The cell membrane (also called the plasma membrane or plasmalemma) is one biological membrane separating the interior of a cell from the outside environment.
The inner membrane is the biological membrane (phospholipid bilayer) of an organelle or Gram-negative bacteria that is within an outer membrane.
In eukaryotic cells, this inner membrane is present within the nuclear envelope, mitochondria and plastids like the chloroplast. The lumen between the inner and outer membranes is referred to as intermembrane space.
In prokaryotic cells such as many Gram-negative bacteria, the space between the inner and outer membrane is commonly referred to as the periplasmic space or periplasm. The inner membrane may also be referred to as the cytoplasmic membrane and it is similar in structure and protein content as the cytoplasmic membrane of other bacteria that contain only one membrane (such as most Gram-positive bacteria).
This structural arrangement of an inner and outer membrane is thought to be similar in Gram-negative bacteria, mitochondria and chloroplasts due to their ancestral relationship, as outlined in the endosymbiotic theory.
The inner membrane of the nuclear envelope is connected to the outer nuclear envelope membrane through nuclear pores. It contains a number of proteins involved in the structural organization of the nucleus and the attachment of chromatin to the nuclear envelope. In metazoan cells, the inner nuclear membrane contains proteins of the nuclear lamina, a protein meshwork underlying the nuclear envelope and providing structural support. Mutations in inner nuclear envelope proteins can cause nuclear envelopathies, a number of genetic disorders in humans.
The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the outer membrane of many Gram-negative bacteria includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and in some bacteria such as E. coli it is linked to the cell's peptidoglycan by Braun's lipoprotein.
Porins can be found in this layer.
If lipid A, part of the LPS, enters the circulatory system it causes a toxic reaction by activating TLR 4. Lipid A is very immunogenic and causes an aggressive response by the immune system. The sufferer will have a high temperature and respiration rate and a low blood pressure. This may lead to endotoxic shock, which may be fatal.
The biogenesis of the outer membrane requires that the individual components are transported from the site of synthesis to their final destination outside the inner membrane by crossing both hydrophilic and hydrophobic compartments. The machinery and the energy source that drive this process are not yet fully understood. The lipid A-core moiety and the O-antigen repeat units are synthesized at the cytoplasmic face of the inner membrane and are separately exported via two independent transport systems, namely, the O-antigen transporter Wzx (RfbX) and the ATP binding cassette (ABC) transporter MsbA that flips the lipid A-core moiety from the inner leaflet to the outer leaflet of the inner membrane. O-antigen repeat units are then polymerised in the periplasm by the Wzy polymerase and ligated to the lipid A-core moiety by the WaaL ligase.
The LPS transport machinery is composed of LptA, LptB, LptC, LptD, LptE. This supported by the fact, that depletion of any of one of these proteins blocks the LPS assembly pathway and results in very similar outer membrane biogenesis defects. Moreover, the location of at least one of these five proteins in every cellular compartment suggests a model for how the LPS assembly pathway is organised and ordered in space.
LptC is required for the translocation of lipopolysaccharide (LPS) from the inner membrane to the outer membrane. LptE forms a complex with LptD, which is involved in the assembly of LPS in the outer leaflet of the outer membrane and is essential for envelope biogenesis.
gr+f/gr+a (t)/gr-p (c)/gr-o
drug (J1p, w, n, m, vacc)
This article incorporates text from the public domain Pfam and InterPro IPR007485
This article incorporates text from the public domain Pfam and InterPro IPR010664
An integral membrane protein (IMP) is a protein molecule (or assembly of proteins) that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents.
IMPs comprise a very significant fraction of the proteins encoded in an organism's genome.
All transmembrane proteins are IMPs, but not all IMPs are transmembrane proteins.
Three-dimensional structures of only ~160 different integral membrane proteins are currently determined at atomic resolution by X-ray crystallography or nuclear magnetic resonance spectroscopy due to the difficulties with extraction and crystallization. In addition, structures of many water-soluble domains of IMPs are available in the Protein Data Bank. Their membrane-anchoring α-helices have been removed to facilitate the extraction and crystallization. Search integral membrane proteins in the PDB (based on gene ontology classification)
IMPs can be divided into two groups:
The most common type of IMP is the transmembrane protein (TM), which spans the entire biological membrane. Single-pass membrane proteins cross the membrane only once, while multi-pass membrane proteins weave in and out, crossing several times. Single pass TM proteins can be categorized as Type I, which are positioned such that their carboxy-terminus is towards the cytosol, or Type II, which have their amino-terminus towards the cytosol.
Integral monotopic proteins, are associated to the membrane from one side but do not span the lipid bilayer completely.
The Protein Structure Initiative (PSI), funded by the U.S. National Institute of General Medical Sciences (NIGMS), has among its aim to determine three-dimensional protein structures and to develop techniques for use in structural biology, including for membrane proteins. Homology modeling can be used to construct an atomic-resolution model of the "target" integral protein from its amino acid sequence and an experimental three-dimensional structure of a related homologous protein. This procedure has been extensively used for ligand-G protein-coupled receptors (GPCR) and their complexes.
IMPs include transporters, linkers, channels, receptors, enzymes, structural membrane-anchoring domains, proteins involved in accumulation and transduction of energy, and proteins responsible for cell adhesion. Classification of transporters can be found in Transporter Classification database.
Examples of integral membrane proteins:
An artificial cell or minimal cell is an engineered particle that mimics one or many functions of a biological cell. The term does not refer to a specific physical entity, but rather to the idea that certain functions or structures of biological cells can be replaced or supplemented with a synthetic entity. Often, artificial cells are biological or polymeric membranes which enclose biologically active materials. As such, nanoparticles, liposomes, polymersomes, microcapsules and a number of other particles have qualified as artificial cells. Micro-encapsulation allows for metabolism within the membrane, exchange of small molecules and prevention of passage of large substances across it. The main advantages of encapsulation include improved mimicry in the body, increased solubility of the cargo and decreased immune responses. Notably, artificial cells have been clinically successful in hemoperfusion.
In the area of synthetic biology, a "living" artificial cell has been defined as a completely synthetically made cell that can capture energy, maintain ion gradients, contain macromolecules as well as store information and have the ability to mutate. Such a cell is not technically feasible yet, but a variation of an artificial cell has been created in which a completely synthetic genome was introduced to genomically emptied host cells. Although not completely artificial because the cytoplasmic components as well as the membrane from the host cell are kept, the engineered cell is under control of a man-made genome and is able to replicate.
Artificial cells were first developed by Thomas Chang at McGill University in the 1960s. These first artificial cells consisted of ultrathin membranes of nylon, collodion or crosslinked protein whose semipermeable properties allowed diffusion of small molecules in and out of the cell. These early artificial cells were micron sized and contained cell, enzymes, hemoglobin, magnetic materials, adsorbents and proteins. Presently, artificial cells range from hundreds of micrometer to nanometer dimensions and can also carry microorganisms, vaccines, genes, drugs, hormones, and peptides. The first clinical use of artificial cells was in hemoperfusion by the encapsulation of activated charcoal. In the 1970s, researchers were able to introduce enzymes, proteins and hormones to biodegradable microcapsules, later leading to clinical use in diseases such as Lesch-Nyhan syndrome. Although Thomas Chang began his initial research focused on artificial red blood cells, it was not until the mid-1990s that biodegradable artificial red blood cells were developed. Most recently, artificial cells have been used extensively in biological cell encapsulation. They were first used in the clinic in 1994 for treatment in a diabetic patient and since then other types of cells such as hepatocytes, adult stem cells and genetically engineered cells have been encapsulated and are being studied for uses in tissue regeneration amongst others. On December 29, 2011, chemists at Harvard University reported the creation of an artificial cell membrane.
Membranes for artificial cells be made of simple polymers, crosslinked proteins, lipid membranes or polymer-lipid complexes. Further, membranes can be engineered to present surface proteins such as albumin, antigens, Na/K-ATPase carriers, or pores such as ion channels. Commonly used materials for the production of membranes include hydrogel polymers such as alginate, cellulose and thermoplastic polymers such as hydroxyethyl methacrylate-methyl methacrylate (HEMA- MMA), polyacrylonitrile-polyvinyl chloride (PAN-PVC), as well as variations of the above mentioned. The material used determines the permeability of the cell membrane, which for polymer depends on the molecular weight cut off (MWCO). The MWCO is the maximum molecular weight of a molecule that may freely pass through the pores and is important in determining adequate diffusion of nutrients, waste and other critical molecules. Hydrophilic polymers have the potential to be biocompatible and can be fabricated into a variety of forms which include polymer micelles, sol-gel mixtures, physical blends and crosslinked particles and nanoparticles. Of special interest are stimuli-responsive polymers that respond to pH or temperature changes for the use in targeted delivery. These polymers may be administered in the liquid form through a macroscopic injection and solidify or gel in situ because of the difference in pH or temperature. Nanoparticle and liposome preparations are also routinely used for material encapsulation and delivery. A major advantage of liposomes is their ability to fuse to cell and organelle membranes.
Many variations for artificial cell preparation and encapsulation have been developed. Typically, vesicles such as a nanoparticle, polymersome or liposome are synthesized. An emulsion is typically made through the use of high pressure equipment such as a high pressure homogenizer or a Microfluidizer. Two micro-encapsulation methods for nitrocellulose are also described below.
In a high-pressure homogenizer, two liquids in oil/liquid suspension are forced through a small orifice under very high pressure. This process shears the products and allows the creation of extremely fine particles, as small as 1 nm.
This technique uses a patented Microfluidizer to obtain a greater amount of homogenous suspensions that can create smaller particles than homogenizers. A homogenizer is first used to create a coarse suspension which is then pumped into the microfluidizer under high pressure. The flow is then split into two streams which will react at very high velocities in an interaction chamber until desired particle size is obtained. This technique allows for large scale production of phospholipid liposomes and subsequent material nanoencapsulations.
In this method, a cell solution is incorporated dropwise into a collodion solution of cellulose nitrate. As the drop travels through the collodion, it is coated with a membrane thanks to the interfacial polymerization properties of the collodion. The cell later settles into paraffin where the membrane sets and is finally suspended a saline solution. The drop method is used for the creation of large artificial cells which encapsulate biological cells, stem cells and genetically engineered stem cells.
The emulsion method differs in that the material to be encapsulated is usually smaller and is placed in the bottom of a reaction chamber where the collodion is added on top and centrifuged, or otherwise disturbed in order to create an emulsion. The encapsulated material is then dispersed and suspended in saline solution.
Artificial cells used for drug delivery differ from other artificial cells since their contents are intended to diffuse out of the membrane, or be engulfed and digested by a host target cell. Often used are submicron, lipid membrane artificial cells that may be referred to as nanocapsules, nanoparticles, polymersomes, or other variations of the term.
Enzyme therapy is being actively studied for genetic metabolic diseases where an enzyme is over-expressed, under-expressed, defective, or not at all there. In the case of under-expression or expression of a defective enzyme, an active form of the enzyme is introduced in the body to compensate for the deficit. On the other hand, an enzymatic over-expression may be counteracted by introduction of a competing non-functional enzyme; that is, an enzyme which metabolizes the substrate into non-active products. When placed within an artificial cell, enzymes can carry out their function for a much longer period compared to free enzymes and can be further optimized by polymer conjugation.
The first enzyme studied under artificial cell encapsulation was asparaginase for the treatment of lymphosarcoma in mice. This treatment delayed the onset and growth of the tumor. These initial findings led to further research in the use of artificial cells for enzyme delivery in tyrosine dependent melanomas. These tumors have a higher dependency on tyrosine than normal cells for growth, and research has shown that lowering systemic levels of tyrosine in mice can inhibit growth of melanomas. The use of artificial cells in the delivery of tyrosinase; and enzyme that digests tyrosine, allows for better enzyme stability and is shown effective in the removal of tyrosine without the severe side-effects associated with tyrosine depravation in the diet.
Artificial cell enzyme therapy is also of interest for the activation of prodrugs such as ifosfamide in certain cancers. Artificial cells encapsulating the cytochrome p450 enzyme which converts this prodrug into the active drug can be tailored to accumulate in the pancreatic carcinoma or implanting the artificial cells close to the tumor site. Here, the local concentration of the activated ifosfamide will be much higher than in the rest of the body thus preventing systemic toxicity. The treatment was successful in animals and showed a doubling in median survivals amongst patients with advanced-stage pancreatic cancer in phase I/II clinical trials, and a tripling in one-year survival rate.
In treatment of genetic diseases, gene therapy aims to insert, alter or remove genes within an afflicted individual's cells. The technology relies heavily on viral vectors which raises concerns about insertional mutagenesis and systemic immune response that have led to human deaths and development of leukemia in clinical trials. Circumventing the need for vectors by using naked or plasmid DNA as its own delivery system also encounters problems such as low transduction efficiency and poor tissue targeting when given systemically.
Artificial cells have been proposed as a non-viral vector by which genetically modified non-autologous cells are encapsulated and implanted to deliver recombinant proteins in vivo. This type of immuno-isolation has been proven efficient in mice through delivery of artificial cells containing mouse growth hormone which rescued a growth-retardation in mutant mice. A few strategies have advanced to human clinical trials for the treatment of pancreatic cancer, lateral sclerosis and pain control.
The first clinical use of artificial cells was in hemoperfusion by the encapsulation of activated charcoal. Activated charcoal has the capability of adsorbing many large molecules and has for a long time been known for its ability to remove toxic substances from the blood in accidental poisoning or overdose. However, perfusion through direct charcoal administration is toxic as it leads to embolisms and damage of blood cells followed by removal by platelets. Artificial cells allow toxins to diffuse into the cell while keeping the dangerous cargo within their ultrathin membrane.
Artificial cell hemoperfusion has been proposed as a less costly and more efficient detoxifying option than hemodialysis, in which blood filtering takes place only through size separation by a physical membrane. In hemoperfusion, thousands of adsorbent artificial cells are retained inside a small container through the use of two screens on either end through which patient blood perfuses. As the blood circulates, toxins or drugs diffuse into the cells and are retained by the absorbing material. The membranes of artificial cells are much thinner those used in dialysis and their small size means that they have a high membrane surface area. This means that a portion of cell can have a theoretical mass transfer that is a hundredfold higher than that of a whole artificial kidney machine. The device has been established as a routine clinical method for patients treated for accidental or suicidal poisoning but has also been introduced as therapy in liver failure and kidney failure by carrying out part of the function of these organs. Artificial cell hemoperfusion has also been proposed for use in immunoadsorption through which antibodies can be removed from the body by attaching an immunoadsorbing material such as albumin on the surface of the artificial cells. This principle has been used to remove blood group antibodies from plasma for bone marrow transplantation and for the treatment of hypercholesterolemia through monoclonal antibodies to remove low-density lipoproteins. Hemoperfusion is especially useful in countries with a weak hemodialysis manufacturing industry as the devices tend to be cheaper there and used in kidney failure patients.
The most common method of preparation of artificial cells is through cell encapsulation. Encapsulated cells are typically achieved through the generation of controlled-size droplets from a liquid cell suspension which are then rapidly solidified or gelated to provide added stability. The stabilization may be achieved through a change in temperature or via material crosslinking. The microenvironment that a cell sees changes upon encapsulation. It typically goes from being on a monolayer to a suspension in a polymer scaffold within a polymeric membrane. A drawback of the technique is that encapsulating a cell decreases its viability and ability to proliferate and differentiate. Further, after some time within the microcapsule, cells form clusters that inhibit the exchange of oxygen and metabolic waste, leading to apoptosis and necrosis thus limiting the efficacy of the cells and activating the host's immune system. Artificial cells have been successful for transplanting a number of cells including islets of Langerhans for diabetes treatment, parathyroid cells and adrenal cortex cells.
Shortage of organ donors make artificial cells key players in alternative therapies for liver failure. The use of artificial cells for hepatocyte transplantation has demonstrated feasibility and efficacy in providing liver function in models of animal liver disease and bioartificial liver devices. Research stemmed off experiments in which the hepatocytes were attached to the surface of a micro-carriers and has evolved into hepatocytes which are encapsulated in a three-dimensional matrix in alginate microdroplets covered by an outer skin of polylysine. A key advantage to this delivery method is the circumvention of immunosuppression therapy for the duration of the treatment. Hepatocyte encapsulations have been proposed for use in a bioartifical liver. The device consists of a cylindrical chamber imbedded with isolated hepatocytes through which patient plasma is circulated extra-corporeally in a type of hemoperfusion. Because microcapsules have a high surface area to volume ratio, they provide large surface for substrate diffusion and can accommodate a large number of hepatocytes. Treatment to induced liver failure mice showed a significant increase in the rate of survival. Artificial liver systems are still in early development but show potential for patients waiting for organ transplant or while a patient's own liver regenerates sufficiently to resume normal function. So far, clinical trials using artificial liver systems and hepatocyte transplantation in end-stage liver diseases have shown improvement of health markers but have not yet improved survival. The short longevity and aggregation of artificial hepatocytes after transplantation are the main obstacles encountered. Hepatocytes co-encapsulated with stem cells show greater viability in culture and after implantation and implantation of artificial stem cells alone have also shown liver regeneration. As such interest has arisen in the use of stem cells for encapsulation in regenerative medicine.
The oral ingestion of live bacterial cell colonies has been proposed and is currently in therapy for the modulation of intestinal microflora, prevention of diarrheal diseases, treatment of H. Pylori infections, atopic inflammations, lactose intolerance and immune modulation, amongst others. The proposed mechanism of action is not fully understood but is believed to have two main effects. The first is the nutritional effect, in which the bacteria compete with toxin producing bacteria. The second is the sanitary effect, which stimulates resistance to colonization and stimulates immune response. The oral delivery of bacterial cultures is often a problem because they are targeted by the immune system and often destroyed when taken orally. Artificial cells help address these issues by providing mimicry into the body and selective or long term release thus increasing the viability of bacteria reaching the gastrointestinal system. In addition, live bacterial cell encapsulation can be engineered to allow diffusion of small molecules including peptides into the body for therapeutic purposes. Membranes that have proven successful for bacterial delivery include cellulose acetate and variants of alginate. Additional uses that have arosen from encapsulation of bacterial cells include protection against challenge from M. Tuberculosis and upregulation of Ig secreting cells from the immune system. The technology is limited by the risk of systemic infections, adverse metabolic activities and the risk of gene transfer. However, the greater challenge remains the delivery of sufficient viable bacteria to the site of interest.
Nano sized oxygen carriers are used as a type of red blood cell substitutes, although they lack other components of red blood cells. They are composed of a synthetic polymersome or an artificial membrane surrounding purified animal, human or recombinant hemoglobin. Overall, hemoglobin delivery continues to be a challenge because it is highly toxic when delivered without any modifications. In some clinical trials, vasopressor effects have been observed for first generation hemoglobin blood substitutes.
Research interest in the use of artificial cells for blood arose after the AIDS scare of the 1980s. Besides bypassing the potential for disease transmission, artificial red blood cells are desired because they eliminate drawbacks associated with allogenic blood transfusions such as blood typing, immune reactions and its short storage life of 42 days. A hemoglobin substitute may be stored at room temperature and not under refrigeration for more than a year. Attempts have been made to develop a complete working red blood cell which comprises carbonic not only an oxygen carrier but also the enzymes associated with the cell. The first attempt was made in 1957 by replacing the red blood cell membrane by an ultrathin polymeric membrane which was followed by encapsulation through a lipid membrane and more recently a biodegradable polymeric membrane. An biological red blood cell membrane including lipids and associated proteins can also be used to encapsulate nanoparticles and increase residence time in vivo by bypassing macrophage uptake and systemic clearance.
A leuko-polymersome is a polymersome engineered to have the adhesive properties of a leukocyte. Polymersomes are vesicles composed of a bilayer sheet that can encapsulate many active molecules such as drugs or enzymes. By adding the adhesive properties of a leukocyte to their membranes, they can be made to slow down, or roll along epithelial walls within the quickly flowing circulatory system.
The German pathologist Rudolf Virchow brought forward the idea that not only does life arise from cells, but every cell comes from another cell; "Omnis cellula e cellula". Until now, most attempts to create an artificial cell have only created a package that can mimic certain tasks of the cell. Advances in cell-free transcription and translation reactions allow the expression of many genes, but these efforts are far from producing a fully operational cell.
The future is in the creation of a protocell, or a cell which has all the minimum requirements for life. Members from the J. Craig Venter Institute have used a top-down computational approach to knock out genes in a living organism to a minimum set of genes. Recently, the team succeeded in creating a replicating strain of Mycoplasma Genitalium using synthetically created DNA deemed to be the minimum requirement for life which was inserted into a genomically empty bacterium. This new strain of bacteria differs from previous artificial cells, for example, an artificial red blood cell which is not alive. It is hoped that the process of top-down biosynthesis will enable the insertion of new genes that would perform profitable functions such as generation of hydrogen for fuel or capturing excess carbon dioxide in the atmosphere.
A bottom-up approach to build an artificial cell would involve creating a protocell de novo, entirely from non-living materials. It is proposed to create a phospholipid bilayer vesicle with DNA capable of self-reproducing using synthetic genetic information. The three primary elements of such artificial cells are the formation of a lipid membrane, DNA and RNA replication through a template process and the harvesting of chemical energy for active transport across the membrane. The main hurdles foreseen and encountered with this proposed protocell are the creation of a minimal synthetic DNA that holds all sufficient information for life, and the reproduction of non-genetic components that are integral in cell development such as molecular self-organization. However, it is hoped that this kind of bottom-up approach would provide insight into the fundamental questions of organizations at the cellular level and the origins of biological life. So far, no completely artificial cell capable of self-reproduction has been synthesized using the molecules of life, and this objective is still in a distant future although various groups are currently working towards this goal.
Another method proposed to create a protocell more closely resembles the conditions believed to have been present during evolution known as the primordial soup. Various RNA polymers could be encapsulated in vesicles and in such small boundary conditions, chemical reactions would be tested for.
Heavy investing in synthetic biology has been done by large companies such as ExxonMobil, who has partnered with Synthetic Genomics Inc; Craig Venter's own biosynthetics company in the development of fuel from algae.
The concept of an Electronic Artificial Cell has been expanded in a series of 3 EU projects coordinated by John McCaskill from 2004-2015.
The European Commission sponsored the development of the "Programmable Artificial Cell Evolution" (PACE) program from 2004-2008 whose goal was to lay the foundation for the creation of "microscopic self-organizing, self-replicating, and evolvable autonomous entities built from simple organic and inorganic substances that can be genetically programmed to perform specific functions" for the eventual integration into information systems. The PACE project developed the first Omega Machine, a microfluidic life support system for artificial cells that could complement chemically missing functionalities (as originally proposed by Norman Packard, Steen Rasmussen, Mark Bedau and John McCaskill). The ultimate aim was to attain an evolvable hybrid cell in a complex microscale programmable environment. The functions of the Omega Machine could then be removed stepwise, posing a series of solvable evolution challenges to the artificial cell chemistry. The project achieved chemical integration up to the level of pairs of the three core functions of artificial cells (a genetic subsystem, a containment system and a metabolic system), and generated novel spatially resolved programmable microfluidic environments for the integration of containment and genetic amplification."Programmable Artificial Cell Evolution" (PACE) The project led to the creation of the European center for living technology which is now continuing similar research.
Following this research, in 2007, John McCaskill proposed to concentrate on an electronically complemented artificial cell, called the Electronic Chemical Cell. The key idea was to use a massively parallel array of electrodes coupled to locally dedicated electronic circuitry, in a two-dimensional thin film, to complement emerging chemical cellular functionality. Local electronic information defining the electrode switching and sensing circuits could serve as an electronic genome, complementing the molecular sequential information in the emerging protocols. A research proposal was successful with the European Commission and an international team of scientists partially overlapping with the PACE consortium commenced work 2008-2012 on the project Electronic Chemical Cells. The project demonstrated among other things that electronically controlled local transport of specific sequences could be used as an artificial spatial control system for the genetic proliferation of future artificial cells, and that core processes of metabolism could be delivered by suitably coated electrode arrays.
The major limitation of this approach, apart from the initial difficulties in mastering microscale electrochemistry and electrokinetics, is that the electronic system is interconnected as a rigid non-autonomous piece of macroscopic hardware. In 2011, McCaskill proposed to invert the geometry of electronics and chemistry : instead of placing chemicals in an active electronic medium, to place microscopic autonomous electronics in a chemical medium. He organized a project to tackle a third generation of Electronic Artificial Cells at the 100 µm scale that could self-assemble from two half-cells "lablets" to enclose an internal chemical space, and function with the aid of active electronics powered by the medium they are immersed in. Such cells can copy both their electronic and chemical contents and will be capable of evolution within the constraints provided by their special pre-synthesized microscopic building blocks. In Sep 2012 work commenced on this project Microscale Chemically Reactive Electronic Agents.
Protocell research has created controversy and opposing opinions, including critics of the vague definition of "artificial life". The creation of a basic unit of life is the most pressing ethical concern, although the most widespread worry about protocells is their potential threat to human health and the environment through uncontrolled replication.
The lipid bilayer
is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus and other sub-cellular structures. The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be. Lipid bilayers are ideally suited to this role because, even though they are only a few nanometers in width, they are impermeable to most water-soluble (hydrophilic) molecules. Bilayers are particularly impermeable to ions, which allows cells to regulate salt concentrations and pH by pumping ions across their membranes using proteins called ion pumps.
Natural bilayers are usually composed of phospholipids, which have a hydrophilic head and two hydrophobic tails each. When phospholipids are exposed to water, they arrange themselves into a two-layered sheet (a bilayer) with all of their tails pointing toward the center of the sheet. The center of this bilayer contains almost no water and excludes molecules like sugars or salts that dissolve in water but not in oil. This assembly process is similar to the coalescing of oil droplets in water and is driven by the same force, called the hydrophobic effect. Because lipid bilayers are quite fragile and are so thin that they are invisible in a traditional microscope, bilayers are very challenging to study. Experiments on bilayers often require advanced techniques like electron microscopy and atomic force microscopy.
Phospholipids with certain head groups can alter the surface chemistry of a bilayer and can, for example, mark a cell for destruction by the immune system. Lipid tails can also affect membrane properties, for instance by determining the phase of the bilayer. The bilayer can adopt a solid gel phase state at lower temperatures but undergo phase transition to a fluid state at higher temperatures. The packing of lipids within the bilayer also affects its mechanical properties, including its resistance to stretching and bending. Many of these properties have been studied with the use of artificial "model" bilayers produced in a lab. Vesicles made by model bilayers have also been used clinically to deliver drugs.
Biological membranes typically include several types of lipids other than phospholipids. A particularly important example in animal cells is cholesterol, which helps strengthen the bilayer and decrease its permeability. Cholesterol also helps regulate the activity of certain integral membrane proteins. Integral membrane proteins function when incorporated into a lipid bilayer. Because bilayers define the boundaries of the cell and its compartments, these membrane proteins are involved in many intra- and inter-cellular signaling processes. Certain kinds of membrane proteins are involved in the process of fusing two bilayers together. This fusion allows the joining of two distinct structures as in the fertilization of an egg by sperm or the entry of a virus into a cell.
A lipid bilayer, also known as the phospholipid bilayer, is a sheet of lipids two molecules thick, arranged so that the hydrophilic phosphate heads point “out” to the water on either side of the bilayer and the hydrophobic tails point “in” to the core of the bilayer. This arrangement results in two “leaflets” which are each a single molecular layer. Lipids self-assemble into this structure because of the hydrophobic effect which creates an energetically unfavorable interaction between the hydrophobic lipid tails and the surrounding water. Thus, a lipid bilayer is typically held together by entirely non-covalent forces that do not involve formation of chemical bonds between individual molecules.
There are a few similarities between this structure and a common soap bubble, although there are also important differences. As illustrated, both structures involve two single-molecule layers of an amphiphilic substance. In the case of a soap bubble, the two soap monolayers coat an intervening water layer. The hydrophilic heads are oriented “in” toward this water core, while the hydrophobic tails point “out” to the air. In the case of a lipid bilayer, this structure is reversed with heads out and tails in. Another important difference between lipid bilayers and soap bubbles is their relative size. Soap bubbles are typically hundreds of nanometers thick, on the same order as the wavelength of light, which is why interference effects cause rainbow colors on a bubble surface. A single lipid bilayer, on the other hand, is around five nanometers thick, much smaller than the wavelength of light and is therefore invisible to the eye, even with a standard light microscope.
The lipid bilayer is very thin compared to its lateral dimensions. If a typical mammalian cell (diameter ~10 micrometre) were magnified to the size of a watermelon (~1 ft/30 cm), the lipid bilayer making up the plasma membrane would be about as thick as a piece of office paper. Despite being only a few nanometers thick, the bilayer is composed of several distinct chemical regions across its cross-section. These regions and their interactions with the surrounding water have been characterized over the past several decades with x-ray reflectometry, neutron scattering and nuclear magnetic resonance techniques.
The first region on either side of the bilayer is the hydrophilic headgroup. This portion of the membrane is completely hydrated and is typically around 0.8-0.9 nm thick. In phospholipid bilayers the phosphate group is located within this hydrated region, approximately 0.5 nm outside the hydrophobic core. In some cases, the hydrated region can extend much further, for instance in lipids with a large protein or long sugar chain grafted to the head. One common example of such a modification in nature is the lipopolysaccharide coat on a bacterial outer membrane, which helps retain a water layer around the bacterium to prevent dehydration.
Next to the hydrated region is an intermediate region which is only partially hydrated. This boundary layer is approximately 0.3 nm thick. Within this short distance, the water concentration drops from 2M on the headgroup side to nearly zero on the tail (core) side. The hydrophobic core of the bilayer is typically 3-4 nm thick, but this value varies with chain length and chemistry. Core thickness also varies significantly with temperature, particularly near a phase transition.
In many naturally occurring bilayers, the compositions of the inner and outer membrane leaflets are different. In human red blood cells, the inner (cytoplasmic) leaflet is largely composed of phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol and its phosphorylated derivatives. By contrast, the outer (extracellular) leaflet is based on phosphatidylcholine, sphingomyelin and a variety of glycolipids, In some cases, this asymmetry is based on where the lipids are made in the cell and reflects their initial orientation. The biological functions of lipid asymmetry are imperfectly understood, although it is clear that it is used in several different situations. For example, when a cell undergoes apoptosis, the phosphatidylserine — normally localised to the cytoplasmic leaflet — is transferred to the outer surface: there it is recognised by a macrophage which then actively scavenges the dying cell.
Lipid asymmetry arises, at least in part, from the fact that most phospholipids are synthesised and initially inserted into the inner monolayer: those that constitute the outer monolayer are then transported from the inner monolayer by a class of enzymes called flippases. Other lipids, such as sphingomyelin, appear to be synthesised at the external leaflet. Flippases are members of a larger family of lipid transport molecules which also includes floppases, which transfer lipids in the opposite direction, and scramblases, which randomize lipid distribution across lipid bilayers (as in apoptotic cells). In any case, once lipid asymmetry is established it does not normally dissipate quickly because spontaneous flip-flop of lipids between leaflets is extremely slow.
It is possible to mimic this asymmetry in the laboratory in model bilayer systems. Certain types of very small artificial vesicle will automatically make themselves slightly asymmetric, although the mechanism by which this asymmetry is generated is very different from that in cells. By utilizing 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 may be lost over time as lipids in supported bilayers can be prone to flip-flop.
At a given temperature a lipid bilayer can exist in either a liquid or a gel (solid) phase. All lipids have a characteristic temperature at which they transition (melt) from the gel to liquid phase. In both phases the lipid molecules are prevented from flip-flopping across the bilayer, but in liquid phase bilayers a given lipid will exchange locations with its neighbor millions of times a second. This random walk exchange allows lipid to diffuse and thus wander across the surface of the membrane. Unlike liquid phase bilayers, the lipids in a gel phase bilayer are locked in place.
The phase behavior of lipid bilayers is largely determined by the strength of the attractive Van der Waals interactions between adjacent lipid molecules. Longer tailed lipids have more area over which to interact, increasing the strength of this interaction and consequently decreasing the lipid mobility. Thus, at a given temperature, a short-tailed lipid will be more fluid than an otherwise identical long-tailed lipid. Transition temperature can also be affected by the degree of unsaturation of the lipid tails. An unsaturated double bond can produce a kink in the alkane chain, disrupting the lipid packing. This disruption creates extra free space within the bilayer which allows additional flexibility in the adjacent chains. An example of this effect can be noted in everyday life as butter, which has a large percentage saturated fats, is solid at room temperature 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 spatially separated regions, rather like an iceberg floating in the ocean. This phase separation plays a critical role in biochemical phenomena because membrane components such as proteins can partition into one or the other phase and thus be locally concentrated or activated. One particularly important component of many mixed phase systems is cholesterol, which modulates bilayer permeability, mechanical strength and biochemical interactions.
While lipid tails primarily modulate bilayer phase behavior, it is the headgroup that determines the bilayer surface chemistry. 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), accounting for about half 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, because these local charges balance, no net charge.
Other headgroups are also present to varying degrees and can include phosphatidylserine (PS) phosphatidylethanolamine (PE) and phosphatidylglycerol (PG). These alternate headgroups often confer specific biological functionality that is highly context-dependent. For instance, PS presence on the extracellular membrane face of erythrocytes is a marker of cell apoptosis, whereas PS in growth plate vesicles is necessary for the nucleation of hydroxyapatite crystals and subsequent bone mineralization. Unlike PC, some of the other headgroups carry a net charge, which can alter the electrostatic interactions of small molecules with the bilayer.
The primary role of the lipid bilayer in biology is to separate aqueous compartments from their surroundings. Without some form of barrier delineating “self” from “non-self” it is difficult to even define the concept of an organism or of life. This barrier takes the form of a lipid bilayer in all known life forms except for a few species of archaea which utilize a specially adapted lipid monolayer. It has even been proposed that the very first form of life may have been a simple lipid vesicle with virtually its sole biosynthetic capability being the production of more phospholipids. The partitioning ability of the lipid bilayer is based on the fact that hydrophilic molecules cannot easily cross the hydrophobic bilayer core, as discussed in Transport across the bilayer below. Nucleus, mitochondria and chloroplasts have two lipid bilayer, and other structures are surrounded by a single lipid bilayer (such as the plasma membrane, endoplasmic reticula, Golgi apparatuses and lysosomes). See Organelle.
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 composed 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 sub-cellular compartments are surrounded by one or more lipid bilayers and, together, typically comprise the majority of the bilayer area present in the cell. In liver hepatocytes for example, the plasma membrane accounts for only two percent of the total bilayer area of the cell, whereas the endoplasmic reticulum contains more than fifty percent and the mitochondria a further thirty percent.
Probably the most familiar form of cellular signaling is synaptic transmission, whereby a nerve impulse that has reached the end of one neuron is conveyed to an adjacent neuron via the release of neurotransmitters. This transmission is made possible by the action of synaptic vesicles loaded with the neurotransmitters to be released. These vesicles fuse with the cell membrane at the pre-synaptic terminal and release its contents to the exterior of the cell. The contents then diffuse across the synapse to the post-synaptic terminal.
Lipid bilayers are also involved in signal transduction through their role as the home of integral membrane proteins. This is an extremely broad 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 linked to the exterior of the cell membrane. An example of this is the CD59 protein, which identifies cells as “self” and thus inhibits their destruction by the immune system. The HIV virus evades the immune system in part by grafting these proteins from the host membrane onto its own surface. Alternatively, some membrane proteins penetrate all the way through the bilayer and serve to relay individual signal events 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 sense its surroundings and, because of this important role, approximately 40% of all modern drugs are targeted at GPCRs.
In addition to protein- and solution-mediated processes, it is also possible for lipid bilayers to participate directly in signaling. A classic example of this is phosphatidylserine-triggered phagocytosis. Normally, phosphatidylserine is asymmetrically distributed in the cell membrane and is present only on the interior side. During programmed cell death a protein called a scramblase equilibrates this distribution, displaying phosphatidylserine on the extracellular bilayer face. The presence of phosphatidylserine then triggers phagocytosis to remove the dead or dying cell.
The lipid bilayer is a very difficult structure to study because it is so thin and fragile. In spite of these limitations dozens of techniques have been developed over the last seventy years to allow investigations of its structure and function.
Electrical measurements are a straightforward way to characterize an important function of a bilayer: its ability to segregate 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 typically quite high]
[ 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 the activity of single ion channels can be resolved.
Electrical measurements do not provide an actual picture like imaging with a microscope can. Lipid bilayers cannot be seen in a traditional microscope because they are too thin. In order to see bilayers, researchers often use fluorescence microscopy. A sample is excited with one wavelength of light and observed in a different wavelength, so that only fluorescent molecules with a matching excitation and emission profile will be seen. Natural lipid bilayers are not fluorescent, so a dye is used that attaches to the desired molecules in the bilayer. Resolution is usually limited to a few hundred nanometers, much smaller than a typical cell but much larger than the thickness of a lipid bilayer.
Electron microscopy offers a higher resolution image. In an electron microscope, a beam of focused electrons interacts with the sample rather than a beam of light as in traditional microscopy. In conjunction with rapid freezing techniques, electron microscopy has also been used to study the mechanisms of inter- and intracellular transport, for instance in demonstrating that exocytotic vesicles are the means of chemical release at synapses.
31P-NMR(nuclear magnetic resonance) spectroscopy is widely used for studies of phospholipid bilayers and biological membranes in native conditions. The analysis of 31P-NMR spectra of lipids could provide a wide range of information about lipid bilayer packing, phase transitions (gel phase, physiological liquid crystal phase, ripple phases, non bilayer phases), lipid head group orientation/dynamics, and elastic properties of pure lipid bilayer and as a result of binding of proteins and other biomolecules.
In addition, a specific H-N...(O)-P NMR experiment (INEPT transfer by scalar coupling 3JH-P~5 Hz) could provide a direct information about formation of hydrogen bonds between amid protons of protein to phosphate of lipid headgroups, which is useful in studies of protein/membrane interactions.
A new method to study lipid bilayers is Atomic force microscopy (AFM). Rather than using a beam of light or particles, a very small sharpened tip scans the surface by making physical contact with the bilayer and moving across it, like a record player needle. AFM is a promising technique because 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. Utilizing this capability, AFM has been used to examine dynamic bilayer behavior 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 of the lipids, since the probe tip interacts mechanically with the bilayer surface. Because of this, the same scan can image both lipids and associated proteins, sometimes even with single-molecule resolution. AFM can also probe the mechanical nature of lipid bilayers.
Lipid bilayers exhibit high levels of birefringence where the refractive index in the plane of the bilayer differs from that perpendicular by as much as 0.1 refractive index units. This has been used to characterise the degree of order and disruption in bilayers using dual polarisation interferometry to understand mechanisms of protein interaction.
Lipid bilayers are complicated molecular systems with many degrees of freedom. Thus atomistic simulation of membrane and in particular ab initio calculations of its properties is difficult and computationally expensive. Quantum chemical calculations has recently been successfully performed to estimate dipole and quadrupole moments of lipid membranes.
Hydrated bilayers show rich vibrational dynamics and are good media for efficient vibrational energy transfer. Vibrational properties of lipid monolayers and bilayers has been investigated by ultrafast spectroscopic techniques and recently developed computational methods.
Most polar molecules have low solubility in the hydrocarbon core of a lipid bilayer and consequently have low permeability coefficients across the bilayer. This effect is particularly pronounced for charged species, which have even lower permeability coefficients than neutral polar molecules. Anions typically have a higher rate of diffusion 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 interior salt concentration is placed in a solution with a low salt concentration it will swell and eventually burst. Such a result would not be observed unless water was able to pass through the bilayer with relative ease. The anomalously large permeability of water through bilayers is still not completely understood and continues to be the subject of active debate. Small uncharged apolar molecules diffuse through lipid bilayers many orders of magnitude faster than ions or water. This applies both to fats and organic solvents like chloroform and ether. Regardless of their polar character larger molecules diffuse more slowly across lipid bilayers than small molecules.
Two special classes of protein deal with the ionic gradients found across cellular and sub-cellular membranes in nature- ion channels and ion pumps. Both pumps and channels are integral membrane proteins that pass through the bilayer, but their roles are quite different. Ion pumps are the proteins that build and maintain the 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 ATP, as is the case for the ATPase+-K+Na. Alternatively, the energy source can be another chemical gradient already in place, as in the antiporter+/Na2+Ca. It is through the action of ion pumps that cells are able to regulate pH via the pumping of protons.
In contrast to ion pumps, ion channels do not build chemical gradients but rather dissipate them in order to perform work or send a signal. Probably the most familiar and best studied example is the channel+voltage-gated Na, which allows conduction of an action potential along neurons. All ion pumps have some sort of trigger or “gating” mechanism. In the previous example it was electrical bias, but other channels can be activated by binding a molecular agonist or through a conformational change in another nearby protein.
Some molecules or particles are too large or too hydrophilic to effectively pass through a lipid bilayer. Other molecules could pass through the bilayer but must be transported rapidly in such large numbers that channel-type transport is impractical. In both cases these types of cargo can be moved across the cell membrane through fusion or budding of vesicles. When a vesicle is produced inside 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 dimple inwards and eventually pinch off, enclosing a portion of the extracellular fluid to transport it 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 this interdependence is the sheer volume of lipid material involved. In a typical cell, an area of bilayer equivalent to the entire plasma membrane will travel through the endocytosis/exocytosis cycle in about half an hour. If these two processes were not balancing each other the cell would either balloon outward to an unmanageable size or completely deplete its plasma membrane within a matter of minutes.
Electroporation is the rapid increase in bilayer permeability 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 across the hydrophobic bilayer core. Because of this, electroporation is one of the key methods of transfection as well as bacterial transformation. It has even been proposed that electroporation resulting from lightning strikes could be a mechanism of natural horizontal gene transfer.
This increase in permeability primarily affects 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 both result from 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 thus chemical in nature. In contrast, during electroporation the lipid molecules are not chemically altered but simply shift position, opening up a pore which acts as the conductive pathway through the bilayer as it is filled with water.
Lipid bilayers are large enough structures to have some of the mechanical properties of liquids or solids. The area compression modulus Ka
, bending modulus Kb
, and edge energy
, can be used to describe them. Solid lipid bilayers also have a shear modulus, but like any liquid, the shear modulus is zero for fluid bilayers. These mechanical properties affect how the membrane functions. Ka
affect the ability of proteins and small molecules to insert into the bilayer, and bilayer mechanical properties have been shown to alter the function of mechanically activated ion channels. Bilayer mechanical properties also govern what types of stress a cell can withstand without tearing. Although lipid bilayers can easily bend, most cannot stretch more than a few percent before rupturing.
As discussed in the Structure and organization section, the hydrophobic attraction of lipid tails in water is the primary force holding lipid bilayers together. Thus, the elastic modulus of the bilayer is primarily determined by how much extra area is exposed to water when the lipid molecules are stretched apart. It is not surprising given this understanding of the forces involved that studies have shown that Ka
varies strongly with osmotic pressure but only weakly with tail length and unsaturation. Because the forces involved are so small, it is difficult to experimentally determine Ka
. Most techniques require sophisticated microscopy and very sensitive measurement equipment.
In contrast to Ka
, which is a measure of how much energy is needed to stretch the bilayer, Kb
is a measure of how much energy is needed to bend or flex the bilayer. Formally, bending modulus is defined as the energy required to deform a membrane from its intrinsic curvature to some other curvature. 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 nearly one so the intrinsic curvature is nearly zero. If a particular lipid has too large a deviation from zero intrinsic curvature it will not form a bilayer and will instead form other phases such as micelles or inverted micelles. Typically, Kb
is not measured experimentally but rather is calculated from measurements of Ka
and bilayer thickness, since the three parameters are related.
is a measure of how much energy it takes to expose a bilayer edge to water by tearing the bilayer or creating a hole in it. The origin of this energy is the fact that creating such an interface exposes some of the lipid tails to water, but the exact orientation of these border lipids is unknown. There is some evidence that both hydrophobic (tails straight) and hydrophilic (heads curved around) pores can coexist.
Fusion is the process by which two lipid bilayers merge, resulting in one connected structure. If this fusion proceeds completely through both leaflets of both bilayers, a water-filled bridge is formed and the solutions contained by the bilayers can mix. Alternatively, if only one leaflet 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 sub-divided by lipid bilayer membranes. Exocytosis, fertilization of an egg by sperm and transport of waste products to the lysozome are a few of the many eukaryotic processes that rely on some form of fusion. Even the entry of pathogens can be governed by fusion, as many bilayer-coated viruses have dedicated fusion proteins to gain entry into the host cell.
There are four fundamental steps in the fusion process. First, the involved membranes must aggregate, approaching each other to within several nanometers. Second, the two bilayers must come into very close contact (within a few angstroms). To achieve this close contact, the two surfaces must become at least partially dehydrated, as the bound surface water normally present causes bilayers to strongly repel. The presence of ions, particularly divalent cations like magnesium and calcium, strongly affects this step. One of the critical roles of calcium in the body is regulating membrane fusion. Third, a destabilization must form at one point between the two bilayers, locally distorting their structures. The exact nature of this distortion is not known. One theory is that a highly curved "stalk" must form between the two bilayers. Proponents of this theory believe that it explains why phosphatidylethanolamine, a highly curved lipid, promotes fusion. Finally, in the last step of fusion, this point defect grows and the components of the two bilayers mix and diffuse away from the site of contact.
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 the 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; some others have only a protein coat).Eukaryotic cells also use fusion proteins, the best studied of which are the SNAREs. SNARE proteins are used to direct all vesicular intracellular trafficking. Despite years of study, much is still unknown about the function of this protein class. In fact, there is still an active debate regarding whether SNAREs are linked to early docking or participate later in the fusion process by facilitating hemifusion.
In studies of molecular and cellular biology it is often desirable to artificially induce fusion. The addition of polyethylene glycol (PEG) causes fusion without significant aggregation or biochemical disruption. This procedure is now used extensively, for example by fusing B-cells with melanoma cells. The resulting “hybridoma” from this combination expresses a desired antibody as determined by the B-cell involved, but is immortalized due to the melanoma component. Fusion can also be artificially induced through electroporation in a process known as electrofusion. It is believed that this phenomenon results from the energetically active edges formed during electroporation, which can act as the local defect point to nucleate stalk growth between two bilayers.
Lipid bilayers can be created artificially in the lab to allow researchers to perform experiments that cannot be done with natural bilayers. These synthetic systems are called model lipid bilayers. There are many different types of model bilayers, each having experimental advantages and disadvantages. They can be made with either synthetic or natural lipids. Among the most common model systems are:
To date, the most successful commercial application of lipid bilayers has been the use of liposomes for drug delivery, especially for cancer treatment. (Note- the term “liposome” is essentially synonymous with “vesicle” except that vesicle is a general term for the structure whereas liposome only refers to artificial, not natural vesicles) The basic idea of liposomal drug delivery is that the drug is encapsulated in solution inside the liposome 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 nearly any hydrophilic drug, can be grafted with molecules to target specific tissues and can be relatively non-toxic since the body possesses biochemical pathways for degrading 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 clearing and phagocytosis. Refinement of the lipid composition to tune fluidity, surface charge density and surface hydration resulted in vesicles that adsorb fewer proteins from serum and thus are less readily recognized by the immune system. The most significant advance in this area was the grafting of polyethylene glycol (PEG) onto the liposome surface to produce “stealth” vesicles which circulate over long times without immune or renal clearing.
The first stealth liposomes were passively targeted at tumor tissues. Because tumors induce rapid and uncontrolled angiogenesis they are especially “leaky” and allow liposomes to exit the bloodstream at a much higher rate than normal tissue would. More recently]
[ work has been undertaken to graft antibodies or other molecular markers onto the liposome surface in the hope of actively binding them to a specific cell or tissue type. Some examples of this approach are already in clinical trials.
Another potential application of lipid bilayers is the field of biosensors. Since the lipid bilayer is the barrier between the interior and exterior of the cell it is also the site of extensive signal transduction. Researchers over the years have tried to harness this potential to develop a bilayer-based device for clinical diagnosis or bioterrorism detection. Progress has been slow in this area and, although a few companies have developed automated lipid-based detection systems, they are still targeted at the research community. These include Biacore Life Sciences, which offers a disposable chip for utilizing lipid bilayers in studies of binding kinetics and Nanion Inc which has developed an automated patch clamping system. Other, more exotic applications 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 commercially viable.
A supported lipid bilayer (SLB) as described above has achieved commercial success as a screening technique to measure the permeability of drugs. This p
ssay PAMPA technique measures the permeability across specifically formulated lipid cocktail(s) found to be highly correlated with Caco-2 cultures, the gastrointestinal tract, blood–brain barrier and skin.
By the early twentieth century scientists had come to believe that cells are surrounded by a thin oil-like barrier, but the structural nature of this membrane was not known. Two experiments in 1925 laid the groundwork to fill in this gap. By measuring the capacitance of erythrocyte 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 of Leiden University approached the problem from a different perspective, spreading the erythrocyte lipids as a monolayer on 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. Later analyses showed several errors and incorrect assumptions with this experiment but, serendipitously, these errors canceled out and from this flawed data Gorter and Grendel drew the correct conclusion- that the cell membrane is a lipid bilayer.
This theory was confirmed through the use of 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 assert that the two dark electron-dense bands were the headgroups and associated proteins of two apposed lipid monolayers. In this body of work, Robertson put forward the concept of the “unit membrane.” This was the first time 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 lipid in organic solvent across an aperture, Mueller and Rudin were able to create an artificial bilayer and determine that this exhibited lateral fluidity, high electrical resistance and self-healing in response to puncture, 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 dried lipid sample to water. This was an important advance since it demonstrated that lipid bilayers form spontaneously via self assembly and do not require a patterned support structure.
The cell membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the phospholipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellular cytoskeleton. Cell membranes can be artificially reassembled.
The cell membrane or plasma membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment. Fungi, bacteria and plants also have the cell wall which provides a mechanical support for the cell and precludes the passage of larger molecules. The cell membrane also plays a role in anchoring the cytoskeleton provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues.
The membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or active, requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. The cell employs a number of transport mechanisms that involve biological membranes:
1. Passive diffusion and osmosis: Some substances (small molecules, ions) such as carbon dioxide (CO2) and oxygen (O2), can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water.
2. Transmembrane protein channels and transporters: Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules are pumped across the membrane by transmembrane transporters or diffuse through protein channels such as Aquaporins (in the case of water (H2O)). These proteins, also called permeases, are usually quite specific, recognizing and transporting only a limited food group of chemical substances, often even only a single substance.
3. Endocytosis: Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles (cell eating or phagocytosis), small molecules and ions (cell drinking or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.
4. Exocytosis: Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell.
Gram-negative bacteria have a plasma membrane and an outer membrane separated by a periplasmic space. Other prokaryotes have only a plasma membrane. Prokaryotic cells are also surrounded by a cell wall composed of peptidoglycan (amino acids and sugars). Some eukaryotic cells also have cells walls, but none that are made of peptidoglycan.
According to the fluid mosaic model of S.J.Singer and G.L. Nicolson (1972), which replaced the earlier model of Davson and Danielli, biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily. Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure. Examples of such structures are protein-protein complexes, pickets and fences formed by the actin-based cytoskeleton, and potentially lipid rafts.
Lipid bilayers form through the process of self-assembly. The cell membrane consists primarily of a thin layer of amphipathic phospholipids which spontaneously arrange so that the hydrophobic "tail" regions are isolated from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the intracellular (cytosolic) and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer. Forces such as van der Waals, electrostatic, hydrogen bonds, and noncovalent interactions all contribute to the formation of the lipid bilayer. Overall, hydrophobic interactions are the major driving force in the formation of lipid bilayers.
Lipid bilayers are generally impermeable to ions and polar molecules. The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (ex. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores, channels and gates.
Flippases and scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Along with NANA, this creates an extra barrier to charged moieties moving through the membrane.
Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis.
The apical membrane of a polarized cell is the surface of the plasma membrane that faces inward to the lumen. This is particularly evident in epithelial and endothelial cells, but also describes other polarized cells, such as neurons. The basolateral membrane of a polarized cell is the surface of the plasma membrane that forms its basal and lateral surfaces. It faces outwards, towards the interstitium, and away from the lumen. Basolateral membrane is a compound phrase referring to the terms "basal (base) membrane" and "lateral (side) membrane", which, especially in epithelial cells, are identical in composition and activity. Proteins (such as ion channels and pumps) are free to move from the basal to the lateral surface of the cell or vice versa in accordance with the fluid mosaic model. Tight junctions join epithelial cells near their apical surface to prevent the migration of proteins from the basolateral membrane to the apical membrane. The basal and lateral surfaces thus remain roughly equivalent][ to one another, yet distinct from the apical surface.
Cell membrane can form different types of "supramembrane" structures such as caveola, postsynaptic density, podosome, invadopodium, focal adhesion, and different types of cell junctions. These structures are usually responsible for cell adhesion, communication, endocytosis and exocytosis. They can be visualized by electron microscopy or fluorescence microscopy. They are composed of specific proteins, such as integrins and cadherins.
The cytoskeleton is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles that extend from the cell. Indeed, cytoskeletal elements interact extensively and intimately with the cell membrane. Anchoring proteins restricts them to a particular cell surface — for example, the apical surface of epithelial cells that line the vertebrate gut — and limits how far they may diffuse within the bilayer. The cytoskeleton is able to form appendage-like organelles, such as cilia, which are microtubule-based extensions covered by the cell membrane, and filopodia, which are actin-based extensions. These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and/or make contact with the substrate or other cells. The apical surfaces of epithelial cells are dense with actin-based finger-like projections known as microvilli, which increase cell surface area and thereby increase the absorption rate of nutrients. Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb.
Cell membranes contain a variety of biological molecules, notably lipids and proteins. Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:
The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and sterols. The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant. In RBC studies, 30% of the plasma membrane is lipid.
The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always "cis". The length and the degree of unsaturation of fatty acid chains have a profound effect on membrane fluidity as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (increasing the fluidity) of the membrane. The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation.
The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Under physiological conditions phospholipid molecules in the cell membrane are in the liquid crystalline state. It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, the exchange of phospholipid molecules between intracellular and extracellular leaflets of the bilayer is a very slow process. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane.
In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane.
Lipid vesicles or liposomes are circular pockets that are enclosed by a lipid bilayer. These structures are used in laboratories to study the effects of chemicals in cells by delivering these chemicals directly to the cell, as well as getting more insight into cell membrane permeability. Lipid vesicles and liposomes are formed by first suspending a lipid in an aqueous solution then agitating the mixture through sonication, resulting in a vesicle. By measuring the rate of efflux from that of the inside of the vesicle to the ambient solution, allows researcher to better understand membrane permeability. Vesicles can be formed with molecules and ions inside the vesicle by forming the vesicle with the desired molecule or ion present in the solution. Proteins can also be embedded into the membrane through solubilizing the desired proteins in the presence of detergents and attaching them to the phospholipids in which the liposome is formed. These provide researchers with a tool to examine various membrane protein functions.
Plasma membranes also contain carbohydrates, predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides). For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane. The glycocalyx is an important feature in all cells, especially epithelia with microvilli. Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing, and many others. The penultimate sugar is galactose and the terminal sugar is sialic acid, as the sugar backbone is modified in the golgi apparatus. Sialic acid carries a negative charge, providing an external barrier to charged particles.
The cell membrane has large content of proteins, typically around 50% of membrane volume These proteins are important for cell because they are responsible for various biological activities. Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms.
The cell membrane, being exposed to the outside environment, is an important site of cell–cell communication. As such, a large variety of protein receptors and identification proteins, such as antigens, are present on the surface of the membrane. Functions of membrane proteins can also include cell–cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane.
Most membrane proteins must be inserted in some way into the membrane. For this to occur, an N-terminus "signal sequence" of amino acids directs proteins to the endoplasmic reticulum, which inserts the proteins into a lipid bilayer. Once inserted, the proteins are then transported to their final destination in vesicles, where the vesicle fuses with the target membrane.
The cell membrane has different lipid and protein compositions in distinct types of cells and may have therefore specific names for certain cell types:
The permeability of a membrane is the rate of passive diffusion of molecules through the membrane. These molecules are known as permeant molecules. Permeability depends mainly on the electric charge and polarity of the molecule and to a lesser extent the molar mass of the molecule. Due to the cell membrane's hydrophobic nature, small electrically neutral molecules pass through the membrane more easily than charged, large ones. The inability of charged molecules to pass through the cell membrane results in pH partition of substances throughout the fluid compartments of the body.
An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consist of two parts, the electrical potential and a difference in the chemical concentration across a membrane. The difference of electrochemical potentials can be interpreted as a type of potential energy available for work in a cell. The energy is stored in the form of chemical potential, which accounts for an ion's concentration gradient across a cell membrane, and electrostatic energy, which accounts for an ion's tendency to move under influence of the transmembrane potential.
Electrochemical potential is important in electroanalytical chemistry and industrial applications such as batteries and fuel cells. It represents one of the many interchangeable forms of potential energy through which energy may be conserved.
In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient. In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force. This potential energy is used for the synthesis of ATP by oxidative phosphorylation.
An electrochemical gradient has two components. First, the electrical component is caused by a charge difference across the lipid membrane. Second, a chemical component is caused by a differential concentration of ions across the membrane. The combination of these two factors determines the thermodynamically favourable direction for an ion's movement across a membrane.
An electrochemical gradient is analogous to the water pressure across a hydroelectric dam. Membrane transport proteins such as the sodium-potassium pump within the membrane are equivalent to turbines that convert the water's potential energy to other forms of physical or chemical energy, and the ions that pass through the membrane are equivalent to water that ends up at the bottom of the dam. Also, energy can be used to pump water up into the lake above the dam. In similar manner, chemical energy in cells can be used to create electrochemical gradients.
The term is typically applied in contexts wherein a chemical reaction is to take place, such as one involving the transfer of an electron at a battery electrode. In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the standard electrochemical potential of that reaction (see also Electrode potential and Table of standard electrode potentials). In instances pertaining specifically to the movement of electrically charged solutes, the potential is often expressed in units of volts. See: Concentration cell.
In biology, the term is sometimes used in the context of a chemical reaction, in particular to describe the energy source for the chemical synthesis of ATP. In more general terms, however, it is used to characterize the tendency of solutes to simply diffuse across a membrane, a process involving no chemical transformation.
With respect to a cell, organelle, or other subcellular compartment, the tendency of an electrically charged solute, such as a potassium ion, to move across the membrane is decided by the difference in its electrochemical potential on either side of the membrane, which arises from three factors:
A solute's electrochemical potential difference is zero at its "reversal potential", the transmembrane voltage at which the solute's net flow across the membrane is also zero. This potential is predicted, in theory, either by the Nernst equation (for systems of one permeant ion species) or by the Goldman-Hodgkin-Katz equation (for more than one permeant ion species). Electrochemical potential is measured in the laboratory and field using reference electrodes.
Transmembrane ATPases or transmembrane proteins with ATPase domains are often used for making and utilizing ion gradients. The enzyme Na+/K+ ATPase uses ATP to make a sodium ion gradient and a potassium ion gradient. The electrochemical potential is used as energy storage. Chemiosmotic coupling is one of several ways a thermodynamically unfavorable reaction can be driven by a thermodynamically favorable one. Cotransport of ions by symporters and antiporter carriers is commonly used to actively move ions across biological membranes.
The proton gradient can be used as intermediate energy storage for heat production and flagellar rotation. In addition, it is an interconvertible form of energy in active transport, electron potential generation, NADPH synthesis, and ATP synthesis/hydrolysis.
The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria, and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force (see chemiosmosis). In this context, protons are often considered separately using units of either concentration or pH.
Two protons are expelled at each coupling site, generating the proton motive force (PMF). ATP is made indirectly using the PMF as a source of energy.
Some archaea, the most notable ones being halobacteria, make proton gradients by pumping in protons from the environment with the help of the solar-driven enzyme bacteriorhodopsin, which is used here for driving the molecular motor enzyme ATP synthase to make the necessary conformational changes required to synthesize ATP.
Proton gradients are also made by bacteria by running ATP synthase in reverse, and are used to drive flagella.
The F1FO ATP synthase is a reversible enzyme. Large enough quantities of ATP cause it to create a transmembrane proton gradient. This is used by fermenting bacteria - which do not have an electron transport chain, and hydrolyze ATP to make a proton gradient - for flagella and the transportation of nutrients into the cell.
In respiring bacteria under physiological conditions, ATP synthase, in general, runs in the opposite direction, creating ATP while using the proton motive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. The same process takes place in mitochondria, where ATP synthase is located in the inner mitochondrial membrane, so that F1 part sticks into the mitochondrial matrix where ATP synthesis takes place.
Health Medical Pharma
Health Medical Pharma