What is passive transport
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Passive transport mechanisms
Some biologically relevant gases such as O pass through simple diffusion2, N2 or CO2 the membrane. Due to their lipid solubility, not only fatty acids but also some vitamins, non-polar pharmaceuticals or toxic substances such as aromatic compounds or halogenated hydrogen can pass through the membrane by simple diffusion. Ethanol or urea can also diffuse through the lipid bilayer. Water can only overcome the lipid barrier through simple diffusion with a low transport rate. The penetration of the water molecules depends on the fluidity (density) and composition of the membrane.
Proteins are embedded in the lipid bilayer, which form water-filled channels. Ions can diffuse through this to the other side of the membrane. Such ion channels can close and open in a controlled manner. By forming weak, non-covalent interactions with the transported ion, they act selectively according to their molecular size and ion charge. A little selective transport occurs through so-called porins, i.e. open, water-filled pores through which substrate molecules of different sizes fit through depending on the pore diameter. For some substrate molecules (e.g. sugar) there are also selectively acting porins. Water molecules are specifically transported through the aquaporins.
Specific membrane proteins, the carriers (also called transporters or permeases), enable larger and / or polar molecules or ions to pass through the membrane. During the transport process, carriers specifically bind to their substrate molecules through weak noncovalent interactions and channel them through the membrane without changing them chemically. They thus catalyze a change in the location of the substrate and are subject to the laws of enzyme kinetics
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