Biomembrane

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A biomembrane is a separating layer that surrounds a cell compartment or, as a cell membrane, separates the interior of a cell from the exterior. Within the cell, differently structured biomembranes separate the interior of organelles or vacuoles from the cytoplasm . Through membrane components, a biomembrane has an active role in the selective transport of molecules and the transmission of information between the two compartments between which this biomembrane is located.

permeability

Since the biomembrane is primarily a separating layer between different areas, it is impermeable to most molecules. Smaller lipophilic molecules, such as carbon dioxide , alcohols and urea, can freely diffuse through the lipid bilayer of the membrane . In order to allow the permeability of the membrane for lipophobic particles such as water or large particles such as ions or sugar molecules , various transport proteins are embedded in the membrane , which are responsible for the transport of certain substances. That is why one speaks of selective permeability .

construction

A biomembrane is always topologically closed and encloses a space. Non-self-contained membranes do not occur in intact cells. Biomembranes are asymmetrical: they have a plasmatic side facing the cytoplasm (P-side) and an extraplasmic side (E-side).

Biomembranes are made up of lipids and proteins . Carbohydrate chains can be linked to the proteins. As a lipid bilayer, the lipid component forms the basic substance of the membrane and is responsible for its special physicochemical properties. In particular, this double layer acts as a passive separating layer. Steroids like cholesterol enter into a hydrophobic interaction with the lipids and solidify the otherwise flexible biomembrane at high concentrations in the biomembrane. In addition, proteins are distributed on and within the membrane, which assume the active functions of the membrane. The proteins only have a very small support function of the biomembrane, as they swim through the lipid layers.

Biomembranes can be characterized based on their density ; it is usually between 1.12 and 1.22 g · cm −3 . The density depends on the weight ratio of the proteins to the lipids: depending on the function of the membrane, values ​​of 0.25 ( myelin membrane , low protein content ), 1.3 (plasma membrane of erythrocytes ), 2.5 (plasma membrane of E. coli ), 2.9 (inner mitochondrial membrane ) up to a value of 5 in the purple membrane found in Halobacterium (high protein content).

In certain types of cell organelles ( nucleus , mitochondrion , plastid ), biomembranes occur as double membranes .

Phospholipid bilayer

The lipid bilayer consists largely of amphiphilic phospholipids , which have a hydrophilic head group and a hydrophobic tail group (mostly hydrocarbon chains ). As a result of the hydrophobic effect , a double layer forms in water , with the hydrophobic tails pointing inwards and the hydrophilic heads pointing outwards. Because of the hydrophobic core, such a lipid bilayer is almost impermeable to water and water-soluble molecules, but at the same time very flexible and mechanically difficult to destroy. For this reason, even a puncture with a pipette does not leave a hole in the membrane. In return, it can be destroyed by lipid solvents and lipases .

Membranes are made up of three main types of lipids: phosphoglycerides, sphingolipids, and cholesterol.

Phospholipids
Phospholipids are characterized by a phosphate group , they make up the main part of membrane lipids. Most of the time they have a basic structure made of glycerine , “across” the membrane, which is why they are called phosphoglycerides. Two of the three hydroxyl groups in glycerol are esterified with hydrophobic fatty acids, the third with a hydrophilic phosphate group. The phosphate group can have a further substituent. If it does not do what almost never occurs in membranes, the molecule would be called phosphatidic acid. Choline , which leads to phosphatidylcholine (PC), or ethanolamine , which leads to phosphatidylethanolamine (PE), serine, leads to phosphatidylserine (PS) or inositol , leads to phosphatidylinositol (PI), is often used as a substituent . It applies that all the molecules described consist of a hydrophilic head group, the phosphate with substituents and a hydrophobic tail, an unbranched fatty acid of 16 to 20 atoms. Depending on the number of double bonds in the fatty acid, a distinction is made between saturated fatty acids (no double bonds ), monounsaturated (one double bond) and even polyunsaturated.
Sphingolipids
A sphingolipid is a compound of a sphingosine that is linked to a fatty acid via its amino group. The hydroxy group can with various groups esterified to be, there are no esterification ceramides , esterification with phosphocholine results sphingomyelin and saccharides result glycosphingolipids . Sphingolipids are also amphipathic and are similar to phospholipids.
cholesterol
Animal membranes can contain up to 50% cholesterol ( percent by mass ), less in plants and not at all in bacteria. Cholesterol is small and not very amphipathic, which is why only the hydroxyl group is on the membrane surface and the rest of the molecule is in the membrane. The rigid ring system of cholesterol hinders the flow of the lipid layer, making it more rigid.
Scheme of the (liquid) lipid bilayer of a biomembrane

The lipid bilayer of a biomembrane is usually liquid; H. the lipids and proteins are quite mobile in the plane of the membrane. An exchange of lipids between the two layers or even a detachment of a lipid from the membrane is very rare. A targeted movement from one side of the membrane to the other ( flip-flop ) is normally only possible with the active participation of special proteins (so-called flippases and floppases ) while consuming adenosine triphosphate (ATP). Flippases transport lipids from the outside of the plasma membrane to the cytosolic side. Floppases are classic ABC transporters and transport membrane lipids from the cytosolic side of the plasma membrane to the outside. Other transporters for membrane lipids are scrambladders , which, however, do not exchange membrane lipids in the direction of their concentration gradient depending on ATP until equilibrium is established.

How fluid the lipid bilayer is depends primarily on the number of double bonds in the hydrophobic hydrocarbon chains of the lipids; some bacteria also use branched chains . The more, the more fluid the membrane is. On the other hand, the degree of liquid is determined by other embedded molecules. Cholesterol, for example, on the one hand reduces fluidity, but at low temperatures prevents the membrane from solidifying like a gel .

Vitamin E is an antioxidant (like vitamin C ); it protects the unsaturated hydrocarbon chains of the phospholipids of the biomembrane from being destroyed by free radicals ( lipid peroxidation ).

Membrane proteins

Model of the cell membrane according to the liquid mosaic model

Different types of membrane proteins , which are embedded in the lipid bilayer, ensure different properties of the biomembranes via protein-lipid interactions . The two sides of a biomembrane can also differ greatly due to the arrangement of the membrane proteins. For example, while receptors for cell-cell communication and for the detection of environmental changes are directed outwards, enzymes involved in reactions point inwards (they are therefore in the cytoplasm).

Many proteins are involved in membrane transport ; H. on the exchange of substances and the transmission of signals via specific receptors. A large number of membrane proteins that characterize different cell types and their stages of maturity and that can differ from individual to individual ( e.g. blood and tissue groups ) have been well studied. This also includes molecules (mostly glycoproteins) that contribute to the distinction between oneself and others according to the lock and key principle .

According to the liquid mosaic model , the membrane proteins are not rigidly fixed in the membrane, but are capable of highly dynamic changes in location within the membrane. This dynamic forms the prerequisite for the triggering of manifold signal chains at the cellular level, both intracellularly and between cooperating cells.

The membrane proteins can be classified according to their anchoring in the lipid bilayer:

Integral proteins
Gene sequencing suggests that 30% of all encoded proteins are integral proteins. Integral proteins protrude as transmembrane proteins through the lipid bilayer, sometimes in multiple loops. Integral proteins, like phospholipids, are amphipathic. Domains within the membrane are hydrophobic, the amino acid residue interacts with the lipid chains. However, these undirected forces alone would not be sufficient for stabilization; In the case of many proteins, a strip of mostly basic residues interacts with the charged head groups of the phospholipids. The other part that protrudes from the membrane interacts with the surrounding water and the substances dissolved in it. Integral proteins are not necessarily firmly anchored in the membrane, but can also move freely.
Peripheral proteins
Peripheral proteins can be located on the inside and outside of the membrane. They are temporarily attached to these or to integral proteins through a mixture of electrostatic and hydrophobic interactions as well as other, non-covalent bonds. The attachment is dynamic, depending on the condition, it can be bound or released. The membrane does not have to be destroyed in order to obtain it; a highly concentrated salt solution is sufficient to bring them into solution, as this weakens the electrostatic interactions. As an example, the best investigated on the cytoplasmic side are proteins that, as fibrils, form something like a skeleton, those that form coatings, and enzymes. Peripheral proteins outside mostly belong to the extracellular matrix. Integral and peripheral proteins can be modified post-translationally by binding to fatty acid residues, prenylation or a GPI anchor .
Lipid Anchored Proteins
Lipid-anchored proteins do not protrude through the membrane either, but are covalently linked to a lipid molecule embedded in the membrane. A distinction is made between different types (including prenylation ( farnesylation , geranylgeranylation ), S-acylation or myristoylation), but many are GPI-anchored . Proteins with a GPI anchor are located on the outside of the plasma membrane.

function

The cytoplasm inside a cell is separated from the outside by a biomembrane. These are called cell membranes , plasma membranes , plasmalemmas or membrana cellularis . Biomembranes have the following tasks:

Compartmentalization
For energetic reasons, each biomembrane represents a gapless layer. With several membranes, there are automatically separate spaces, so-called compartments. Most cells contain reaction and storage spaces ( compartments ), such as cell organelles and vacuoles with very different chemical properties. There are very different substances in the different compartments. Thus, very different, z. Sometimes even opposing processes are possible at the same time that do not affect each other, such as carbohydrate build-up and breakdown. Furthermore, individual regulation is possible.
Scaffold for biochemical activity
For specific reactions, the exact alignment of the molecules with each other is necessary, as certain interactions have to be entered into. This exact alignment is not possible in solution. Biomembranes now offer a framework on which molecules can effectively interact and react with one another. Otherwise important reactions would not be possible; the multi-enzyme complex of the respiratory chain and photosynthesis , for example, are anchored in the membrane.
Selective permeability
Particles do not penetrate membranes unhindered, but can be selected and possibly retained.
Transport of dissolved substances
Molecules can be transported from one side of the membrane to the other, even against a concentration gradient (i.e. actively). In this way, nutrients can be enriched in the cell. Ions can also be transported across the membrane, which plays a major role in nerves and muscles.
Response to external signals
The plasma membrane is important for a reaction to external stimuli (i.e. for signal transmission). There are receptors in the membrane. If a certain molecule diffuses in their vicinity (a ligand), both can combine due to their complementary structure and the receptor sends a signal to the cell. Different receptors recognize different ligands , so that the cell can absorb information about its environment. Reactions to the environment would have to adapt the metabolism by changing the enzyme activity, release storage materials or even commit suicide.
Intercellular interaction
The plasma membrane is the outer layer of the cell. In multicellular cells, one cell interacts with its neighboring cells via the plasma membrane. So cells can e.g. B. stick together or exchange signals and material.
Energy conversion
Membranes are involved in energy conversions such as photosynthesis and the breakdown of carbohydrates. In eucaryotes, the former takes place in the chloroplasts , the latter in the mitochondria .
Surface enlargement
Small protuberances of the biomembrane, so-called microvilli , enlarge the cell surface and thus the area that can be worked on, since the metabolism takes place particularly intensively on the biomembrane.

Fluidity

Influence of unsaturated fatty acids on the membrane structure

The fluidity of a biomembrane depends on the temperature . A membrane made of phosphatidylcholine and phosphatidylethanolamine, the fatty acid residues of which are saturated, would be quite fluid at 37 ° C. In this state the membrane could be viewed as a two-dimensional liquid crystal . The long axes of the phospholipids are aligned parallel, the phospholipids themselves can rotate and move freely in the plane. Up to a certain temperature, the transition temperature, the movement of the phospholipids is severely restricted and the membrane properties change, the state now resembles that of a frozen gel . The transition temperature depends on the type of lipids; the shorter they are and the more double bonds they contain, the lower it is. Cholesterol disrupts the normal structure of the membrane and reduces the mobility of membrane lipids. The transition temperature can then no longer be clearly determined. In animal cells, the lipid cholesterol ensures that the fluid level is maintained even with temperature fluctuations.

meaning

The fluidity of a biomembrane lies between rigid and liquid and thus allows a certain structure. Membrane proteins can lengthen into functional units and later separate again. This is important for photosynthesis, for example. Fluidity also plays a major role in membrane genesis and is important for many basic processes such as cell division , cell growth , secretion, etc. While the temperature often fluctuates, the membrane fluidity must remain constant. To achieve this, the membrane lipids can be modified: an exchange of phospholipids is possible; Desaturases can form double bonds from single bonds, the phosphate backbone and lipid tails of the phospholipids can be redistributed and a higher proportion of unsaturated fatty acids can be produced than before. In this way, especially cold-blooded creatures can adapt to the environment.

Lipid rafts

Lipid molecules are not evenly distributed in the biomembrane, but there are microdomains with a special lipid composition. Cholesterol and sphingolipids are particularly prone to such an association. Some proteins, such as those with GPI anchors, accumulate in such areas, while others are particularly rarely found there. Lipid rafts are presumably very small and in a constant process of dissolution and regeneration.

history

Scheme of a lipid bilayer with peripheral proteins (sandwich model)
Scheme of the experiment by Frye and Edidin from 1972
  • 1895 Charles Ernest Overton assumes that the biomembranes consist of lipids. He concluded this from observations that lipophilic (fat-soluble) substances, for example certain anesthetics, can get into cells much more easily than substances that are lipophobic.
  • 1917 Irving Langmuir suspects that phospholipids float on the surface of the water.
  • In 1925, the Dutch scientists Gorter and Grendel developed the bilayer model : Phospholipids with hydrophilic groups are arranged as a double layer in the membrane in such a way that the hydrophilic groups of the lipids point outwards and the hydrophobic groups in the interior of the double layer. However, with their model, the two researchers completely ignored the large protein content of the biomembrane.
  • In 1935, JF Danielli and H. Davson presented the classic model of the structure of a biomembrane: The biomembrane consists of a bimolecular lipid layer . The hydrophobic tails of the lipids face each other, the hydrophilic heads are coated with proteins . In short: protein - lipid bilayer - protein (sandwich structure). Electron microscopic images of biomembranes reveal a three-layer structure: two outer layers (each 2.5 nm thick) and a middle layer (3 nm thick). This membrane model is a unit membrane called (ger .: unit membrane).
  • 1972 developed Seymour Jonathan Singer and GL Nicolson , the fluid mosaic model (fluid mosaic model) a biomembrane: Globular protein molecules to "float" in a bimolecular lipid film. The lipid film behaves like a viscous two-dimensional liquid , so lipid molecules and proteins can diffuse unhindered in the membrane plane . There are two types of membrane association of proteins. Integral proteins, also called transmembrane proteins, extend through the membrane. Peripheral proteins , also called associated proteins, are deposited on the lipid bilayer.
  • 1972: At the same time as Singer and Nicolson, Frye and Edidin concluded from experiments with two cells in which certain membrane proteins were labeled that the membrane cannot be static, but is in constant motion. They combined the marked cells and the marked areas of the membrane that were only present separately mixed.
  • In 1983 Mouritsen and Bloom introduced the mattress model of the cell membrane. It says that the hydrophobic part of the membrane proteins that is embedded in the membrane is not always exactly the same size as the cell membrane and that lipids of different chain lengths are therefore suitably stored around certain membrane proteins.
  • Since Singer and Nicholson set up the liquid mosaic model in 1972, numerous indications have been discovered which led to the formulation of the dynamically structured mosaic model . Various studies have shown that the proteins and various lipid molecules are by no means evenly distributed on the surface of the membrane, as would be expected in a pure liquid. Instead, there seem to be areas with a high concentration of certain proteins (so-called receptor islands ) or certain types of lipids (so-called lipid rafts ) that are constantly regrouping, dissolving and coming back together.

Inner membrane system

The inner membrane system comprises various cell compartments or organelles in eukaryotic cells, which are surrounded by biomembranes. These include: the nuclear membrane , the endoplasmic reticulum , the Golgi apparatus , lysosomes , vesicles , endosomes, and the plasma membrane . Most proteins that are intended for organelles of the inner membrane system are cotranslationally transported into the endoplasmic reticulum and from there transported via the secretory pathway in the inner membrane system.

Web links

Individual evidence

  1. ↑ Biomembrane I: Selective Permeability of Membranes . ( Memento of February 8, 2008 in the Internet Archive )
  2. ↑ H. Kleinig, P. Sitte: Cell Biology . 2nd Edition. Stuttgart 1986, ISBN 3-437-30528-X , p. 33.
  3. ↑ Hans Kleinig, Uwe Maier: Kleinig / Sitte Zellbiologie . Verlag Gustav Fischer, 1999, ISBN 3-437-26010-3 .
  4. ↑ a b c Gerald Karp, Kurt Begin: Molecular Cell Biology. Springer, 2005, ISBN 3-540-27466-9 , pp. 157-230.
  5. ↑ T. Kaneda: Iso- and anteiso-fatty acids in bacteria: biosynthesis, function, and taxonomic significance. In: Microbiol. Rev. 55 (2), June 1991, pp. 288-302. PMID 1886522
  6. ^ S. Tan, HT Tan, MC Chung: Membrane proteins and membrane proteomics . In: Proteomics . tape 8 , no. October 19 , 2008, p. 3924-3932 , doi : 10.1002 / pmic.200800597 , PMID 18763712 .
  7. ↑ E. Gorter, F. Grendel: On bimolecular layers of lipoids on the chromocytes of the blood. In: Journal of Experimental Medicine . Volume 41, 1925, pp. 439-443.
  8. ^ SJ Singer, GL Nicolson: The fluid mosaic model of the structure of cell membranes. In: Science . Volume 175, 1972, pp. 720-731. PMID 4333397 .
  9. ↑ OG Mouritsen, M. Bloom: Mattress model of lipid-protein interactions in membranes . In: Biophys. J. Band 46 , no. 2 , August 1984, p. 141–153 , doi : 10.1016 / S0006-3495 (84) 84007-2 , PMID 6478029 , PMC 1435039 (free full text).
  10. ↑ G. Vereb u. a .: Dynamic, yet structured: The cell membrane three decades after the Singer-Nicolson model. In: Proc. Natl. Acad. Sci. UNITED STATES. Volume 100, 2003, pp. 8053-8058. PMID 12832616 , PMC 166180 (free full text).