds a threshold known as the melting temperature, which depends on the length of the fatty acid chains, as well as their degree of unsaturation.

The constituent lipids of the membrane are of different nature and, therefore, can have different melting temperatures. Due to this, at different temperatures solid and fluid phases can coexist in the same bilayer.

Other features

Lipid bilayers, thanks to covalent interactions and attractive van der Waals forces, have a tendency to be extensive, as well as to close in on themselves so that there are no exposed ends. Its ability to self-repair is also characteristic, because a lack of continuity is not energetically favorable to its structure.

Structure

There are different models to explain the structure of the lipid bilayer:

Davson and Danielli model

It was proposed in 1935, and it maintains that membranes contain a continuous hydrocarbon phase, contributed by the lipids that make up the membrane.

Davson and Danielli cell membrane model. Taken and edited from: miguelferig.

Unit membrane model

Erected by JD Robertson, this hypothesis is a modification of the Davson and Danielli model. He postulated that the unit membrane was made up of a double layer of mixed polar lipids.

These lipids were oriented with the hydrocarbon chains inward, forming a continuous layer of hydrocarbon, while the hydrophilic heads pointed in the opposite direction.

Additionally, this unitary membrane was covered on both sides by a single layer of protein molecules arranged in an extended manner.

Globular model

Also known as the subunit model. According to this model, the membranes would be constituted by a mosaic of repetitive lipoprotein subunits between 4.0 and 9.0 nm.

Fluid mosaic pattern

It was proposed by SJ Singer and GL Nicholson in 1972 and is the most accepted model. According to it, the phospholipids of the membrane are arranged in double layers, forming a matrix of liquid crystals.

According to this model, individual lipid molecules can move freely laterally, which would explain the flexibility, fluidity, electrical resistance and selective permeability that these membranes present.

The proteins that are part of the bilayer, according to the model, must be globular. Additionally, some proteins would be partially embedded in the bilayer, while others would be totally embedded in it.

The degree of penetration of globular proteins into the bilayer would be defined by their amino acid sequence, as well as by the presence of nonpolar R groups on the surface of these amino acids.

Composition

Natural bilayers are composed mainly of phospholipids. These are compounds derived from glycerol that are characterized by having a hydrophilic head and two hydrophobic tails.

When phospholipids come into contact with water they can organize in different ways. The most stable form is as a bilayer with the tails facing inward and the heads facing the outside of the bilayer.

Glycolipids are also part of the lipid bilayer. These compounds, as their name indicates, are lipids associated with sugars, derived in the case of animals from a compound known as sphingocin.

Another important constituent of the membrane is cholesterol, an unsaponifiable lipid. It is present in both the inner and outer layers of the bilayer. It is more abundant in the plasma membrane than in the membrane of the organelles.

Membranes are also associated with many kinds of proteins, which can be of two types, extrinsic or intrinsic. Extrinsic or peripheral proteins are loosely bound to the membrane and can be easily separated from them.

The intrinsic or integral proteins are strongly associated with the bilayer and do not detach from it easily. They represent about 70% of membrane proteins. Some of them function as receptors for signals from outside the cell and their transmission to the interior.

Other proteins are associated with the fusion of two different bilayers. Among them are those that allow the union of the sperm with the ovum during fertilization; also those that allow viruses to penetrate host cells.

Additionally, ion pumps are integral proteins that cross the bilayer that allow the exchange of ions between the interior and the exterior of the cell, through the lipid bilayer, against a gradient.

Features

The main biological function of the lipid bilayer is the separation of aqueous compartments with different compositions, such as separating the cellular protoplasm from its environment. Without this physical delimitation between compartments, life as we know it would be impossible.

This function is so important that practically all living beings have a membrane made up of a lipid bilayer. The exception is represented by some species of archaea, in which the membrane is a lipid monolayer.

Lipid bilayers participate in the transmission of the interneuronal nerve impulse. Neurons are not physically attached to each other, but separated by a short space called a synapse. Neurotransmitter loaded vesicles intervene to bridge this interneuronal space.

Another function of the bilayer is to serve as a structural base or support skeleton, to which some transport systems as well as some enzymes have been strongly attached.

Organelles with lipid bilayer

In prokaryotes, the lipid bilayer is present only in the cell membrane, while in Eukaryotes there are different organelles or organelles that may have one or two lipid bilayers.

-Organelles with two lipid bilayers

Core

Cellular organelle present in eukaryotic cells and containing most of the genetic material organized in chromosomes.

The nuclear membrane is made up of two lipid bilayers separated by a space called perinuclear. Both layers are called the outer and inner nuclear membrane and are differentiated by their protein composition.

Mitochondria

Organelle in charge of cellular respiration, a process by which the energy necessary for cellular activity is supplied. It has a double membrane, the outer smooth and the inner folded, forming laminar or finger-like chalk.

The function of such folds is to increase the internal surface area, which is the place where metabolic reactions occur.

Mitochondria. Taken and edited from: LadyofHats.

Chloroplast

Organelle present in higher plants and other photoautotrophic eukaryotic organisms. It has two concentric lipid bilayers separated by an intermembrane space. The outer layer is more porous than the inner layer due to the presence of proteins called porins.

-Organelles with a lipid bilayer

Aside from the plasma membrane, which has been discussed extensively in this article, other organelles, such as the endoplasmic reticulum, the Golgi apparatus, and lysosomes have a single lipid bilayer.

Endoplasmic reticulum (ER)

Cytoplasmic membrane complex associated (rough ER) or not (smooth ER) with ribosomes, and that participates in the synthesis of lipids and phospholipids (smooth ER) or of peptides and proteins (rough ER), thanks to the ribosomes attached to their walls.

Golgi apparatus

A complex of smooth-walled membranes involved in the storage, modification, and packaging of protein substances.

Lysosomes

Vesicular organelles that contain enzymes involved in the degradation of foreign materials. They also degrade unnecessary cellular components and even damaged or dead cells.

Applications

The main application of lipid bilayers is in the field of medicine. Liposomes are vesicular structures delimited by lipid bilayers. They are artificially formed by sonic oscillation of aqueous phosphoglyceride suspensions.

If ions or molecules are included in the aqueous suspension, then some of these elements will be contained within the liposomes. Based on these principles, drugs have been encapsulated in solution within liposomes.

The drug-containing liposomes are delivered injected into the patient. Once inside they travel through the blood system, until they reach the target place. In the place of destination they break and release their content.

The use of lipid bilayers as biosensors for disease diagnosis has also been tested; as well as for the possible detection of biological weapons. Lastly, it has been successfully tested for drug permeability.

References

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7. MS Bretscher (1972). Asymmetrical Lipid Bilayer Structure for Biological Membranes. Nature New Biology.