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Selective Transport

Transmembrane proteins permit certain molecules to pass through the lipid bilayer. This type of transport is highly specific for molecules and ions Therefore, the cell needs many different kinds of transport proteins.
In general, all transport proteins function in a similar fashion. The molecule to be transported first binds to the protein. This causes the protein to change its shape. In essence, it “closes” behind the molecules and “opens” to the opposite side of the membrane (remember, molecules can be transported in either direction: from the inside out or from the outside in). This is often referred to as a gated channel. Once the protein has allowed the molecule to pass through the lipid bilayer, the molecule is released.
There are two types of selective transport:


Passive transport (or facilitated diffusion): Transmembrane proteins facilitate the diffusion of a solute down its concentration gradient. Remember our discussion earlier: this process will continue until an equilibrium is reached. However, this rarely happens. For example, once a molecule is transported into a cell, it is usually metabolized right away. Therefore, the concentration gradient does not change and the molecule will continue to diffuse into the cell.active transport:  Molecules can be pumped across the cell membrane against their concentration gradient, i.e. from an area of low concentration to an area of high concentration. Since this is not the normal behavior for molecules, active transport requires energy, usually in the form of ATP. Once ATP binds, it alters the conformation of the transmembrane protein and the molecule can bind. The molecule is transported across the membrane and is released. The protein returns to its original conformation, preventing the molecule from making the return trip across the membrane.

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