Which membrane protein spans the entire lipid bilayer?

• A. Transmembrane proteins
• B. Peripheral proteins
• C. Glycoproteins
• D. Channels that do not span fully

Answer: (A)

Proteins that cross the lipid bilayer are transmembrane proteins. The membrane’s interior is hydrophobic, so a protein that spans from the outside to the cytosol must have hydrophobic segments that traverse the bilayer. These integral proteins can form channels, transporters, or receptors, and their structure allows them to interface with both sides of the membrane to enable signaling, transport, or enzymatic activity across the boundary.

Peripheral proteins stay attached to the membrane surface rather than crossing it; they interact with the membrane or with other membrane proteins but do not span the bilayer. Glycoproteins describe membrane proteins with carbohydrate chains, often on the extracellular surface; many are transmembrane, but glycosylation alone doesn’t indicate that a protein crosses the membrane. A channel that does not span fully wouldn’t provide a continuous passage across the membrane, so it isn’t a transmembrane protein.