Transmembrane proteins are integral membrane proteins that span the entire lipid bilayer of a cell's membrane. They play crucial roles in various cellular functions, including transport, signaling, and maintaining the cell's structure. These proteins have hydrophobic regions that interact with the lipid bilayer and hydrophilic regions that extend into the aqueous environment inside and outside the cell. These proteins can function as channels, allowing specific molecules to pass through the membrane, or as receptors, transmitting signals from the outside to the inside of the cell. Examples of transmembrane proteins include ion channels, G protein-coupled receptors, and transporters, all of which are essential for cellular communication and homeostasis.