Transmembrane domains are segments of proteins that span the lipid bilayer of cell membranes. These domains are typically composed of hydrophobic amino acids, allowing them to interact favorably with the lipid environment while anchoring the protein in place. They play a crucial role in the function of membrane proteins, including receptors and channels. These domains can vary in length and structure, often forming alpha-helices or beta-barrels. The arrangement of transmembrane domains is essential for the protein's ability to transport molecules across the membrane or transmit signals, impacting cellular communication and metabolism.