Are there impermeable membranes in plant cells?

A Biomembrane serves as a separating layer (membrane) between different areas within a living cell (intracellular) or between the interior of a cell and the cell exterior (in the case of the cell membrane; intercellular). Within the cell, differently structured biomembranes separate the interior of organelles or vacuoles from the cytoplasm. However, a biomembrane is not just a passive separating layer, but through membrane components (e.g. transmembrane proteins) it plays an active role in the selective transport of molecules and the transmission of information between the two compartments between which this biomembrane is located.

construction

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 such as cholesterol enter into a hydrophobic interaction with the lipids and solidify the otherwise flexible biomembrane. In addition, proteins are distributed on and within the membrane, which take on 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).[1]

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

Lipid 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. To do this, 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 of glycerol are esterified with hydrophobic fatty acids, the third with a hydrophilic phosphate group. The phosphate group can carry a further substituent. If it does not do what almost never occurs in membranes, the molecule would be called phosphatidic acid. A frequent substituent is choline, which leads to phosphatidylcholine (PC), or ethanolamine, which leads to phosphatidylethanolamine (PE), serine, leads to phosphatidylserine (PS) or inositol, leads to phosphatidylinositol (PI). 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 polyunsaturated.
  • Sphingolipids: A sphingolipid is a compound of a sphingosine that is linked to a fatty acid via its amino group. The hydroxyl group can be esterified with various groups, without esterification ceramides result, esterification with phosphocholine produces sphingomyelin and with saccharides glycosphingolipids result. Sphingolipids are also amphipathic and are similar to phospholipids in this respect.
  • 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.[2]
Scheme of the (liquid) lipid bilayer of a biomembrane
The lipid bilayer of a biomembrane is normally liquid, i.e. the lipids and proteins are quite mobile in the plane of the membrane. However, 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 with the active participation of special proteins (so-called. Flipasen) with consumption of adenosine triphosphate (ATP) possible.

How fluid the lipid bilayer is depends primarily on the number of double bonds in the hydrophobic hydrocarbon chains of the lipids, some bacteria[3] also use chain branches. 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 it 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 that are embedded in the lipid bilayer ensure different properties of the biomembrane. 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 (i.e. they are located in the cytoplasm).

Many proteins are involved in membrane transport, i.e. in the exchange of substances and signal transmission 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 here 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 be freely mobile.

Peripheral proteins

Peripheral proteins are located outside 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 loosened. 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 studied 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.[2]

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[4].

function

The cytoplasm inside a cell is separated from the outside by a biomembrane. These are called the cell membrane, Plasma membrane, Plasma lemma or Cellular membrane. 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. This means that very different, sometimes even contradicting processes are possible at the same time that do not affect each other, such as the build-up and breakdown of carbohydrates. Furthermore, an individual regulation possible.
  • Scaffold for biochemical activity: For specific reactions, the exact alignment of the molecules with respect to each other is necessary, as certain interactions have to be entered into. This exact alignment is not possible in solution. Biomembranes now provide 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. active). 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.
  • Reaction 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. For example, cells can adhere to one another 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, as the metabolism takes place particularly intensively on the biomembrane.[2]

Fluidity of Biomembranes

The fluidity of a biomembrane is related to temperature. For example, 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. From 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.

Importance of fluidity

The fluidity of a biomembrane is a middle ground between rigid and liquid and allows a certain breakdown. 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

In the biomembrane, lipid molecules are not evenly distributed, 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 probably very small and in a constant process of dissolution and regeneration.

History of the model development

Scheme of a lipid bilayer with peripheral proteins (sandwich model)
Experimental scheme of the experiment of Frye and Edidine from 1972
  • 1895 Charles Overton assumes that the biomembranes are made up of lipids. He concluded this from observations that lipophilic (fat-soluble) substances, for example certain anesthetics, can enter cells much more easily than substances that are lipophobic.
  • 1917 Irving Langmuir suspects that phospholipids float on the surface of the water.
  • 1925 was made by Dutch scientists Gorter and Grendel the Bilayer model developed[5]: 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, the hydrophobic groups in the interior of the double layer. However, with their model, the two researchers completely disregarded the large protein content of the biomembrane.
  • In 1935 J. F. Danielli and H. Davson 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 microscope images of biomembranes reveal a three-layer structure: two outer layers (each 2.5 nm thick) and a middle layer (3 nm thick).
  • Developed in 1972 Seymour Jonathan Singer and G. L. Nicolson the Liquid mosaic model(fluid mosaic model) a biomembrane[6] : Globular protein molecules “Swim” 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 closed Frye and Edidine from experiments with two cells in which certain membrane proteins were marked 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 presented the mattress model of the cell membrane. It states 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.[7]
  • Since the establishment of the liquid mosaic model by Singer and Nicholson in 1972, numerous indications have been discovered that can be used to formulate the dynamically structured mosaic model[8] led. 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 seems to be areas with a high concentration of certain proteins (so-called Receptor islands) or to give certain types of lipids (so-called Lipid rafts), which constantly regroup, dissolve and come back together again.

Individual evidence

  1. ↑ Hans Kleinig, Uwe Maier, Kleinig / Custom Cell Biology. Verlag Gustav Fischer, 1999. ISBN 3-437-26010-3
  2. 2,02,12,2Karp, Gerald; To begin, Kurt: Molecular cell biology. Springer, 2005, 978-3-540-27466-7, pp. 157-230.
  3. ↑ 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 (free full text access)
  4. ↑ Biomembrane I: Selective Permeability of Membranes
  5. ↑ Gorter, E. & Grendel, F. (1925): On bimolecular layers of lipoids on the chromocytes of the blood. The Journal of Experimental Medicine. Vol. 41, pp. 439-443.
  6. ↑ Singer, S.J. & Nicolson, G.L. (1972): The fluid mosaic model of the structure of cell membranes. Science. Vol. 175, pp. 720-731. PMID 4333397.
  7. Mouritsen OG, Bloom M: Mattress model of lipid-protein interactions in membranes. In: Biophys. J.. 46, No. 2, August 1984, pp. 141-53. doi: 10.1016 / S0006-3495 (84) 84007-2. PMID 6478029. Full text at PMC: 1435039.
  8. ↑ Vereb, G. et al. (2003): Dynamic, yet structured: The cell membrane three decades after the Singer-Nicolson model. Proc. Natl. Acad. Sci. UNITED STATES. Vol. 100, pp. 8053-8058. PMID 12832616PDF

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