Answer:
The correct answer is ''proteins.''
Explanation:
Transport phenomena are essential in living beings, either to carry a hydrophobic molecule through an aqueous medium (transport of oxygen or lipids through the blood) or to transport polar molecules through hydrophobic barriers (transport through the cell membrane). Biological transporters are always proteins. Proteins are capable of carrying out specific functions such as the selective transport of molecules, or certain enzymatic reactions, among others. Membrane proteins can be peripheral or partially or totally integrated into the membrane, being, generally, globular or fibrous. Its activity is essential for the maintenance of transmembrane gradients. Thanks to membrane proteins, the exchange of molecules such as nutrients or waste products takes place, as well as the reception of external signals. Among its functions are channel-forming: Integral proteins arranged in the membrane in such a way that they give rise to the formation of a pore or channel, usually hydrophilic. These pores or channels allow the entry and exit of certain substances from the cell.
