Membrane transport

Under membrane transport is understood by a biomembrane in the biology of transport of different substances. At the same time be doing parts of the membrane itself moves, it is sometimes referred to separately as membrane flux.

The interior of an enclosed area of a biological membrane (such as the cytoplasm of a cell) represents a biologically active, substantially self-contained unit

This (selective and self-directed ) isolation from the environment is to allow the cell to build an object indispensable for its function cell environment, and to preserve the constructed differences from the environment.

The existing phospholipid bilayer of the membrane is permeable only for gases and very small, mostly uncharged (and thus hydrophobic ) molecules " by itself ". For ions as well as the most biologically active substances it is an insurmountable barrier without help, because such molecules are polar and thus hydrophilic.

However, all life processes and specific cell functions are dependent on the cell or the compartments of which are in contact with their environment, communicate. Communication means, among other things, selective substance or particle exchange. Therefore, mechanisms must be in place, the highly selective allow molecules to pass through the membrane, such as channels and so-called carriers.

  • 2.1 endocytosis
  • 2.2 exocytosis
  • 2.3 transcytosis

Trans- membrane transport

For most transport processes that can be considered difficult to separate in complex systems (they are depending on what " around it happened," for example, from an upstream transport process ), both concentration and Ladungsgradienten act in different weighting, and partly synergistically, partly antagonistic.

Simple diffusion

Lipophilic, and small non-polar molecules pass through the diffusion through the membrane. They always follow their concentration gradient to balance the desire of this. If the concentration inside and outside the cell is equal to a flow equilibrium is established (see Figure 1, A). When charged particles and the membrane potential in the establishment of the equilibrium plays a role.

Passive transport

Even when passive transport molecules overcome the membrane without any input of energy from outside or from the cell in the direction of a concentration or potential gradient. Finally, the passive transport is only a special case of diffusion: larger molecules and ions, such as sugars, amino acids or nucleotides to which the membrane is insurmountable Here, by means of membrane transport proteins transported from one side to the other. There are two options: the free diffusion through a plasma membrane and facilitated diffusion through channel proteins or carrier proteins.

Passive transport through channel proteins

For the next while passive transport used channels are transmembrane proteins ( also called channel proteins ), which span the membrane like a tunnel. To the inside of the channel they carry polar amino acids. Characterized small polar or charged particles such as ions can be transported through these channels in the cell. Different channels have different specificity in terms of conductivity for certain ions or molecules.

Most channels open only to go to a specific signal. Ligand-gated channels respond to the binding of a neurotransmitter, such as a hormone. Voltage- gated channels respond to the change in membrane potential. Mechanically gated channels, for example, with changes in cell shape is regulated by interactions with the cytoskeleton.

If the channels once open, the molecules diffuse along the concentration gradient across the plasma membrane. This is done either to the concentration of the transported substance on both sides of the membrane is equal, so that the net flux is zero, or until the channels close again ( See Figure 1, B).

Porins are similar in structure to ion channels, however, allow the passage of much larger molecules. One example is the so-called aquaporins. This form water- conducting channels.

Passive transport by carrier proteins

The passive transport by carrier proteins, the molecule of carriers is transported from one side of the membrane to the other. Carrier specialize in very specific molecules, for which they - like enzymes - have a binding site. When connecting the carrier to the substrate, it changes its conformation. This rearrangement by the molecule in question is passed through the membrane and on the other hand are released ( see Fig 1, e ). Each to be transported substance is dependent on its corresponding carrier protein. While some carriers can carry only one molecule at a time ( Uniport ) have different binding sites for two different molecules. They change their conformation only when both binding sites are occupied. The transport takes place for both molecules in the same ( symport ) or in opposite directions ( antiport ). Note that in contrast to the secondary active transport is not dependent on an electrical gradient.

Active Transport

Active transport is defined as a transport process which only takes place in the particular system when power is supplied from the outside. With their help, then molecules against a chemical concentration gradient or ions can be transported against an electrical potential gradient.

For the energy balance of the transport of the most charged particles of both charge and concentration aspects play a role: both the reduction of the entropy of a system ( construction / reinforcement of a concentration gradient ) and charge transport against the electric field, here the resting membrane potential requires the input of energy. It should be noted that although there are about energy and charge balances in the system under consideration (here, a volume by the transporter ) is that particle concentrations and their changes due to the semi- / selectively permeable cell membrane but are certainly to be considered separately.

This energy is (often a combination of these ) made ​​essentially on three types:

  • Chemical bond energy, a typical example is the hydrolysis of ATP;
  • Degradation of a Ladungsgradienten as a " driving force ", ie electrical energy;
  • Increase of entropy in a communicating system, eg the degradation of another concentration gradient.

A transport process, which is carried in the balance sheet against the electrical gradient is referred to as electrogenic (vs. electrically neutral ). Regarding the origin of the energy and the type of work performed, a distinction is primary, secondary and tertiary active transport as well as the special case of group translocation.

  • In primary active transport of protons and inorganic ions by transport ATPases by the cytoplasmic membrane are pumped out of the cell with ATP consumption. H -ATPase operates in plant cells, for example, as a proton pump. An ion is pumped through a so-called ion pump (Fig. 1 E), from the lower side of the side of higher concentration. The energy comes from the hydrolysis of ATP to ADP and inorganic phosphate (see: Figure 1 D). An important application for the primary active transport is the sodium-potassium pump, integrated in the cell membrane protein, pumped with consumption of ATP, three positively charged sodium ions out of the cell and in the same cycle also two positively charged potassium ion pumped into the cell. Characterized the rest potential is maintained in nerve cells (neurons), which is necessary for the generation and transmission of action potentials.
  • Secondary active transport of a particle conveyed (usually an ion ) passively along an electrochemical gradient and uses the potential energy of this gradient, a second substrate opposite the electrical or concentration gradient in the same direction ( symport, for example, sodium - glucose symport in the small intestine, sodium iodide symporter into the thyroid gland ) or in the opposite direction ( anti- port, such as the sodium -calcium anti port to transport through the sodium -calcium exchanger) (See Figure 1, C).
  • The tertiary active transport, the concentration gradient is used, the a secondary active transport has built up on the basis of a primary active transport. This form of active transport di-and tripeptides are absorbed by the peptide transporter 1 in the small intestine, for example.
  • In the group translocation mostly monosaccharides such as glucose and mannose, or sugar alcohols such as mannitol or glucitol are passed through a membrane, the material to be transported chemically modified ( phosphorylated iA ) and thus is no concentration gradient. The best-studied Gruppentranslokationssystem is the so-called PEP -PTS ( phosphoenolpyruvic phosphotransferase system ) in E. coli. The necessary energy comes from ATP instead of PEP ( phosphoenolpyruvic ). This form of active transport has been found only in bacteria.

Verlagernder membrane transport

Endocytosis

As endocytosis is called a Einstülpungsvorgang the biomembrane, in which a single cell or compartment with a drop of liquid, certain solutes, macromolecules or larger food particles to smaller other cells incorporated. At the end of a so-called Einstülpungsvorgangs endosome is pinched off or repelled into the cell interior and is now part of the endomembrane system. Thus, the cell receives a portion of the surrounding medium in her heart on (See Fig 1, F).

There are two different forms of endocytosis:

  • Phagocytosis (also called " cell eating " ) should be included in the solid particles,
  • The pinocytosis (the " cell drinking" ) should be included in the dissolved particles.

Further, the receptor-mediated (or receptor -controlled ) endocytosis of importance in the specific receptors ( Asialoglykoproteinrezeptoren ) on the cell surface are responsible for the recognition of the particles to be included. So for example, carry LDL the apolipoprotein B-100 on their surface, which binds to the LDL receptor of the cell, and so causes the recording of the particle. In this way, cholesterol is included in the cell, for example. After binding to the receptor, the cell membrane invaginates, and forms a coated pit (English: coated pit, a depression, which is lined with the protein clathrin ). At the neck of the vesicle thereby increasing the protein dynamin outsourced to. This recognizes with its pleckstrin homologous domain ( pleckstrin homology domain, PH) the specific Phosphainositol from the membrane. In the arrangement to a dynamin - supramolecule Ampiphysin which binds with its SH3 domain, the proline-rich domain (PRD ) of dynamin and thereby recruited more Dynaminmoleküle helps. In the GTP-bound state, the supramolecule is as right-handed helix around the Vesikelnacken. During the interaction of the GED domain and the GTPase domain of dynamin GTP is hydrolyzed and the dynamin molecule Supra makes a conformational change. In " Poppase " theory of this is an increase in the pitch of the Dynaminhelix, which results in that the vesicle is repelled from the membrane. In " Pinchase " theory, it is this conformation which leads to the reduction of the helix diameter and thus to pinch-off of Vesikelnackens.

Exocytosis

The exocytosis is a process in which substances are released from the cell to the cell environment. These substances can be formed either in the cell or be indigestible remnants of the cell digestion. Basically always merges in a transport vesicle exocytosis ( exosome ) with the cell membrane (See Figure 1, G). The exosome has a simple lipid bilayer ( biomembrane ) as an outer coating, which consists of the cell membrane. Most Exozytosen associated with endocytosis ( exocytosis coupled endocytosis ). This is necessary to avoid that the membrane is increased freely. On the other hand, the cell saves hereby also the de novo synthesis of transport vesicles and the associated membrane proteins. This process is called vesicle recycling.

Transcytosis

Transcytosis ( = Zytopempsis ) is a receptor- dependent transport of extracellular material through the cell, and thus a combination of endocytosis and exocytosis. The vesicle is passed on to a neighboring cell or transported into the extracellular space, without that its content is changed. It occurs in the epithelial cells of the vessels and in the epithelial cells of the intestine, since the spaces are blocked by tight junctions.

An example of a transcytosis receptor for a family of Fc receptors. You are in the placenta and on the apical side of the intestinal epithelium and childlike transport by transcytosis maternal IgG to the fetus or to the infant.

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