The Cell Membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer") is a selectively permeable lipid bilayer found in all cells. A semipermeable membrane, also termed a selectively-permeable membrane, a partially-permeable membrane or a differentially-permeable membrane, is a membrane A lipid bilayer or bilayer lipid membrane ( BLM) is a membrane composed of Lipid molecules (usually Phospholipids. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called ^[1] It contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes such as cell adhesion, ion channel conductance and cell signaling. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton and, if present, the cell wall. cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. A cell wall is a tough flexible and sometimes fairly rigid layer surrounding a cell, located external to the Cell membrane, which provides the cell with structural

Illustration of a cell membrane

Function

The cell membrane surrounds the cytoplasm of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment, thereby serving a function similar to that of skin. The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. The skin is the outer covering of living tissue of an animal (or plant In fungi, some bacteria, and plants, an additional cell wall forms the outermost boundary; however, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary. A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Plants are living Organisms belonging to the kingdom Plantae. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix to help group cells together in the formation of tissues. cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. In Biology, the extracellular matrix ( ECM) is the Extracellular part of animal tissue that usually provides structural support to the cells Tissue is a cellular organizational level intermediate between cells and a complete organism

The barrier is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. A semipermeable membrane, also termed a selectively-permeable membrane, a partially-permeable membrane or a differentially-permeable membrane, is a membrane A membrane transport protein (or simply transporter) is a Protein involved in the movement of Ions small molecules or macromolecules such as another 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 moving it. The membrane also maintains the cell potential. The Membrane potential, or better Membrane Voltage, is the difference of Electric potentials between two Aqueous solutions separated by a (

Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to communicate with each other. Protein receptors are found ubiquitously and function to receive signals from both the environment and other cells. In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling These signals are transduced into a form that the cell can use to directly effect a response. In Biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another Other proteins on the surface of the cell membrane serve as "markers" that identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the immune system. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor

Structure

Lipid bilayer

Diagram of the arrangement of amphipathic lipid molecules to form a lipid bilayer. A lipid bilayer or bilayer lipid membrane ( BLM) is a membrane composed of Lipid molecules (usually Phospholipids. The yellow polar head groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular environments. "Polar molecule" and "Non-polar" redirect here

The cell membrane consists primarily of a thin layer of amphipathic phospholipids which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. Amphiphile (from the Greek αμφις amphis both and φιλíα Philia: love friendship is a term describing a Chemical compound possessing both Phospholipids are a class of Lipids and are a major component of all Biological membranes All phospholipids contain a Diglyceride, a Phosphate This forms a continuous, spherical lipid bilayer approximately 7 nm thick, barely discernible with a transmission electron microscope. A lipid bilayer or bilayer lipid membrane ( BLM) is a membrane composed of Lipid molecules (usually Phospholipids. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a ^[1]

The arrangement of hydrophilic and hydrophobic heads of the lipid bilayer prevent polar solutes (e. g. 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 and gates. A transmembrane protein is a Protein that spans the entire Biological membrane.

Flippases and Scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Flippases (rarely flipases) are Enzymes located in the membrane responsible for aiding the movement of Phospholipid molecules between the two Scramblase is a hypothetical Protein thought to be responsible for transportation of Phospholipids between the two monolayers of a Lipid bilayer of Phosphatidylserine (abbreviated Ptd-L-Ser or PS is a Phospholipid component usually kept on the inner-leaflet the cytosolic side of cell membranes by an enzyme called Translocase Along with NANA, this creates an extra barrier to charged moities moving through the membrane. Sialic acid is a generic term for the N - or O -substituted derivatives of Neuraminic acid, a Monosaccharide with a nine- Carbon backbone

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.

Integral membrane proteins

The cell membrane contains many integral membrane proteins, which pepper the entire surface. An Integral Membrane Protein ( IMP) is a Protein Molecule (or assembly of proteins that is permanently attached to the Biological membrane. These structures, which can be visualized by electron microscopy or fluorescence microscopy, can be found on the inside of the membrane, the outside, or membrane spanning. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image A fluorescence microscope (colloquially synonymous with epifluorescent microscope) is a light Microscope used to study properties of organic or inorganic substances A transmembrane protein is a Protein that spans the entire Biological membrane. These may include integrins, cadherins, desmosomes, clathrin-coated pits, caveolaes, and different structures involved in cell adhesion. Integrins are Cell surface receptors that interact with the Extracellular matrix (ECM and mediate various intracellular signals. Cadherins are a class of type-1 Transmembrane proteins They play important roles in Cell adhesion, ensuring that cells within tissues are bound together A desmosome, also known as macula adherens or macula adherentes ( Latin: adhering spot) is a cell structure specialized for cell-to-cell Clathrin is a Protein that is the major constituent of the 'coat' of the clathrin- Coated pits and coated Vesicles formed during Endocytosis In Biology, caveolae ( Latin for little caves) are small (50&ndash100 Nanometer) invaginations of the Plasma membrane in many vertebrate Cellular adhesion is the binding of a cell to another cell or to a Surface or matrix.

Membrane skeleton

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. cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. 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. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes The cytoskeleton is able to form appendage-like organelles, such as cilia, which are covered by the cell membrane and project from the surface of the cell. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately The apical surfaces of the aforementioned epithelial cells are dense with finger-like projections, called microvilli, which increase cell surface area and thereby increase the absorption rate of nutrients. Microvilli (singular microvillus) are microscopic cellular membrane protrusions that increase the surface area of cells and are involved in a wide variety of functions including The cell membrane acts as a protecting body.

Structure and the Fluid mosaic model

According to the fluid mosaic model of S. J. Singer and Garth Nicolson, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less freely^[2]. This picture may be valid in the space scale of 10 nm. However, the plasma membranes contain different structures or domains that can be classified as (a) protein-protein complexes; (b) lipid rafts, (c) pickets and fences formed by the actin-based cytoskeleton; and (d) large stable structures, such as synapses or desmosomes. A lipid raft is a Cholesterol -enriched microdomain in Cell membranes Properties of lipid rafts Rietveld & Simons related lipid rafts in model cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm.

The fluid mosaic model can be seen when the membrane proteins of two cells (e. g. , a human cell and a mouse cell) are tagged with different-coloured fluorescent labels. When the two cells are fused, the two colours intermix, indicating that the proteins are free to move in the 2D plane.

Composition

Cell membranes contain a variety of biological molecules, notable lipids and proteins. Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:

• Fusion of intracellular vesicles with the membrane (exocytosis) not only excretes the contents of the vesicle but also incorporates the vesicle membrane's components into the cell membrane. Exocytosis (ek-soh-sy-TOH-sis Greek: Έξω - external and κύτος - cell is the durable process by which a cell directs secretory vesicles out of the Cell The membrane may form blebs around extracellular material that pinch off to become vesicles (endocytosis). Endocytosis is a process where cells absorb material ( Molecules such as proteins from the outside by engulfing it with their Cell membrane.
• If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
• Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), exchange of molecules with this small reservoir is possible.

In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called homeostasis, area regulation or tension regulation. Homeostasis (from Greek: ὅμος hómos, "equal" and ιστημι istēmi, "to stand" lit

Lipids

Examples of the major membrane phospholipids and glycolipids: phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer). Phosphatidylcholines are a class of Phospholipids which incorporate Choline as a headgroup Phosphatidylethanolamine ( cephalin, sometimes abbreviated PE) is a Lipid found in Biological membranes. Phosphatidylinositol (abbreviated PtdIns or PI is a minor Phospholipid component in the cytosolic side of eukaryotic Cell membranes. Phosphatidylserine (abbreviated Ptd-L-Ser or PS is a Phospholipid component usually kept on the inner-leaflet the cytosolic side of cell membranes by an enzyme called Translocase

The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and steroids. Amphiphile (from the Greek αμφις amphis both and φιλíα Philia: love friendship is a term describing a Chemical compound possessing both Phospholipids are a class of Lipids and are a major component of all Biological membranes All phospholipids contain a Diglyceride, a Phosphate Glycolipids are Carbohydrate -attached Lipids. Their role is to provide Energy and also serve as markers for cellular recognition A steroid is a Terpenoid Lipid characterized by a Carbon skeleton with four fused rings generally arranged in a 6-6-6-5 fashion The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant. ^[3] In RBC studies, 30% of the plasma membrane is lipid. Red blood cells are the most common type of Blood cell and the Vertebrate body's principal means of delivering Oxygen to the body tissues via the Blood

The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. 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 acids chains have a profound effect on membranes fluidity^[4] as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting point (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 maintenance of proper Cell membrane fluidity is of critical importance for the function and integrity of the cell, essential for the mobility and function of embedded

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. A noncovalent bond is a type of Chemical bond, typically between Macromolecules that does not involve the sharing of pairs of electrons but rather involves more dispersed Phospholipid molecules in the cell membrane are "fluid" in the sense that they are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, movement of phospholipid molecules between layers is not energetically favourable and does not occur to an appreciable extent. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane. A lipid raft is a Cholesterol -enriched microdomain in Cell membranes Properties of lipid rafts Rietveld & Simons related lipid rafts in model In Biology, caveolae ( Latin for little caves) are small (50&ndash100 Nanometer) invaginations of the Plasma membrane in many vertebrate Cholesterol is a Lipid found in the Cell membranes and transported in the Blood plasma of all Animals It is an essential component of mammalian

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. ^[1]

Carbohydrates

About 5% of the plasma membrane weight is carbohydrate, predominantly glycoprotein, but with some lipoprotein (cerebrosides and gangliosides). Carbohydrates (from ' Hydrates of Carbon ' or saccharides ( Greek σάκχαρον meaning " Sugar " are the most Not to be confused with Peptidoglycan. Glycoproteins are proteins that contain Oligosaccharide chains ( Glycans) covalently attached A lipoprotein is a biochemical assembly that contains both Proteins and Lipids The lipids or their derivatives may be covalently or non-covalently bound Cerebrosides are Glycosphingolipids which are important components in animal Muscle and Nerve Cell membranes Myelin is the most well Ganglioside is a compound composed of a Glycosphingolipid ( Ceramide and Oligosaccharide) with one or more Sialic acids (AKA N-acetylneuraminic For the most part, no glycosylation occurs on other unit membranes, and only ever occurs on the extracellular surface of cell membranes. Glycosylation is the enzymatic process that links Saccharides to produce glycans, either free or attached to Proteins and Lipids This enzymatic

The glycocalyx is an important feature in all cells, especially epithelia with microvilli. Glycocalyx is a general term referring to extracellular Polymeric material produced by some Bacteria, Epithelia and other cells In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body 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. Galactose (Gal (also called brain sugar) is a type of Sugar which is less sweet than Glucose. Sialic acid is a generic term for the N - or O -substituted derivatives of Neuraminic acid, a Monosaccharide with a nine- Carbon backbone The Golgi apparatus (also called the Sialic acid carries a negative charge, providing an external barrier to charged particles.

Proteins

The cell membrane plays host to a large amount of protein that is responsible for its various activities. Peripheral membrane proteins are Proteins that adhere only temporarily to the Biological membrane with which they are associated Peripheral membrane proteins are Proteins that adhere only temporarily to the Biological membrane with which they are associated The amount of protein differs between species and according to function, however the typical amount in a cell membrane is 50%. ^[4] These proteins are undoubtedly important to a cell: Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance Yeasts are a growth form of eukaryotic Microorganisms classified in the kingdom Fungi, with about 1500 Species currently described ^[3]

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. In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of Antibodies and can cause an immune response Functions of membrane proteins can also include cell-cell contact, surface recognition, cytoskeleton contact, signalling, enzymic 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. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle Once inserted, the proteins is then transported to its final destination in vesicles, where the vesicle fuses with the target membrane/

Variation

The cell membrane has slightly different composition in different cell types and has therefore different denominations in different cell types:

• Sarcolemma in myocytes
• Oolemma in oocytes. A cell type is a distinct morphological or functional form of cell. The sarcolemma is the Cell membrane of a muscle cell It consists of a true cell membrane called the plasma membrane and an outer coat made up of a thin layer of polysaccharide A myocyte (also known as a myocardial cell is the type of cell found in heart muscles. An oocyte, ovocyte, or rarely ocyte, is a female Gametocyte or Germ cell involved in reproduction.

Permeability

The permeability of membranes is the ease of molecules to pass it. This depends mainly on electric charge and, to a slightly lesser extent, on the molar mass of the molecule. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. Molar mass, symbol M, is the Mass of one mole of a substance ( Chemical element or Chemical compound) Electrically-neutral and small molecules pass the membrane easier than charged, large ones.

The electric charge phenomenon results in pH parturition of substances throughout the fluid compartments of the body. In Pharmacokinetics, a compartment is a defined volume of Body fluids.

See also

• Ammonium transporter
• AP2 adaptors
• Bacterial cell structure
• Cell adhesion
• Efflux (microbiology)
• Elasticity of cell membranes
• Gram-negative bacteria
• Gram-positive bacteria

References

External links

• Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology
• Cell membrane protein extraction protocol
• Membrane homeostasis, tension regulation, mechanosensitive membrane exchange and membrane traffic
• 3D structures of proteins associated with plasma membrane of eukaryotic cells