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Typus
KomplettPlagiat
Bearbeiter
SleepyHollow02
Gesichtet
Yes.png
Untersuchte Arbeit:
Seite: 27, Zeilen: 1 ff. (entire page)
Quelle: Anézo 2003
Seite(n): 40 f., Zeilen: 40: 20 ff.; 41: 1 ff.
I.3. Biomembrane transport

I.3.1 Transport pathways and mechanisms

One of the most important roles of biomembranes is to moderate the passage of substances between compartments of organisms. In spite of their chemical and structural differences, they all act as highly selective permeability barriers, separating internal and external fluids. The transport processes that have evolved to overcome these barriers are at least as important as the barrier functions of biomembranes. Solute transport can occur via a transcellular pathway (across the cell) or via a paracellular pathway (between the cells). The lipid matrix, structural framework of all biological membranes, constitutes the main permeation barrier to transcellular transport. The adhesion of adjacent cells is ensured by junctions, whose structure influences paracellular transport. Paracellular diffusion is, by definition, solely passive, while transcellular transport involves various mechanisms. These mechanisms can be divided in two main classes. In the first class, the solute moves from a region of relatively high chemical potential to a region of lower chemical potential by passive or facilitated diffusion. For this type of transport, no cellular energy, such as that in the form of ATP, is required. The second form of transport involves active processes, which do require an input of energy and generally occur against a concentration gradient. These processes are all protein-mediated. In each class of transport, a wide variety of mechanisms elucidate how solute flux across membranes is achieved.

I.3.2 Passive transport

Passive diffusion across the lipid matrix represents the simplest form of transport and refers to the movement of a molecule by random processes through the lipid bilayers portion of a membrane without an expenditure of energy. A net flux of the transported species across the membrane is only achieved when a difference in chemical potential of the species is observed between both sides of the membrane. This chemical potential difference usually results from a difference in concentration (or activity) of the species on [one side versus the other.]

1.3 Biomembrane transport

1.3.1 Transport pathways and mechanisms

One of the most important roles of biomembranes is to moderate the passage of substances between compartments of organisms. [...] In spite of their chemical and structural differences, they all act as highly selective permeability barriers, separating internal and external fluids. [...] The transport processes that have evolved to overcome these barriers are at least as important as the barrier functions of biomembranes. Solute transport can occur via a transcellular pathway (across the cell) or via a paracellular pathway (between the cells). The lipid matrix, structural framework of all biological membranes, constitutes the main permeation barrier to transcellular transport. The adhesion of adjacent cells is ensured

[page 41:]

by junctions, whose structure influences paracellular transport. [...] Paracellular diffusion is, by definition, solely passive, while transcellular transport involves various mechanisms. These mechanisms can be divided in two main classes. In the first class, the solute moves from a region of relatively high chemical potential to a region of lower chemical potential by passive or facilitated diffusion. For this type of transport, no cellular energy, such as that in the form of ATP, is required. The second form of transport involves active processes, which do require an input of energy and generally occur against a concentration gradient. These processes are all protein-mediated. In each class of transport, a wide variety of mechanisms elucidates how solute flux across membranes is achieved. The principal transport mechanisms are briefly described in this section.

Passive diffusion across the lipid matrix Passive diffusion across the lipid matrix represents the simplest form of transport and refers to the movement of a molecule by random processes through the lipid bilayer portion of a membrane without an expenditure of energy. A net flux of the transported species across the membrane is only achieved when a difference in chemical potential of the species is observed between both sides of the membrane. This chemical potential difference usually results from a difference in concentration (or activity) of the species on one side versus the other.

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Sichter
(SleepyHollow02), Klgn

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