Final answer:
Ion channels are proteins that form pores in the cell membrane, allowing charged atoms to pass through. They have selective permeability, allowing specific charged particles to cross based on their charge. This is achieved through the presence of charged side chains in the pore.
Step-by-step explanation:
Proteins called ion channels form pores in the cell membrane through which charged atoms can pass. These ion channels are capable of selectively allowing specific charged particles to cross the membrane in response to an existing concentration gradient. Channels for cations (positive ions) will have negatively charged side chains in the pore, while channels for anions (negative ions) will have positively charged side chains in the pore. This selective permeability is called electrochemical exclusion.