Lipid bilayer
The lipid bilayer is a thin polar membrane made up of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells and their structures. The cell membranes of all cellular organisms and some viruses are composed of a lipid bilayer, as are the membranes that surround the cell nucleus and other subcellular structures. The lipid bilayer is the barrier that keeps ions, proteins, and other molecules where they are needed, preventing their dispersion. They are nanometers thick, are impermeable to most water-soluble molecules (hydrophilic molecules), are impermeable to ions allowing cells to regulate salt and pH concentrations by transporting ions across their protein membranes called ion pumps or ion channels.
Biological bilayers are composed of amphiphilic phospholipids, have a hydrophilic phosphate head and a hydrophobic tail consisting of two fatty acid chains. Phospholipids with certain groups on their head can alter the chemical surface of a bilayer and can serve as "anchor" for other molecules in cell membranes.
As well as the heads, the tails of lipids can also affect the properties of the membrane that determine the phase of the bilayer. The bilayer can adopt a solid gel phase state at low temperatures, but can undergo a phase transition from a fluid state at higher temperatures, the chemical properties of the lipid tails influencing at which temperature this occurs. The packing of lipids within the bilayer affects its mechanical properties, including resistance to stretching and bending. Many of the properties have been studied with artificial "model" produced in a laboratory. Vesicles made by artificial bilayers are also used clinically to deliver drugs.
Biological membranes include several types of molecules other than phospholipids, a particularly important example in animal cells is cholesterol, as it helps to strengthen the bilayer by decreasing its permeability and regulating the activity of certain integral membrane proteins. Integral membrane proteins function when incorporated into a lipid bilayer, and are tightly attached to the lipid bilayer with the help of an annular lipid shell. Because the bilayers define the boundaries of the cell and its compartments, these membrane proteins are involved in many intracellular and intercellular signaling processes. Certain types of membrane proteins are involved in bilayer fusion processes. This fusion allows the union of two different structures as in the fertilization of an egg by sperm or the entry of a virus into a cell. Because lipid bilayers are quite fragile and invisible under a traditional microscope, they are difficult to study, which is why experiments require advanced techniques such as electron microscopy and atomic force microscopy.
Structure and organization
When phospholipids are exposed to water, they self-assemble into a two-layer sheet with the hydrophobic tails pointing toward the center of the sheet. This arrangement results in two "flakes" which are a single molecular layer, the center of this bilayer contains almost no water and excludes molecules such as sugars or salts that dissolve in water. The assembly process is driven by the interactions between the hydrophobic molecules (also called the hydrophobic effect), an increase in the interactions between the hydrophobic molecules (causing the clustering of hydrophobic regions) allows the water molecules to bind more freely with each other, increasing the entropy of the system. This complex process includes non-covalent interactions, such as Van der Waals, electrostatic bonding, and hydrogen bonding.
Cross Section Analysis
The lipid bilayer is very thin compared to its lateral dimensions, if a typical mammalian cell (diameter ~10 micrometers) is magnified to the size of a watermelon (~1 ft/30 cm), it forms the plasma membrane about the size of a sheet of office paper. Despite being only a few nanometers thick, the bilayer is composed of chemical regions across its cross section, these regions and their interactions with the surrounding water are characterized with X-ray reflectometry, neutron scattering, and imaging techniques. nuclear magnetic resonance.
The first region on both sides of the bilayer is the hydrophilic head group, it is fully hydrated and is typically around 0.8-0.9 nm thick. In phospholipid bilayers, the phosphate group lies within the hydrated region, approximately 0.5 nm outside of the hydrophobic core. In some cases, the hydrated region can extend much further, for example, in lipids with a large protein or a long sugar chain grafted onto the head. A common example of such a modification in nature is the lipopolysaccharide layer on a bacterial outer membrane, which helps retain a layer of water around the bacteria to prevent dehydration.
Next to the hydrated region is a partially hydrated intermediate region, this boundary layer is about 0.3 nm thick. Within this short distance, the concentration of water droplets drops from 2M on the head group side to almost zero on the tail side (intermediate region).
. The hydrophobic middle region of the bilayer is typically 3-4 nm thick, but this value varies with chain length and chemistry. The middle region thickness also varies significantly with temperature, particularly in a phase transition.
Asymmetry
Most naturally occurring bilayers have different compositions of the inner and outer membrane layers. In human red blood cells, the inner (cytoplasmic) layer is composed primarily of phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol and their phosphorylated derivatives. In contrast, the outer (extracellular) layer is based on phosphatidylcholine, sphingomyelin, and glycolipids. In some cases, the asymmetry is based on where the lipids are created in the cell and reflects their initial orientation. lipid asymmetry are imperfectly differentiated, although it is clear that they are used in different situations. For example, when a cell undergoes apoptosis, phosphatidylserine located in the cytoplasmic layer is transferred to the outer surface: There, it is recognized by a macrophage which then actively neutralizes the dying cell.
Lipid asymmetry arises from the fact that most phospholipids are synthesized and initially inserted into the inner monolayer: those that make up the outer monolayer are subsequently transported out of the inner monolayer by a class of enzymes called flippases. Others lipids, such as sphingomyelin, appear to be synthesized in the outer layer. Flippases are members of a family of large lipid-transporting molecules that includes flopases, which transfer lipid in the opposite direction, and scramblases, which randomize the distribution of lipids across lipid bilayers (as in apoptotic cells). In either case, once lipid asymmetry is established, it does not normally dissipate quickly because the spontaneous flip-flop process of lipids between sheets is extremely slow.
It is possible to mimic asymmetry in the laboratory in two-layer model systems. Certain types of very small artificial vesicles automatically become slightly asymmetric, although the mechanism by which this asymmetry is generated is very different from that of cells.
By using two different monolayers in Langmuir-Blodgett deposition or a combination of Langmuir-Blodgett and vesicle rupture deposition it is also possible to synthesize an asymmetric planar bilayer. This asymmetry can be lost over time as the lipids in the supported bilayers can be prone to the flip-flop process.
Phases and phase transitions
At a given temperature a lipid bilayer can exist in liquid form or a gel (solid) phase. All lipids have a characteristic temperature at which they transform (melt) from a gel to a liquid phase. In both phases lipid molecules are prevented from flip-flopping across the bilayer, but in liquid phase bilayers a given lipid will exchange places with its neighbor millions of times per second. This random walk exchange allows lipids to diffuse and thus wander across the surface of the membrane. Unlike liquid-phase bilayers, lipids in a gel-phase bilayer are locked in place.
The phase behavior of lipid bilayers is largely determined by the strength of attractive Van der Waals interactions between adjacent lipid molecules. Lipids with longer tails have more area on which to interact, increasing the strength of this interaction and, as a consequence, decreasing lipid mobility. Therefore, at a given temperature, a short-tailed lipid will be more fluid than an identical long-tailed lipid.
The transition temperature can also be affected by the degree of unsaturation of the lipid tails. An unsaturated double bond can cause a narrowing in the alkane chain, which disrupts lipid packing. This disruption creates extra free space within the bilayer that allows for greater flexibility in adjacent strands. An example of this effect can be seen in everyday life as butter, which has a large percentage of saturated fat, is solid at room temperature. environment, while vegetable oil, which is mostly unsaturated, is liquid.
Most natural membranes are a complex mixture of different lipid molecules. If some of the components are liquid at a given temperature, while others are in the gel phase, the two phases can coexist in separate regions in space, like an iceberg floating in the ocean. This phase separation plays a fundamental role in biochemical phenomena because membrane components such as proteins can partition into one phase or another and are therefore locally concentrated or activated. A particularly important component of many mixed-phase systems is cholesterol, which modulates bilayer permeability, mechanical strength, and biochemical interactions.
Surface Chemistry
While the lipid tails primarily modulate the phase behavior of the bilayer, it is the head group that determines the chemistry of the bilayer surface. Most natural bilayers are composed primarily of phospholipids, although sphingolipids such as sphingomyelin and sterols such as cholesterol are also important components. Of the phospholipids, the most common headgroup is phosphatidylcholine (PC), which accounts for about half of the phospholipids in most mammalian cells. PC is a zwitterionic headgroup, as it has a negative charge on the phosphate group and a positive charge on the amine, but, due to local charge balance, no net charge.
Other head groups are also present to varying degrees and may include phosphatidylserine (PS), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG). These alternate headgroups often confer specific biological functionality that is highly context dependent. For example, the presence of PS on the extracellular membrane side of erythrocytes is a marker of apoptosis, whereas PS in growth plate vesicles is required for hydroxyapatite crystal nucleation and subsequent bone mineralization. Unlike PC, some of the other headgroups carry a net charge, which can alter the electrostatic interactions of the small molecules with the bilayer.
Biological roles
Containment and separation
The main function of the lipid bilayer in biology is to separate aqueous compartments from their surroundings. Without some form of delineation between "myself" and "something else", it is difficult to define even the concept of an organism or life. This barrier takes the form of a lipid bilayer in all known life forms except for a few species of archaea that use a specially adapted lipid monolayer. It has even been proposed that the first life form may have been a simple lipid vesicle. its biosynthetic capacity being the production of more phospholipids. The partitioning capacity of the lipid bilayer is based on the fact that hydrophilic molecules cannot easily cross the core of the hydrophobic bilayer, as discussed in transport across the lipid bilayer. bilayer below. The nucleus, mitochondria, and chloroplasts have two lipid bilayers, while other subcellular structures are surrounded by a single lipid bilayer (such as the plasma membrane, endoplasmic reticulum, Golgi apparatus, and lysosomes). See Organelles.
Prokaryotes have only one lipid bilayer - the cell membrane (also known as the plasma membrane). Many prokaryotes also have a cell wall, but the cell wall is made up of proteins or long-chain carbohydrates, not lipids. In contrast, eukaryotes have a range of organelles, including the nucleus, mitochondria, lysosomes, and endoplasmic reticulum. All of these subcellular compartments are surrounded by one or more lipid bilayers and, together, typically comprise the majority of the bilayer zone present in the cell. In liver hepatocytes, for example, the plasma membrane accounts for only two percent of the cell's total two-layer surface area, while the endoplasmic reticulum contains more than fifty percent and the mitochondria thirty percent more.
Signage
Probably the most familiar form of cell signaling is synaptic transmission, in which a nerve impulse that has reached the end of one neuron is transported to an adjacent neuron through the release of neurotransmitters. This transmission is made possible by the action of synaptic vesicles loaded with the neurotransmitters that are released. These vesicles fuse with the cell membrane at the pre-synaptic terminal and release their contents to the outside of the cell. The content then diffuses across the synapse to the post-synaptic terminal.
Lipid bilayers are also involved in signal transduction through their role as the home for integral membrane proteins. This is an extremely large and important class of biomolecule. It is estimated that up to a third of the human proteome may be membrane proteins. Some of these proteins are related to the exterior of the cell membrane. An example of this is the CD59 protein, which identifies cells as "self" and therefore inhibits its destruction by the immune system. The HIV virus evades the immune system, in part, by grafting these host membrane proteins onto its own surface. Alternatively, some membrane proteins penetrate all the way through the bilayer, and serve to transmit host membrane events. individual signal from the outside to the inside of the cell. The most common class of this type of protein is the G protein-coupled receptor (GPCR). GPCRs are responsible for much of the cell's ability to perceive its environment, and because of this important function, approximately 40% of all modern drugs are targeted at GPCRs.
In addition to protein and solution-mediated processes, it is also possible that lipid bilayers are directly involved in signaling. A classic example of this is alarm phosphatidylserine phagocytosis. Normally, phosphatidylserine is distributed asymmetrically in the cell membrane and is present only on the inner side. During programmed cell death, a protein called scramblase balances this distribution, phosphatidylserine is present on the bilayer extracellular face. The presence of phosphatidylserine then triggers phagocytosis to remove dead or dying cells.
Characterization methods
The lipid bilayer is a very difficult structure to study because it is so thin and fragile. Despite these limitations, dozens of techniques have been developed over the past seventy years to allow investigation of their structure and function.
Electrical measurements are a direct way to characterize an important function of a bilayer: its ability to separate and prevent the flow of ions in solution. By applying a voltage across the bilayer and measuring the resulting current, the resistance of the bilayer is determined. This resistance is usually quite high (108 Ohm-cm^2 or more) since the hydrophobic core is impermeable to charged species. The presence of even a few nanometer-scale holes results in a dramatic increase in current. The sensitivity of this system is such that even individual ion channel activity can be resolved.
Electrical measurements do not provide a true image like imaging with a microscope can. Lipid bilayers cannot be seen under a traditional microscope, as they are too thin. In order to observe the bilayers, researchers often use fluorescence microscopy. A sample is excited with one wavelength of light and observed at a different wavelength, so only fluorescent molecules with a matching excitation and emission profile will be seen. Natural lipid bilayers do not fluoresce, so a dye is used that binds to the desired molecules in the bilayer. Resolution is generally limited to a few hundred nanometers, much smaller than a typical cell but much larger than the thickness of a lipid bilayer.
Electron microscopy provides a higher resolution image. In an electron microscope, a beam of focused electrons interacts with the sample instead of a beam of light as in traditional microscopy. In combination with quick freezing techniques, electron microscopy has also been used to study the mechanisms of inter- and intracellular transport, for example in demonstrating that exocytotic vesicles are the means of chemical release at the synapse.
31P-NMR (nuclear magnetic resonance) spectroscopy is widely used for studies of phospholipid bilayers and biological membranes under native conditions.
31P-NMR analysis of lipids could provide a wide range of information about lipid bilayer packing, phase transitions (gel phase, physiological liquid crystal phase, the rippling phases, non-bilayer phases), lipid headgroup orientation/dynamics, and the elastic properties of the pure lipid bilayer and as a result of binding of proteins and other biomolecules.
A new method for studying lipid bilayers is atomic force microscopy (AFM). Instead of using a beam of light or particles, a small sharp point explores the surface by making physical contact with the bilayer and moving across it, like a needle player. AFM is a promising technique, as it has the potential to image with nanometer resolution at room temperature and even under water or physiological buffer, conditions necessary for natural bilayer behavior. Using this capability, AFM has been used to examine the dynamic behavior of the bilayer including the formation of transmembrane pores (holes) and phase transitions in supported bilayers. Another advantage is that AFM does not require fluorescent or isotopic labeling. lipids, since the probe tip mechanically interacts with the bilayer surface. Because of this, the same image can scan both lipids and associated proteins, sometimes even down to single-molecule resolution. AFM can also probe the natural mechanics of lipid bilayers.
Lipid bilayers exhibit high levels of birefringence in which the refractive index in the plane of the bilayer differs from the perpendicular by 0.1 refractive index units. This has been used to characterize the degree of order and disruption in bilayers using dual polarization interferometry to understand the mechanisms of protein interaction.
Lipid bilayers are complicated molecular systems with many degrees of freedom. Thus the atomistic simulation of the membrane and, in particular, the ab initio calculations of its properties is difficult and computationally expensive. Quantum chemistry calculations have recently been successfully performed to estimate dipole and quadrupole moments of lipid membranes.
Transport through the bilayer
Passive diffusion
Most polar molecules have low solubility in the hydrocarbon core of a lipid bilayer and, as a consequence, have low coefficients of permeability through the bilayer. This effect is particularly pronounced for charged species, which have even lower permeability coefficients than polar neutral molecules. Anions typically have a greater diffusion rate through bilayers than cations.
Compared to ions, water molecules actually have a relatively large permeability through the bilayer, as evidenced by osmotic swelling. When a cell or vesicle with a high concentration of salt inside is placed in a solution with a low concentration of salt it will swell and eventually burst. Such a result will not be observed unless the water is able to pass through the bilayer with relative ease. The anomalously large permeability of water through bilayers is still not fully understood and remains the subject of active debate.
Small uncharged nonpolar molecules diffuse through lipid bilayers many orders of magnitude faster than ions or water. This applies to both fats and organic solvents like chloroform and ether. Regardless of their polar character, larger molecules diffuse more slowly through lipid bilayers than small molecules.
Ion Channels and Pumps
Two special classes of proteins with the ionic gradients found across cell and subcellular membranes in nature's ion channels and ion pumps. Both pumps and channels are integral membrane proteins that pass through the bilayer, but their functions are very different. Ion pumps are the proteins that build and maintain chemical gradients by utilizing an external energy source to move ions against the concentration gradient to an area of higher chemical potential. The energy source can be from ATP, as is the case with Na + -K + ATPase. Alternatively, the energy source may be another chemical gradient already in place, as in the Ca2+/Na+ antiporter. It is through the action of ion pumps that cells are able to regulate pH by pumping protons.
In contrast to ion pumps, ion channels do not build chemical gradients, but rather they dissipate in order to do work or send a signal. Probably the best-known and best-studied example is the voltage-gated Na+ channel, which allows action potential conduction along neurons. All ion bombs have some sort of 'gating' mechanism. In the example above it was electrical polarization, but other channels can be activated by the binding of a molecular agonist or through a conformational change in another nearby protein.
Endocytosis and exocytosis
Some molecules or particles are too large or too hydrophilic to pass through a lipid bilayer. Other molecules could pass through the bilayer, but they must be transported rapidly in such large numbers that channel-type transport is impractical. In both cases, these types of cargo can move across the cell membrane by means of fusion or budding of vesicles. When a vesicle is produced within the cell and fuses with the plasma membrane to release its contents into the extracellular space, this process is known as exocytosis. In the reverse process, a region of the cell membrane will be dimpled inward and eventually punctured, entrapping some of the extracellular fluid for transport into the cell. Endocytosis and exocytosis rely on very different molecular machinery to function, but the two processes are intimately linked and could not work without each other. The primary mechanism of this interdependence is the large amount of lipid material involved. In a typical cell, the area of the bilayer equivalent to the entire plasma membrane will travel through the endocytosis/exocytosis cycle in half an hour. If these two processes do not balance each other, the cell would balloon outward to an unwieldy size or completely deplete its plasma membrane in a matter of minutes.
Exocytosis in prokaryotes: Vesicular membrane exocytosis, popularly known as membrane vesicle trafficking, winner of the Nobel Prize (year 2013) is traditionally considered a prerogative of eukaryotic cells. However, this myth was broken with the revelation that nanovesicles, popularly known as bacterial outer membrane vesicles, released by gram-negative microorganisms, carry bacterial signal molecules to host or target cells. to carry out multiple processes in favor of the secreting microbe, for example, in the invasion of the host cell and in microbe-environment interactions, in general.
Electroporation
Electroporation is the rapid increase in permeability in the bilayer induced by the application of a large artificial electric field across the membrane. Experimentally, electroporation is used to introduce hydrophilic molecules into cells. It is a particularly useful technique for large, highly charged molecules such as DNA, which would never passively diffuse through the core of the hydrophobic bilayer. Because of this, electroporation is one of the main methods of transfection, as well as bacterial transformation.. It has even been proposed that electroporation resulting from electrical storms could be a natural mechanism of horizontal gene transfer.
This increased permeability primarily affects the transport of ions and other hydrated species, indicating that the mechanism is the creation of nm-scale water-filled holes in the membrane. Although electroporation and dielectric breakdown result from the application of an electric field, the mechanisms involved are fundamentally different. In dielectric breakdown, the barrier material is ionized, creating a conductive pathway. The material alteration is therefore chemical in nature. In contrast, during electroporation the lipid molecules are not chemically altered but simply shift position, opening a pore that acts as the conductive pathway through the bilayer as it fills with water.
Mechanics
Lipidic bicapas are quite large structures to have some of the mechanical properties of liquids or solids. The Ka area compression module, Kb bending module, and edge energy .... {displaystyle Lambda }, can be used to describe them. Solid lipid bilayers also have a cutting module, but like any liquid, the cutting module is zero for fluid bicapas. These mechanical properties affect the operation of the membrane. Ka and Kb affect the capacity of small proteins and molecules to insert into the bicapa, and the mechanical properties of the bicapa have been shown to alter the function of mechanically activated ionic channels. Bicapa mechanical properties also establish which types of stress a cell can withstand without breaking. Although lipid bicaps can be folded easily, most can't stretch more than a small as much percent before breaking up.
As discussed in the Structure and Organization section, the hydrophobic attraction of lipid tails in water is the primary force holding lipid bilayers together. Therefore, the elastic modulus of the bilayer is primarily determined by the amount of additional area that is exposed to water in which the lipid molecules are stretched apart. Not surprisingly, given this understanding of the forces involved that studies have shown that Ka varies strongly with osmotic pressure but weakly with tail length and unsaturation. Because the forces involved are so small, Ka is difficult to determine experimentally. Most microscopy techniques require sophisticated, highly sensitive measurement equipment.
In contrast to Ka, which is a measure of the amount of energy it takes to stretch the bilayer, Kb is a measure of the amount of energy it takes to bend or flex the bilayer. Formally, a bending modulus is defined as the energy required to deform a membrane from its intrinsic curvature to some other curvature. The intrinsic curvature is defined by the ratio of the diameter of the head group to that of the tail group. For two-tailed PC lipids, this ratio is close to one, so the intrinsic curvature is close to zero. If a particular lipid has too large a deviation from intrinsic curvature from zero, a bilayer will not form and other phases such as micelles or inverted micelles will form instead. The addition of small hydrophilic molecules such as sucrose into mixed-lipid lamellar liposomes based on thylakoid- and galactolipid-rich membranes destabilize the bilayers into a micellar phase. Typically Kb it is not measured experimentally but is calculated from measurements of Ka and the thickness of the bilayer, since the three parameters are related.
.... {displaystyle Lambda } is a measure of the amount of energy that is needed to expose a bicapa edge to water by scraping the bicapa or creating a hole in it. The origin of this energy is the fact that, when creating such an interface, it exposes some of the lipid tails to the water, but the exact orientation of these lipids of the edge is unknown. There is some evidence that so many hydrophobic pores (right wings) and hydrophilic (curved heads around) can coexist.
Fusion
Fusion is the process by which two lipid bilayers fuse together, resulting in a connected structure. If this fusion proceeds completely through both sheets of both bilayers, a water-filled bridge is formed and the solutions contained by the bilayers can mix. Alternatively, if only one sheet from each bilayer is involved in the fusion process, the bilayers are said to be hemifused. Fusion is involved in many cellular processes, particularly in eukaryotes, since the eukaryotic cell is extensively subdivided by membrane lipid bilayers. Exocytosis, fertilization of an egg by sperm, and transport of waste products to the lysosome are some of the many eukaryotic processes that depend on some form of fusion. Even pathogen entry can be governed by fusion, as many bilayer-coated viruses have dedicated fusion proteins in order to gain entry into the host cell.
There are four fundamental steps in the fusion process. First, the membranes involved must aggregate, coming within several nanometers of each other. Second, the two bilayers must come into very close contact (within a few angstroms). To achieve this contact, the two surfaces must become at least partially dehydrated, since the boundary surface water normally present causes bilayers to repel each other strongly. The presence of ions, in particular divalent cations such as magnesium and calcium, strongly affects this step. One of calcium's critical roles in the body is the regulation of membrane fusion. Third, a destabilization must form at a point between the two bilayers, distorting their structures. The exact nature of this distortion is not known. One theory is that a "stem" A highly curved lipid must form between the two bilayers. Proponents of this theory believe that this explains why phosphatidylethanolamine, a highly curved lipid, promotes fusion. Eventually, in the last stage of fusion, this dot defect grows and the components of the two bilayers mix and diffuse away from the contact site.
The situation is further complicated when considering fusion in vivo since biological fusion is almost always regulated by the action of membrane-associated proteins. The first of these proteins to be studied were viral fusion proteins, which allow an enveloped virus to insert its genetic material into the host cell (enveloped viruses are those surrounded by a lipid bilayer, and some others only have a lipid bilayer). protein). Eukaryotic cells also use fusion proteins, the best studied being the SNARE proteins that are used to direct all intracellular vesicular traffic. Despite years of study, much remains to be discovered about the function of this class of proteins. In fact, there is still an active debate as to whether SNAREs are linked to early docking or participate later in the fusion process, facilitating hemifusion.
In molecular and cell biology studies it is often desirable to artificially induce fusion. Addition of polyethylene glycol (PEG) causes fusion without significant aggregation or biochemical disruption. This procedure is now widely used, for example by fusing B cells with melanoma cells. "Hybridism" resulting from this combination expresses a desired antibody as determined by the B cells involved, but is immortalized due to the melanoma component. Fusion can also be artificially induced by means of electroporation in a process known as electrofusion. This phenomenon is believed to result from energetically active edges formed during electroporation, which may act as the local defect point for nucleating stem growth between two bilayers.
Model Systems
Lipid bilayers can be created artificially in the laboratory to allow researchers to perform experiments that cannot be done on natural bilayers. Many types of model bilayers exist, each with experimental advantages and disadvantages. They can be made with either synthetic lipids or natural lipids. Among the most common model systems are:
- Black lipidic membranes
- Lipidic sheaths supported
- Tethered lipidic bilayer membranes (tBLM)
- Movies
Commercial applications
To date, the most successful commercial application of lipid bilayers has been the use of liposomes for drug delivery, especially for the treatment of cancer. (Note-the term "liposome" is, in essence, synonymous with "vesicle", except that vesicle is a general term for the structure, whereas liposomes refer to only vesicles artificial, not natural) The basic idea of liposomal drug delivery is that the drug is encapsulated in solution within the liposome and then injected into the patient. These drug-loaded liposomes travel through the system until they bind at the target site and rupture, releasing the drug. In theory, liposomes should make an ideal drug delivery system since they can isolate almost any hydrophilic drug, can be grafted with the molecules to specific target tissues, and can be relatively non-toxic since the body possesses biochemical pathways to break down lipids..
The first generation of drug delivery liposomes had a simple lipid composition and suffered from several limitations. Circulation in the bloodstream was extremely limited due to both renal compensation and phagocytosis. Refinement of lipid composition to fluidity, hue, surface charge density, and surface hydration resulted in vesicles that absorb less serum protein and thus are less readily recognized by the immune system. The most significant advance in this area it was the grafting of polyethylene glycol (PEG) onto the surface of the liposome to produce "hidden" vesicles, which circulate for long times without immune or renal clearance.
The first hidden liposomes were passively attacked in tumor tissues. Because tumors induce rapid and uncontrolled angiogenesis they are especially "leaky" and allow the liposomes to exit the blood at a much higher rate than normal tissue would. More recently when work has been done with grafting antibodies or other molecular markers to the liposome surface in the hope that actively bind to a specific cell type or tissue. Some examples of this approach are already in clinical trials.
Another potential application of lipid bilayers is in the field of biosensors. For not only is the lipid bilayer the barrier between the inside and outside of the cell, but it is also the site of extensive signal transduction. Researchers in recent years have tried to harness this potential to develop a device based on the bilayer for clinical diagnosis or bioterrorism detection. Progress has been slow in this area, and although some companies have developed automated lipid-based detection systems, they are being targeted at the research community. These include Biacore (now GE Healthcare Life Sciences), which offers a disposable chip for the use of lipid bilayers in binding kinetics studies, and Nanion Inc., which has developed an automated patch attachment system. Other applications More exotic ones are also being pursued, such as the use of lipid bilayer membrane pores for DNA sequencing by Oxford Nanolabs. To date, this technology has not proven to be commercially viable.
A supported lipid bilayer (SLB) as described above has achieved commercial success as a screening technique for measuring drug permeability. This PAMPA parallel artificial membrane permeability assay measures permeability through the specifically formulated lipid cocktail, where a high correlation has been found with Caco-2 cultures, the gastrointestinal tract, blood-brain barrier, and the skin.
History
By the early 20th century, scientists had come to believe that cells are surrounded by a thin barrier similar to oil, but the structural nature of this membrane was not known. Two experiments in 1925 laid the groundwork to fill this gap. By measuring the capacitance of red blood cell solutions, Hugo Fricke determined that the cell membrane was 3.3 nm thick.
Although the results of this experiment were accurate, Fricke misinterpreted the data to mean that the cell membrane is a single molecular layer. Prof. Dr. Evert Gorter (1881–1954) and F. Grendel from the University of Leiden approached the problem from a different perspective, diffusion of erythrocyte lipids as a monolayer over a Langmuir-Blodgett trough. When they compared the area of the monolayer to the surface area of the cells, they found a ratio of two to one. Further analysis showed several errors and incorrect assumptions with this experiment, but, coincidentally, these errors canceled out and from these data Gorter and Grendel pointed to the correct conclusion, that the cell membrane is a lipid bilayer.
This theory was confirmed using electron microscopy in the late 1950s. Although he did not publish the first electron microscopy study of lipid bilayers, J. David Robertson was the first to state that the two dark bands of electrons high in density were the head groups and associated proteins of two opposing lipid monolayers. In this body of work, Robertson proposed the concept of the "membrane unit." This was the first time that the bilayer structure had been universally assigned to all cell membranes, as well as organelle membranes.
Around the same time, the development of model membranes confirmed that the lipid bilayer is a stable structure that can exist independently of proteins. By "painting" a solution of lipids in an organic solvent through an opening, Mueller and Rudin were able to create an artificial bilayer and determine that it exhibits lateral fluidity, high electrical resistance, and self-healing in response to perforation, all of which are properties of a natural cell membrane. A few years later, Alec Bangham showed that bilayers, in the form of lipid vesicles, could also be formed simply by exposing a dry lipid sample to water. This was an important advance, as it was shown that lipid bilayers are formed spontaneously through self-assembly and do not require a designed support structure.
See also
- Fluid mosaic model.
- Linked mosaic model.
- Coacervado.