Biological membrane

A biomembrane acts 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 biomembranes separate different design the interior of organelles or vacuoles from the cytoplasm. However, a biomembrane is not just a passive barrier layer, but has membrane components (eg, transmembrane proteins) play an active role in the selective transport of molecules and the transmission of information between the two compartments between which these biomembrane is.

• 4.1 The importance of the fluidity
• 4.2 lipid rafts

Construction

Biomembranes are composed of lipids and proteins. Carbohydrate chains may be attached to the proteins. The lipid content is formed as the lipid bilayer matrix of the membrane and is responsible for its specific physicochemical properties. In particular, this double layer acts as a passive interface. Steroids such as cholesterol are entering into a hydrophobic interaction with the lipids and solidify at high concentrations in the biomembrane, the otherwise flexible biomembrane. In addition, distributed on and within the membrane proteins, which take over the active functions of the membrane. The proteins have only a very small support function of the biomembrane as they float through the lipid layers.

Biomembranes can be characterized by their density; it is usually from 1.12 to 1.22 g · cm -3. The density is the weight ratio of proteins to lipids dependent: depending on the function of the membrane are values ​​of 0.25 ( myelin membrane, low protein content), 1.3 ( plasma membrane of red blood cells ), 2.5 ( plasma membrane of E. coli), 2.9 (Inner mitochondrial membrane ) to a value of 5 in the purple membrane in Halobacterium ( high protein content) found.

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

Lipid bilayer

The lipid bilayer is composed largely of amphiphilic phospholipids (mostly hydrocarbon chains) have a hydrophilic head group and a hydrophobic tail group. In water is formed, as a result of the hydrophobic effect, a double layer, in which the hydrophobic tails point inwards and the hydrophilic heads towards the outside. Because of the hydrophobic core, such a lipid bilayer is virtually impermeable to water and water-soluble molecules, but at the same time very flexible and mechanically difficult to destroy. For this reason, even leaving a puncture with a pipette no hole in the membrane. For this, they can be destroyed by the lipid solvent, and lipases.

Membranes are composed of three major types of lipids: phosphoglycerides, sphingolipids and cholesterol.

The lipid bilayer of a biological membrane is normally liquid, ie the lipids and proteins are fairly mobile in the plane of the membrane. However, the exchange of lipids between the two layers, or even loosening of a lipid membrane is very rare. A specific movement of one membrane side to the other ( flip-flop ) is normally only active Contribute of specific proteins (known as Flipasen ) with the consumption of adenosine triphosphate (ATP ) is possible.

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

Vitamin E is an antioxidant ( such as vitamin C), it protects the unsaturated hydrocarbon chains of the phospholipids of biological membranes from destruction by free radicals ( lipid peroxidation).

Membrane proteins

Different types of membrane proteins that are embedded in the lipid bilayer, provide different properties of biomembranes. The two sides of a biological membrane can differ greatly by the arrangement of membrane proteins. Whereas for example the receptors for cell-cell communication and the detection of changes in the outside environment are directed towards exhibit responses to enzymes involved in the inside ( ie, they are in the cytoplasm ).

Many proteins are involved in membrane transport, ie the mass transfer and the signal transfer through specific receptors. Well studied is a variety of membrane proteins, characterize the different cell types and their stages of maturation and differ from individual to individual can (for example, blood and tissue groups). These include molecules ( mostly glycoproteins ) that contribute for the key - lock principle for self - foreign distinction.

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

A classification of membrane proteins is possible after anchoring in the lipid bilayer:

Integral proteins

Through gene sequencing is thought that 30 % of all encoded proteins are integral proteins. Integral proteins protrude as transmembrane proteins through the lipid bilayer through, some of them multiple times in a loop. Integral proteins are as phospholipids amphipathic. Domains within the membrane are hydrophobic, amino acid residue interacting with the lipid chains here. This undirected forces alone would not be sufficient for stabilization; in many proteins, a strip interacts mostly basic residues with the charged head groups of the phospholipids. The other portion, which protrudes out from the membrane, interacts with the surrounding water and the substances dissolved. Integral proteins are not necessarily anchored in the membrane, but can also be free to move.

Peripheral proteins

Peripheral proteins are located outside of the membrane, they are attached temporarily by a combination of electrostatic and hydrophobic interactions and other noncovalent bonds to them or to integral proteins. The addition is dynamic, depending on the condition they can be bound or loosed. To win they will not destroy the membrane; a highly concentrated salt solution is enough to bring them in solution, as this weakens the electrostatic interactions. As an example, the best studied are on the cytoplasmic side of proteins that form fibrils than something like a skeleton, those which form coatings and enzymes. Peripheral proteins outside mostly belong to the extracellular matrix. Integral and peripheral proteins can be post-translationally modified by binding to fatty acid residues, prenylation or a GPI anchor.

Lipid -anchored proteins

Lipid -anchored proteins also do not protrude through the membrane, but are covalently linked to a membrane embedded in the lipid molecule. There are different types (including prenylation ( farnesylation, geranylgeranylation ), S- acylation or myristoylation ), but many are GPI - anchored.

Permeability (permeability)

Since the biomembrane is primarily a barrier layer between the various areas, it is impermeable to most molecules. Small lipophilic molecules can diffuse freely through the lipid bilayer of the membrane, such as carbon dioxide, alcohol and urea. To enable the permeability of the membrane for particles lipophobic such as water, or large particles, such as ions or glucose molecules, various transport proteins are embedded in the membrane, which are responsible for the transport of certain materials. Therefore it is called a selective permeability.

Function

The cytoplasm within a cell is defined by a biomembrane outward. This is called the cell membrane, plasma membrane, plasma membrane or membrane cellularis. Biomembranes have the following tasks:

Fluidity of biomembranes

The fluidity of biological membranes is related to the temperature. For example, would be a membrane made of phosphatidylcholine and phosphatidylethanolamine, whose fatty acid residues are saturated, at 37 ° C rather fluid. In this state, one could consider the membrane as a two-dimensional liquid crystal. The longitudinal axes of the phospholipids align themselves parallel, the phospholipids themselves may rotate and move freely in the plane. Up to a certain temperature, the transition temperature, the movement of phospholipids is severely restricted and the membrane properties change, the state now more closely resembles that of a frozen gel. The transition temperature depends upon the nature of the lipid; the shorter they are, and the more double bonds they contain, the less it is. Cholesterol disturbs the normal structure of the membrane and reduces the mobility of the membrane lipids. The transition temperature is then no longer be clearly determined.

Importance of fluidity

The fluidity of a biomembrane is a middle ground between rigid and fluid and allows some division. Membrane proteins can be composed henchmen to functional units and separate them later. This is for instance important for photosynthesis. Fluidity also plays a major role in the genesis of membrane and is important for many fundamental processes such as cell division, cell growth, secretion, etc. While the temperature often varies, membrane fluidity must remain constant. To achieve this, the membrane lipids may be modified: The possible exchange of phospholipids; Can desaturases from single bonds form double bonds, the phosphate backbone and the lipid tails of the phospholipids can be redistributed and there may be a higher proportion of unsaturated fatty acids are produced than before. So is especially cold-blooded creatures an environmental adaptation possible.

Lipid rafts

In the biomembrane lipid molecules are not evenly distributed, but there are microdomains with specific lipid composition. Especially cholesterol and sphingolipids tend to such a merger. Some proteins, such as GPI-anchored, accumulate in such areas, while others can be found there very rare. Presumably, lipid rafts are understood very small and in a constant process of dissolution and reformation.

History of Model Development

• Since the preparation of the fluid mosaic model of Singer and Nicholson in 1972 numerous references were discovered, which led to the formulation of the dynamically structured mosaic model. Various studies have shown that the proteins and lipid molecules are different by no means uniformly distributed over the surface of the membrane, as would be expected in a pure liquid. Instead, there seems to be, or certain types of lipids (so-called lipid rafts ) that are constantly regroup, dissolve and come together again areas with a high concentration of certain proteins (called receptor Islands).