Final answer:
In an integrin, a transmembrane helix crosses the lipid bilayer, allowing the protein to integrate with and be anchored in the cell membrane.
Step-by-step explanation:
The structure that crosses the lipid bilayer in an integrin is a transmembrane helix. Integrins are integral membrane proteins that are embedded in the cell membrane. These proteins have hydrophobic transmembrane segments that consist of alpha-helices to allow them to integrate with the hydrophobic core of the phospholipid bilayer. Their hydrophilic regions can be exposed to either extracellular fluid or the cytosol, whereas their hydrophobic regions remain adjacent to the phospholipid tails within the membrane, anchoring the protein in place.