Final answer:
Transmembrane proteins are the components of the cell membrane that serve as channels across the lipid bilayer, facilitating the transport of substances. Peripheral proteins, carbohydrates, and cholesterol play other roles but do not form such channels.
Step-by-step explanation:
The channel across the lipid bilayer may be served by transmembrane proteins. These proteins are integrated within the bilayer, allowing substances to pass through the otherwise impermeable lipid membrane. Transmembrane proteins are a type of integral proteins that can form channels or pores through which ions and molecules can diffuse passively, or they may facilitate active transport mechanisms.
While carbohydrate chains are associated with cell recognition and cholesterol provides membrane flexibility, it is the transmembrane proteins that actually form the channels for transporting substances. Peripheral proteins may be involved in recognition and signaling processes, yet they typically do not span across the lipid bilayer to form channels. Ion channels are a specific example of transmembrane proteins, highlighting how integral they are to the function of the cell membrane in selectively permitting the passage of substances.